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From the French of F. Dronin . 

Editor of The Practical Photographer. 

Percy Lund & 

Co., The Country Press, Bradford; 


Hall, Ludgate Circus, London. 




TV] O book dealing with the theory and practice 
^ of the stereoscope and stereoscopic photo- 
graphy has been published in this country since 
about i860, when Sir David Brewster gave to the 
world his now historic work. The active revival of 
stereoscopy brings with it the need of a modern 
treatise on the subject, more adapted to the times 
than the book of thirty years ago ; for if in optical 
knowledge but few strides have been made, in 
matters photographic our progress has been 
enormous. The present work is almost a literal 
translation from "Le Stereoscope et la Photo- 
graphie Stereoscopique" of M. Drouin, only a few 
alterations and interpolations having been made 
in the necessary anglification. The metric system 
was adopted by the author, and it has been deemed 
advisable in most instances not to convert into 
the cruder English method. 


Binocular Vision : The Perception of Relief - 5 

The Pseudos.cope. The Purpose of Judgment in 

Vision - - - - - - - -13 

The Telestereoscope and the Iconoscope - - 19 

The Stereoscope -------- 29 

Panoramic Stereoscopes. Various Combinations - 69 
Examination of Stereoscopic Pictures Without a 

Stereoscope ------- 73 

Stereoscopes of Projection - - - - - 81 

Obtaining Relief by a Single Picture - - - 91 

Applications of the Stereoscope - - - - 99 

Stereoscopic Photography ------ 109 

Stereoscopic Photography by Displacing the 

Object- 121 

Stereoscopic Photography by Successive Exposures 131 

Stereoscopic Photography by Simultaneous Ex- 
posures - - - - - - - - 1 39 

Stereoscopic Photography without Lenses - - [49 
Stereoscopic Photography by Artificial Eight - 155 
Stereoscopic Negatives - - - - - - 161 

Stereoscopic Positives - - - - - 165 

A Few Words of History ------ 175 





A A / HEN we are looking at a flat picture, such 
as a drawing or a photograph, we grasp, 
without difficulty, the form and relative propor- 
tion of the objects which compose it, and we are 
equally able to form an idea of the distances of 
these objects, in the direction perpendicular to the 
picture. Yet, the impression received is very 
incomplete when compared with that given by a 
view of the object itself. However perfect the 
drawing, the picture always appears flat ; the 
relief is wanting. If two objects, the dimensions of 
which are not already known, are represented in a 
picture, without being accompanied by something 
which might give an idea of their respective 




positions, it is often difficult to tell which of them 
is nearer, and which further away. 

The same impression is received when looking 
at the objects themselves, with one eye, which 
suggests the idea that the sight of both eyes is 
indispensable to the complete perception of relief. 
The accuracy of this long-established opinion has 
been confirmed by the discovery of the stereoscope, 
an apparatus which, with the aid of two pictures 
of the same thing, gives precisely the impression 
of relief, which would be produced by looking at 
the thing itself. 

We must state at the outset, that the two 
pictures formed in the right and left eyes, when 
looking at the same thing, are slightly different; 
for the simple reason that they are not seen from 
the same point of view. In order then, to obtain 
by these pictures the semblance of relief, we must 
present to each eye a different picture, similar to 
the one it would see in looking at the object 
itself. In other words, these two pictures (which 
we will call from now stereoscopic pictures) should 
be taken from two points of view, between which 
the distance is equal to the separation of the eyes. 
We shall see further on, that in some cases there 
is an advantage in augmenting this distance, in 
order to accentuate the relief ; the conditions 
which these pictures ought to fulfil, and the 
manner in which they can be^ presented to the 



eyes, form, however, the subject of this book. 

A description of the eye* is not necessary, nor 
is a list of the various hypotheses already set forth 
in explanation of why the two impressions received 
upon the retina only give one single picture, and 
why it results in the perception of relief. 

Physiologists, moreover, are not entirely 
agreed on the mechanism of the vision, and an 
intimate knowledge of these phenomena is not 

*We give herewith figures relating to the principal 
elements of the eye They serve to show the point of view 

of the formation of pictures on the retina. 

Radius of average curve of the anterior 

face of the crystalline lens 10 mm. 

Radius of average curve of the posterior 

face of the crystalline lens 6 mm. 

Average thickness of the crystalline lens, 

variable according to accommodation 

(Helmholz) 3 mm. 5. 

Diameter of crystalline lens 9 to 10 mm. 

Points of refraction ^ Exterial layers 1.405. 

crystalline lens * Middle layers 1,429. 

(W. Krause) I Nucleus I -454- 

Distance of retina from optical centre of 

crystalline lens 20 mm. 

Focal length of crystalline lens in repose 45 mm. 

cornea 32 mm. 

Radius of curves of ( Anterior face 10,075. 

the cornea (Vallee) I Posterior face 8,68. 

Point of refraction of cornea i,33- 

,, aqueous humour (Sappey) 1,337. 

Average diameter of pupil 3 to 4 mm. 

Average distance between the eyes, .about 65 mm. 

Minimum distance of distinct vision ,, 20 cm. 



necessary to attain our object, which is simply to 
present to each eye a picture as far as possible in 
the identical conditions offered by nature. Besides, 
we propose to look at things exclusively from a 
practical point of view. 

It is easy to ascertain by very simple experi- 
ments, that binocular vision is necessary to the 
complete perception of relief, but that it is not the 
only thing necessary; in other words, that the 
distances can be roughly estimated with one eye, 
but that both eyes are necessary to estimate them 
with true approximation. 

It is in this way that the distances of objects 
in a drawing are estimated, by looking on the 
dimensions given to them by perspective. It is 
the same in monocular vision. 

A one-eyed person knows the distances of 
objects around him, because he has an idea of the 
size of those objects. 

In the same way, if one eye be closed, objects 
near at hand can be easily grasped, but it would 
be very difficult to thread a needle, because the 
distance between the thread and the needle has to 
be exactly estimated in order to bring them 

Suspend a small ring on a string, and take a 
long stick — a fishing rod for instance, — stand at a 
distance from the ring, and close one eye ; then 
approach the ring, trying at the same time to put 


the end of the rod through it ; you may try a long 
time before succeeding, if, after each failure you 
resume your original position and in no way allow 
yourself to be guided by the preceding trial. 

Here is a much more simple experiment : 
Stick a pin into the table, at some distance from 
you, but within reach of your hand, place your 

Fig. i. 

arm at your side, then, with one eye closed, try, 
without hesitation, to place the tip of the finger on 
the pin. Very probably you will put it either on 
one side, further away, or nearer to you. 

Another experiment, suggested by Necker de 
Saussure, shows how difficult it is for one picture 
to give an exact representation of an object. 

If a cube be drawn (Fig. i) it can be seen in 



two different ways. In looking at the point A, 
the face AFG H appears to be in front ; but if, 
on the contrary, the point B is regarded, the face 
B E D C appears to be in front. 

A similar design is shown in Fig. 2. By 
looking at face A, or at face B, as the case may 
be, the stairs are seen either in relief or hollow. 


These experiments can be repeated in another 
form, by using solid objects, and looking at them 
with one eye. We have constructed a cube of 
wire, similar to Fig. 1. Painted white, and 
placed against a black background, it can be re- 
versed at will, as easily as the drawing, if looked 


at with one eye.* At whatever distance it may- 
be placed, even at the minimum of distinct vision, 
the change can be easily produced, with one eye. 
If the cube be too near, the perspective of the 
picture so reversed would be false. If, while looking 
at a picture so reversed, the other eye be gradually 
opened, the cube will at once assume its true 
position, and it afterwards becomes impossible to 
see it otherwise. If a retreat be now made to 
some distance, the reversal can be produced as 
easily while using both eyes.t This is explained 
by the fact that the two pictures formed in the 
eyes are less different at some distance from the 
object, and that beyond a certain distance they 
are sufficiently alike for the appearance of relief to 
disappear almost completely. 

* A very curious observation arises from this experiment : 
We will suppose that in reality (Fig. i) the face C B E D is 
in front, and that the reversing is effected so as to bring the 
face A F G H forward. If the head be moved, or if a turn 
be taken round the cube, it will appear to move at the same 
time. This illusion is easily explained by the fact that the 
vertical edges in the primitive cube have become oblique in 
the turned cube, and as they are bound to remain parallel 
with each other, the angle they make, along with the 
horizontal edges, varies when the position of the eye is 
changed, whence the illusion of rotation. 

fThe side of the cube which we used measured 30 mm. 
at a distance of about 3 m. 50, and it is possible to obtain 
the reversing when using both eyes. 



These simple experiments, which show that 
the use of both eyes is necessary for the perfecft 
appearance of relief, may be varied indefinitely; 
but if we wish to penetrate more deeply into the 
mechanism of binocular vision, we must have 
recourse to several instruments, which are, how- 
ever, very simple, and which allow the conditions 
to be varied. 



I N looking at an object with one eye its depth 
is perceived by looking at it perspectively. 
In looking at it with both eyes the relief is 
complete, and becomes much more striking, 
because of the perception of two different pictures. 
But it is only by experience founded on preliminary 
experiments, that we learn to make use of this 
double phenomenon. 

We have all seen that a young child in trying 
to seize an object will often join his hands in front 
of it, because he has not a true idea of his distance 
from it. It is only by many similar mistakes, and 
by comparisons between the senses of touch and 
sight, that he at last finds out the distances by 
sight alone. 

The presence of a double impression (per- 
spective and binocular effect) makes this estimate 
sufficiently sure. In cases where by some artifice 
the two impressions are rendered incompatible, 


the judgment becomes fixed for a more lengthy 
period on that impression which predominates. 

Suppose we are looking at the figure of a 
hexagonal pyramid. The picture impressed on 
the left eye will be A (Fig. 3), that on the right 
eye will be B. If the pyramid, instead of being 
in relief were hollow, the reverse would be the 
case, i.e., B would be the left picture and A the 

Fig. 3. 

right. We .can then suppose that if the right 
picture be presented to the left eye, and vice versa, 
the form of the object will be seen unlike its real 
one, i.e., the hollow parts will appear in relief, and 
those in relief will appear hollow. But if the 
experiment be made, it will be found that this 
reversal takes place, though not in every case ; 
because in order to produce the change it must be 


as easy to picture the reverse form of a thing as 
the primitive one. 



merit is formed by two rectangular flint prisms, 
P P' (Fig. 4), whose oblique faces measure about 
fifteen millimeters. * These two prisms are placed 
in such a position that their hypothenuses make 
between them an angle similar to that of the eyes. 
The object is looked at through these two prisms, 
which are set in such a way that the two pictures 
are superposed. In reality, the substitution of 
the right for the left picture does not take place, 
but on account of the reversal, the right picture 
takes the form of the left one. Take, for example, 
the pyramid (Fig. 3) of which we have already 
spoken. The right picture seen by means of the 
pseudoscope will be picture B turned round, i.e., 
picture A. In this way we get the illusion of the 
hollow pyramid. At the same time, there are, as 
we have already said, certain objects which cannot 
be turned, because the reverse form is not con- 
ceivable. Thus in looking through a pseudoscope 
at a plaster medallion, it is easy to think that it 
has the form of a mould, i.e., that it is hollow ; 
but it is impossible to get the reversal of a living 
figure. It also often happens that the effects of 
shadow give inadmissible pictures. A medal with 
the face strongly lighted is a very good object 
with which to make the experiment. 

Helmholz has suggested, also, the use of a 

* The apparatus represented in Fig. 71 might be used 
as a Pseudoscope. 


graduated glass guage, with the divisions on the side 
next the observer (the convex side). Examined 
through a pseudoscope, the guage will appear to 
be divided on the other (concave) side. 

A pseudoscope which really presents the right 
picture to the left eye, and vice versa, is easily 
made. We only need four mirrors, or still better, 
four prisms of total reflection. We place prism P 
(Fig. 5) with its face to 
the left eye, and set it in 
such a position that it reflects 
the picture into another 
prism, P' placed before the 
right eye. We shall obtain 
the same results by using 
Wheatstone's instrument, 
except that objects will be 
seen in their true form. 

We shall see further on 
that the reversal of relief 
is also effected through the 
stereoscope by presenting 
to both eyes, not the objects 
themselves, but photographs of them. 

Fig. 5- 


I HE perception of relief obtained by binocular 
vision proceeds from the difference in the 



Fig. 6. 

form of the two pictures. If, by the aid of an 
appropriate instrument, we present to both eyes 



two pictures seen from the same point, and 
therefore alike, the appearance of relief will 
immediately disappear. The iconoscope, invented 
by Javal (1866), helps to prove this. This instru- 
ment (Fig. 6) is formed of a double mirror, MM, 
which reflects the pictures of objects into two 
other mirrors, m m', placed at a distance from 
each other equal to the separation of the eyes. 
Objects looked at with this instrument assume the 
appearance of a flat painting, even in a more 
pronounced degree than in monocular vision. 

On the contrary, a 
drawing looked at with 
the iconoscope gains 

We have con- 
structed an iconoscope 
of a different form. 
Fig. 7 shows its 
principle. A drum, 
T, with a vertical axis 
rapidly rotates round 
this axis. The object, 
P, which is to be 
observed is fixed in the centre of the drum and 
turns with it. Through a hole, f, which is made 
near a generator, we can see the object, P. The 
eyes, placed at O O', will successively see the 
object, P, through the hole, f, as the drum rotates; 


and as they will each see it in exactly the same 
position, the image will appear to be without 
relief. In order that the object, P, should be 
lighted in the same way in both cases, the source 
of light should be fixed in such a way that it will 
turn with the drum. 

Fig. 8. 

Fig. 8 illustrates this iconoscope. The drum 
is set in action by a small electric motor, and the 
object is fixed in the interior by means of screws 
and holdfasts. It is lighted by an incandescent 

lamp which rotates along with it, and which is 


2 2 





supplied from outside by the aid of a plate which 
rubs against an insulated coil. The speed of 
rotation should be from thirty to forty turns in a 
second. A most interesting experiment is made 
by placing in the apparatus the small wire cube , 
A already mentioned (Fig. i). 

This cube, seen in the icono- 
scope, and at the minimum 
distance of distinct vision, is 
absolutely without relief, and 
can be seen reversed as easily 
as in the case of the drawing. 

The relief of an object 
seen with both eyes becomes 
more pronounced the more the 
two pictures differ ; in other 
words, it augments at the same 
rate as the angle O AO' (Fig. 
9), or the optical angle. This 
explains why the form of an 
object near at hand is so much 
more easily seen than that of 
one at a distance, even when 
the latter is greatly enlarge^ 
(as by means of an opera glass). The most perfect 
perception of relief is obtained when the object is 
placed at the minimum of distinct vision. From 
a longer distance the relief is, practically speaking, 
no longer perceived ; and if we attempt to estimate 

0/ „J > 

o 3 h 

Fig. 9. 


2 3 

the distances between any objects it is by first 
deciding on their probable dimensions. 

These facts are made use of in arranging 
panoramas (such as Niagara in London, Battle 
of Waterloo, etc.), where the nearer parts are 
represented by real objects, and those further 
away by the painting, arranged in such a way that 

©,■ i <t> 

Fis. 10. 

the spectator can scarcely tell where the reality 
terminates and the painting begins. 

The telestereoscope, invented by Helmholz, 1857, 
is an apparatus the function of which is to increase 
the relief of distant objects by enlarging the optical 
angle in the following manner : — 

Two mirrors, M M' (Fig. 10), are placed at a 

2 4 


distance from each other much greater than that 
of one eye from the other. Two other mirrors, 
m m', reflect into the eyes the pictures shown in 
the mirrors, M M'. The perception of relief is as 
great as if the separation of the eyes equalled 
that of M M\ It is evident, therefore, that the 
telestereoscope produces precisely the opposite 
effect: to the iconoscope. Telestereoscopes have 

Fig. ii. — Telestereoscope. 

been constructed by simply mounting mirrors on 
supports (Fig. n), but it is better to enclose the 
whole in united tubes or in a box (Fig. 12). 

Claudet in i860 fitted opera glasses with four 
prisms forming a telestereoscope, thus magnifying 
the depth as well as the length and breadth. In 


using an [ordinary glass the appearance of depth 
in distant objects seems to be reduced, which 

Fig 12. — Telestereoscope. 

proves that the optical angle is less than it would 
be if the objects were seen directly and in the 
same dimensions. 


The relief of an object can be varied in a very 
simple manner by looking at it through two thick 

Fig. 13. Fig. 14. 

glasses, inclined in accordance with the eyes. 
The relief is increased by looking at the object 
from the interior of the angle formed by the two 



glasses (Fig. 13). On the contrary, by looking at 
it from the exterior the relief is diminished (Fig. 
14). On referring to the figures opposite, it will 
be seen that the optical angle is really increased 
in the first, and diminished in the second case. 


I F we could obtain two pictures of the same 
object, such as would be seen by the two eyes, 
and could look at each picture with the corres- 
ponding eye, a single picture would be seen, giving 
an effect of relief as though the object itself were 
before our eyes. We get this result by the use of 
the Stereoscope, 

Primarily it may be thought that once the 
images are obtained, it is sufficient to place one 
of them before each eye to secure immediately 
the resultant picture ; but such is not the case ; 
the eyes converge naturally and fix on the same 
point, refusing to look in a parallel direction so 
that each may see a separate picture. Even if a 
screen be placed between the two pictures, so as 
to hide from each eye the one it ought not to see» 
both eyes would instinctively converge towards a 
point. It is possible to make the eyes look 
parallel, but the performance involves painful 
optical gymnastics and to avoid this the stereo- 



scope has been invented. The stereoscope is,, 
then, an apparatus which, having two pictures 
placed in it, while permitting one to be seen by 
each eye, allows the two eyes to converge at a 
certain angle, as in ordinary sight. 

The stereoscope has been made in numerous 
forms ; we shall describe the most notable without 
maintaining any historical order. Stereoscopes of 
projection will be dealt with in a subsequent 
chapter, at present we are only concerned with 
stereoscopes of dived vision. 

Refvatting stereoscopes are only employed some- 
what exclusively, and photographers have been 
led to mount stereoscopic views in the ordinary 
manner, for the use of these instruments. We 
shall, however, describe various other forms of the 
stereoscope, which, while not always permitting 
the use of ordinary stereographs, possess numerous 
interesting peculiarities. 

The Reflecting Stereoscope. — This instru- 
ment which formed the turning-point of stereoscopic 
science was invented by Wheatstone in 1832. It 
is formed of two vertical mirrors M M' (Fig. 15) 
inclined to one another at an angle of 90 degrees. 

The pictures to be examined are placed at 
E E', and a screen pierced by two holes t f, 
indicates the position of the eyes, and only allows 
the rays forming the corresponding picture to pass 


to each eye. A glance a Fig. 15 shows that two 
similar points g d, belonging to each picture, com- 
bine at v in the true picture. 

Fig. 16 represents the usual form of reflecting 
stereoscope. The pictures G D are placed on 
supports, movable horizontally and vertically, so 
that the position may be exactly regulated. A 

6 1 6 

Fig. 15- 

and B are mirrors (their angle may be varied in 
some instruments). E and F are openings and 
are sometimes furnished with lenses intended to 
magnify the pictures. 

The stereoscopes used by Wheatstone were 
at first made for the examination of drawings ; 
as the pictures were reversed by the mirrors they 
had to be drawn accordingly. Photographs can 



be examined by it equally well ; but it is necessary 
in that case also to use reversed prints. Although 
there is little difficulty in obtaining reversed 
photographs, it is also possible to use ordinary 
prints in a reflecting stereoscope, by putting the 
right hand picture to the left eye and vice versa. 

The result will then be seen reversed and 
possessing true relief. 

In order to understand this transference of 
the pictures, let g d be views of the end of a pencil 
seen flat (Fig. 17). 

Suppose then that the left picture g is placed 
on the left in the stereoscope. After the reflection 
in the mirror, the perspective of this picture will 
be inverted, and it will appear as though seen by 
the right eye. In the same way, the right picture 
d will take the left perspective. In the case of a 

Fig. 16. 


symmetrical object seen full face, the substitution 
of the pictures can be made without altering the 
form of the object ; such would be the case with 
the end of the pencil, of which we are speaking ; 
but if it be a landscape, it will be seen reversed 
unless, as before mentioned, reversed pictures be 
used. The inconveniences of the instrument, 
together with the necessity of experimenting in 

Fig. 17. 

order to obtain combination, have led gradually to 
its disuse. It has, however, the advantage of 
allowing pictures of any dimensions to be used. 
At the same time, in the case of very large subjects, 
it would be necessary to increase the distance, 
and the size of the instrument to such an extent 
that it would become cumbrous. To guard 
against this inconvenience, it has been suggested 



that the pictures should be hung on opposite walls 
of a room, and nothing but the central part of the 
instrument retained. In this case there is not 
only a difficulty in regulating, but the lighting 
of the pictures is unequal. 

The instrument is generally made with mirrors 
silvered on the under side, with the result that 
a double picture is often evident ; this fault may 
be remedied by the use of metallic mirrors. 
Brewster has remedied it still more perfectly, in 
replacing the mirrors by prisms of total reflection. 

Wheatstone's stereoscope is perfectly adapted 
for the examination of transparencies ; the only 
modification necessary is to replace the two 
supports by frames fitted with ground glass. 
Flashed opal glass gives a still better effect. The 
lighting is secured in this case either by two lamps 
placed one on each side of the stereoscope, or two 
joined mirrors. In all cases it is better to mount 
the apparatus on an adjustable stand, so that the 
transparencies can be brought to the height of an 
ordinary lamp. This elevation of the stereoscope 
makes it easier to use on a table, as it places the 
mirrors on a level with the eyes of a seated person. 

Total Reflection Stereoscope. — Duboscq 
invented this stereoscope for the examination of 
large pictures. It is formed of two isosceles 
prisms P P', the use of which will be readily 


understood by aid of Fig. 18. The two images 
g d give at r the resulting picture. 

The pictures can be enlarged by lenses placed 
before the eyes. It will be seen that this instru- 

© j o ' 

Fig. 18. 

ment permits the use of two pictures placed on 
the same level. It is better, therefore, to mount 
them on one card, in order to avoid adjustment 
afterwards, the pictures being once for all put into 
position in reference to each other. 



Direct Stereoscopes. — Eliott & Waterston 
invented a stereoscope which brings about the 
perception of relief without the interposition before 
the eyes of any instrument either reflective or 

It is composed of a box B (Fig. 19) open at 
CC. In the opposite end two holes are pierced, 
having a distance from each 
other corresponding with 
that of the eyes. The 
dimensions and position of 
the opening C C are so 
determined that the right 
eye can only see the picture 
placed at the left, and the 
left eye that on the right. 
The sides of the opening 
C C are fitted with keys, 
with which to regulate the 
dimensions according to size 
and distance of the objects. 
This instrument profits by 
the natural tendency of the 
eyes, in ordinary vision, to 
converge towards the same 
point, but, as it exacts the 
convergence at a certain 
distance, some effort is 
necessary to bring about the desired result. In 

Fig. 19. 



fact, the eyes, accustomed to look at the plane 
g d, must converge as if they were looking at the 
plane C C, which is much nearer. It is therefore 
evident that, at first, it will prove difficult, the 
conditions differing from those of ordinary vision. 
To facilitate the adaptation a small object should 
be placed at C C\ and looked at first, so that 
the eyes become forced to take the desired 

Steinhanser constructed a similar stereoscope, 
furnished with either lenses or portions of concave 
lenses, in such a way as to allow a normal accom- 
modation for a given convergence. This instru- 
ment really constitutes a refracting stereoscope 
contrary to that of Brewster. 

Volpicelli also invented a direct stereoscope, 
which he named the Diaphragmatic Stereoscope. It 
is composed of a box 62 centimetres long, 20 broad 
and 11 in height, of which one of the small ends is 
pierced with two holes, the opposite end receiving 
two stereoscopic pictures. Two vertical screens, 
movable round the vertical angles of the first end, 
serve to limit the compass of each eye. 

Telescopic Stereoscope. — H. de la Blan- 
chere gave his name to a stereoscope of his inven- 
tion, similar to the preceding ones, but more 
perfect, because it allows accommodation, corres- 
ponding to the convergence to be obtained. 




Its appearance reminds us of an opera-glass 
(Fig. 20). The two lenses of which it is formed 
are mounted on hinges which allow the adjust- 
ment : — 

1 st. Of the distance between them to suit 
the distance between the eyes. 

2nd. Of their inclination, in such a way as 
to make both look at the picture. 
3rd. Of their convergence. 

When the apparatus has 
been properly regulated, and 
the two glasses put in posi- 
tion by these numerous move- 
ments, superposition is easily 

Zinelli (1857) used ordin- 
ary opera-glasses for examin- 
ing stereographs, but it is 
evident that if these are fixed 
for looking at an ordinary 
object an effort will be essen- 
tial to bring about superposition. 

Double Total Reflection Stereoscope. — 
Girard-Teulon (1861) invented a stereoscope whose 
form recalls the telestereoscope of Helmholtz. It 
consists of four total reflection prisms arranged 
as indicated in Fig. 21. Their duty is to separate 
the two visual axes at such a distance as to allow 


the examination of large pictures, the two prints 
G D being placed in the usual position. Two 
ordinary prisms P P' placed before the eyes 
produce the desired convergence. 

J z. 

Fig 21. 

Refractive Stereoscopes. — These instru- 
ments are those almost exclusively employed at 
the present time. They allow the use of prints 

4 o 


mounted on one card,* and placed in their natural 
position, that is, the right picture at the right, and 
the left at the left of the observer. 

g d are the two 
pictures and P P' two 
prisms fixed together 
by their angles (Fig. 
22). If the angle and 
position of each prism 
be conveniently fixed, 
two similar points of 
the pictures g d, will 
be seen at the same 
point r, and then 
superposition and 
stereoscopic relief will 
be obtained. 

In the apparatus 
formed simply of two 
prisms, each eye natur- 
ally sees both pictures, 
the left eye will see 
them refracted to g 1 d 1 
and the right eye to 
g* d 2 (Fig. 23). (The 
two pictures have been 

Fig. 22. 

* The name stereograph is often given to the right and 
left pi&ures thus mounted side by side. 


drawn one over the other for greater clearness.) 



Fig. 23. 

superpose each other to 

The pictures g 1 and d' 
give relief, but the 
pictures d and g 2 re- 
main visible at each 
side of the former. 
This causes no serious 
inconvenience, but the 
lateral images can be 
done away with by 
placing between the 
two prisms an opaque 
partition C, perpen- 
dicular to the picture 
(Fig. 22). 

Brewster (1844) 
improved this appara- 
tus by replacing the 
two prisms with two 
prismatic lenses, which 
both magnify the pictures and produce the desired 
refraction (Fig. 24). 

Fig. 24. 

4 2 


These prismatic lenses are made as follows : — 
Cut in two, following a diameter D D' a lens L of 
about 10 centimetres in diameter and 20 centi- 
meters focus. Then trim the two halves as 

indicated in 
Fig. 25, so as 
to make them 
r e ct angu 1 ar, 
and fix them to- 
gether by their 
small edges. 
When thus 
fixed they 
should be about 
80 millimetres 
wide (3£in.), the 
least separation 
oftheeyes being 
65 millimetres 
(2% in.) The 
sharp edges of 
the two half len- 
ses can be left 
and inserted in 
the mounting 
(Fig. 26). Brewster inserted two halves of the same 
lens into one stereoscope so as to be certain of 
obtaining the same focal length on each side. In 
practice it is not necessary for this condition to be 

Fig. 25. 


exactly adhered to. In looking by ordinary sight 
in a lateral direction at the object M (Fig. 27), the 
eyes O O' are at different distances from it, and 
the two pictures reflected on the retina are, there- 

Fig. 26. 

fore, not of the same size, but ihis does not 
prevent the easy perception of relief. 

We should therefore expect to find the same 
elasticity in stereoscopic vision, and this is, in 
fact, the case. Although the two pictures may 

Fig. 27. 

differ slightly in size there is no difficulty in seeing 
them properly in relief. Complete lenses may be 
employed instead of halves, and this may facilitate 
the construction of the apparatus. It is interest- 
ing to notice that a bi-convex lens acts in regard 


to divergence in the same way as a prism, the 
deflection of which is nil in the centre, but 
increases in crossing till it reaches a maximum at 



Fig. 23. 

the side (Fig. 28). It follows therefore that in 
varying the position of the eye, with reference to 

Fig. 29. 

the centre of the lens, the divergence sustained by 
the picture will be varied at the same time. By 
making the two lenses movable laterally, the 



divergence can be regulated, and the exact 
superposition obtained without effort. 

We must now describe the various forms in 
which Brewster's stereoscope has been con- 
structed, and they are numerous. 

Fig. 29 shows its original form. A wooden 
box holds the prismatic lenses P P' on its front 

The pictures are placed at the opposite end, 
a slit being left in the side for the insertion. 

One part of the top of the box consists of a 
door on hinges, which can be opened to allow 
light to fall on the picture, and the interior face of 
this door is fitted with a mirror. The use of the 
box is to keep out all light except that reflected 
by the mirror, in order that the face of the picture 
may be well lighted. 

On the under side of the lenses there is a 
hollow, indented to fit the nose of the observer, 
so as to allow the instrument to be brought close to 

Fig. 30. 



the eyes. This precaution, often neglected, is, 
however, of real use. 

Figs. 30 and 31 are models of stereoscopes 
with prismatic lenses, which until comparatively 
recently were the forms usually sold in this 

They are made like the preceding one. 

Fig- 3i. 

The front can be removed or opened, for the 
prisms to be cleaned ; the opposite end is fitted 
with ground glass," so that the apparatus serves 
either for transparencies or for ordinary photo- 

* Duboscq was the first to construct a stereoscope with 
ground glass at one end, for the use of transparencies. 



Fig. 32 represents a stereoscope with entire 
lenses. These are mounted in a double frame- 
work, movable by the aid of a screw or rack, so 
that the instrument may be adapted to all 
sights. In reality the focussing is not very neces- 
sary, as the stereoscope once focussed for normal 
sight, will serve for either a short or long-sighted 
person, if they wear their usual spectacles. The 

Fig. 32. 

adjustment of the lenses is not, however, without 
its use, for by varying their distance from the 
picture, the refraction of the rays is also varied, 
so that generally a distance can be found at which 
the two pictures are superposed perfectly and 
without effort. We have frequently found that 
in this kind of stereoscope, the changing of the 
lenses is much more useful for this stereoscopic 
adaptation than for its proper use of focussing. 


All these stereoscopes can either be used 

Fig. 33- 

the hand (Fig. 33) or mounted on a stand. 


M. Donnadieu has improved the ordinary 
stereoscope* so as to allow it to be used for other 
pictures besides those mounted on a card of the 
ordinary shape. To do this he has mounted the 
roughened glass on a movable frame, which can 
be separated from the box of the stereoscope. 

Fig. 34- 

Fig. 34 shows another form of refraction 
stereoscope, also useful for examining pictures 
mounted in any sort of way. There is no box, 
but the whole apparatus is fixed on an upright 

.* A. L. Donnadieu, " Traite de Photographie Stereo- 



Fig. 35 represents a pattern in which the 
separation of the lenses can be varied, one of 
them sliding laterally. This modification is very 
important : it is the only one known to us which 
permits a rational adaptation to all sights. The 
idea is by no means new, arid it is strange that 
this stereoscope is not in greater demand. 

Fig. 35- Fi 8- 36. 

Fig. 36 represents another old form of 
stereoscope, in which the box is absent. The 
apparatus thus gains in elegance, without losing 
many of its other qualities. 

The American stereoscope consists of a flat 
wooden base board, furnished with a handle. At 
one end the prismatic lenses are fixed, at the other 
is a frame intended to hold the photographs. 



This frame can be moved backwards and forwards 
on the baseboard for the purpose of focussing. 

(Fig- 37). 

This form of stereoscope has now become 
enormously popular in England and America, and, 
in fa dl, all over the civilized world. The demand 
for it has been extraordinary, and comparatively 
few households are without one. The cost of this 

Fig. 37- 

pattern is very low, about half-a-crown, or even 
two shillings for the cheaper makes, is the usual 
retail price. 

Duboscq (1857) invented an interesting mod- 
ification, which renders adaptation to all sights 
possible without varying the distance between the 
two optical systems. In his stereoscope the lenses 
are only used for magnifying, and the refraction is 
produced by prisms placed in front of them. The 



lenses can be moved backwards and forwards for 
the necessary focussing, and each prism is in 
reality divided into two, which permits the re- 
fraction to be varied by a process as follows : — 

If the two prisms, P P' (Fig. 38), having the 
same angle, a (the cylindrical form is used to 
facilitate mounting in the frames), are placed one 

Fig. 38. 

on the other, they will, when united, form a plate 
with parallel faces, and consequently without 
refraction, if they are placed to fit each other, as 
in position 1. 

If the prism P' be turned 180 degrees round 
the axis, A A', the two prisms will then have an 
angle, 2 a, and will produce the maximum refraction 


(position 2). Between these two positions lie all 
the intermediate refractions. This system, already 
applied by Rochon to his diasporametve, has been 
adapted by Duboscq to his stereoscope with 
separate prisms and lenses. For the proper 
working, the edges of the frames are toothed, and 
fitted with a toothed wheel. A pinion touches 
both wheels at once, so that by a single turn 
symmetrical regulation is effected. Fig. 39, how- 
ever, shows the whole of the mechanism. The 
side button, C, serves for focussing ; the under 

Fig- 39.— Duboscq's Stereoscope. 

button does the regulating. In Duboscq's stereo- 
scopes the angle a in each prism was 12 degrees. 
The rotation thus varies the angle from o to 24 
degrees. Duboscq proposed the use of prisms 
achromatised by each other, which would suppress, 
or, at any rate, diminish chromatic aberration in 
the white parts of the pictures. It has been 
noticed, also, that the aberration of the lenses 
tended to distort the pictures in a way contrary 
to the distortion produced by the prisms, so that 

one corrected the other. 

e - 



A short time before (1857) M. Jequezel also 
described a stereoscope with separate lenses and 

Gerard (1859) placed behind the stereoscope 
coloured glasses which could be raised or lowered, 
in front of the photographs, to produce different 

Fig 40. — The "Column'' Stereoscope. 

Cassaignes (1863) produced the same result 
by colouring the prisms or lenses, or by interposing 
coloured glasses before the eyes. 

Grillet (1855) suggested the idea of putting a 
diaphragm inside the stereoscope, so as to allow 
the piclure only to be seen without the surrounding 
white card. 


Fig. 40 represents the " column" stereoscope. 
It is formed of a high box, in the upper part of 
which are placed the lenses of a stereoscope. A 
circular chain, worked from outside by a button, 
holds the photographs, whiph can be passed 
successively before the stereoscope. Some instru- 
ments will hold as many as 200 photographs. 
These stereoscopes, which really form a piece of 
drawing-room furniture, are generally made orna- 
mentally, in accordance with their surroundings. 

A pleasant variation applied by M. Meheux 
to the "column " stereoscope, is made by fitting 
the instrument with two sets of stereoscopic lenses, 
mounted on the opposite sides of the box. Instead 
of putting only one photograph in each frame, two 
are fixed back to back, so that two persons can 
use the apparatus at the same time. After looking 
at one half of the pictures, the other half can be 
seen by using the other stereoscope. 

Under the name of gvaphoscope a combination 
has been designed, consisting of an ordinary 
stereoscope and a magnifying lens of large size 
(Fig. 41) : this lens being intended for the 
examination of ordinary photographic views. 

The instrument is represented open, and when 
closed looks like a flat box, the lid of which is 
formed by the end containing the lenses. 

In 1857 Duboscq construe! ed & portable stereoscope 
made in cloth, which opened and closed by the 


same mechanism as used in an opera hat. 

A pocket stereoscope can be made in the same 
way, formed of a box without bottom, whose 

Ffg. 41. — Graphoscope. 

sides fold up. When closed it occupies no more 
room than an ordinary pocket-book. 

We have devised the stereoscope represented 
in Fig. 42 to meet the requirements of all sights, 




and to permit the use of all photographs, whether 
mounted on cards in the ordinary way, collected 
in an album, or printed in a book. It is formed of 
two lenses of variable distance. The framework 
is fixed on two feet, P P'. We prefer instruments 
of this unrestricted kind to those where the photo- 
graph cannot of necessity be moved vertically, 

Fig. 43- 

for very often the photographs which one buys 
are not fixed on the cards at exactly the same 
height. The stereoscope has then to be slightly 
leaned, in order to obtain the relief more easily. 
The adjustment in width is found by varying the 
separation of the lenses with the aid of the button 
B, which acts on two racks. 


In the more simple model shown by Fig. 43, 
the lenses B B' are moved by the hand. The 
instrument can be made higher or lower on the 
feet which hold it, so as to regulate the focussing 
for all sights. These feet can be removed, so that 
the dismounted stereoscope can be packed into a 
small case. 

rig. 44. 

Fig. 44 shows us another form of stereoscope 
of variable separation, which answers the same 
purpose, but is not dismountable. The base is 
formed of a frame on which the photographs are 
fixed by two clips. The plane of the lenses is 
joined to this frame by two supports, which can 
be more or less bent for focussing purposes. To 
effect this, a band of black cloth (which forms at 



the same time a screen between the two pictures) 
extends from the top to the bottom, where it is 
rolled on a small eccentric wheel. Focussing is 
effected by this wheel. The eccentric wheel is to 
prevent the cloth unrolling under the action of 

the supports, the balance of the whole being 
restored after each complete turn. 

Sundry Stereoscopes. — We have now 
examined the more usual forms of the stereoscope; 
but there are others which, though not very 


practical, are still very interesting, and worthy of 
brief mention. 

Fig. 45 shows a scheme for the construction 
of a stereoscope with a single mirror. The right 
eye looks directly at the right picture d, placed at 
the left. The left sees in a flat mirror M, the left 

Fig. 46. 

picture g placed at the right. This last picture 
should be reversed, so that it can be seen in its 
true form, after being reflected in the mirror M. 
To allow for the difference in the distance from 
the two eyes, the picture g is placed a little nearer 
the mirror. 


Corbin (1861) made a stereoscope with a 
single mirror differently constructed (Fig. 46). The 
photographs are placed at g and d, perpendicularly 
to each other. The picture g is seen directly by 
the left eye O ; the picture d is seen in a mirror M. 
This latter should be reversed. Larger pictures 

Fig. 47. Fig. 48. 

than ordinary can be used with this instrument. 

It may be mentioned, that in stereoscopes of 
this description, the position of the photographs 
can be changed by merely reversing the pictures. 

Brewster designed stereoscopes with a single 
prism (Fig. 47). But it is better to produce the 


refraction by two prisms, the distortion and 
chromatic aberration being less. . 

Brewster also designed a stereoscope with 
two prisms joined together by their bases (Fig. 48), 
which can also be used as a pseudoscope. To 
obtain relief the right picture d should be looked 

Fig. 49. 

at with the left eye, and the picture g through the 
prism P\ The apparatus then acts in the same 
way as the preceding stereoscope with a single 

The pseudoscopic effect is obtained by looking 
at the right picture with the left eye through the 

6 4 


prism P, while the right eye looks directly at the 
left picture (Fig. 49). 

M. Ducos du Hauron invented a stereoscope 
for coloured pictures, formed of two glasses of com- 
plementary colours (red and green). 

The pictures to be looked at are also drawn 
in two colours, a transparent colour being used for 
one of them, and they are superposed. The 
convergence is not perfect because the form of the 
two pictures is slightly different. If the left 
picture be red, and the left eye be furnished with 
a green glass, this eye will see the picture in black 
on a green background; the right eye furnished 
with a red glass will see the right picture (green) 
in black on a red background, so that the resulting 
picture in relief will be black on a white or greyish 
ground. It is evident that this instrument can 
only be used for drawings or printed pictures. 
It is interesting to notice that in using this 
stereoscope, the optical axes of the eyes converge 
naturally, as in ordinary sight, because the pictures 
are superposed. We shall allude further on to 
stereoscopes of projection — one of which has 
furnished the principle of the preceding apparatus 
— and which also permit this natural convergence. 

Stereoscopes for a Single Picture. — We 
shall here describe those instruments which are 
more rightly " stereoscopic curiosities," than 


stereoscopes properly so called ; for they are 
constructed for the purpose of giving stereoscopic 
relief, by using only one picture ; and it is very 
evident that this is nonsense. Their use, however, 
is very restricted, and any sort of picture cannot 
be examined by them. However, the following is 
the artifice employed : — 

We will suppose an object C, seen from the 
front, whose two halves are symmetrical, in 

Fig. 50. 

accordance with a vertical plane PP' (Fig. 50) 
(in the present case it is the end of a pencil seen 
flat). The right and left pictures of this object 
are alike ; but, if, while looking at two points 
situated in the plane PP' from two different 
distances (for instances, the centres of the two 
bases), one of them be diverted, in accordance 
with the other, to a distance d in the left picture 



G, the same point in the right picture D would 
also be diverted to the same distance, but in the 

opposite direction. 
The same thing would 
happen to all the similar 
points in the two 
pictures; in other 
words, the right and 
left pictures are 
symmetrical in accord- 
ance with the line of 
intersection of the 
plane PP' with the 
plane of the pictures. 
(It is evident that this 
symmetry only exists 
in the forms, and not 
in the shadows of the 
objects). The first 
and second pictures 
seen in a mirror would 
be alike. 

Brewster based on 
this idea, a stereo- 
scope for a single 
picture. Fig. 51 
suffices to show the 
way in which it is disposed. I is the single 
picture seen directly by the left eye, and P is a 


prism of total reflection, intended to give the 
exact picture seen by the right eye. By slightly 
inclining the prism P convergence is easily pro- 
duced. The picture seen through the prism is 
rather smaller than the other, because of the 
difference in the distance, but it is so slight that it 


Fig. 52. 

does not spoil the stereoscopic effect. Brewster, 
however, proposed to correct this by the use of a 
lens, or, still better, by enlarging the two pictures 
by using two lenses of rather different focus. He 
also proposed to use, in place of a prism, the half 
of a bi-convex lens (Fig. 52), the other half being 



employed for enlarging the picture seen directly. 

It is extremely easy to repeat this experiment 
for the perception of relief by a single picture. 
We give (Fig. 53) the left picture of a symmetrical 
object ; the reader can, by using a single total 
reflection prism, held in the hand before the right 

Fig- 53- 

eye, obtain relief in this picture. It will be noticed 
that in refracting the right picture to separate it 
from the left, it is rather weaker than the left ; 
this is due to the absorption of light by the prism ; 
but its only inconvenience is the very slight 
diminution of intensity in the resulting picture. 


| jUBOSCQ invented two stereoscopes for the 
examination of pictures of great length, 
such as panoramic views. 

The first of these 
instruments (Fig. 54) is 
composed of two mirrors 
M M', whose positions 
can be regulated so that 
they will reflect the 
photographs (placed A 
and E one above another) 
in two frames B B', 
before which the eyes are 
brought. Thus, in this 
instrument the pictures 
are seen on the surface opposite to the observer. 
Unlike other stereoscopes, the dimensions of the 
pictures are limited in height, but not in length. 

The plan of a second apparatus is shown in 
Fig. 55. The pictures face the observer ; one of 


Fig- 54- 



them is looked at directly by the right eye ; the 
other is seen by the left eye through two total 
reflection prisms, one of which is movable for the 
purpose of exact regulation. 

The first of these stereoscopes has been 
combined with the phenakisticope in the following 
manner : A hollow vertical cylinder holds the two 

series of pictures, placed one exactly beneath the 
other. The cylinder is pierced with holes, cor- 
responding to the frames fixed on the exterior, 
The mirrors are placed inside. Duboscq named 
this instrument the steveophantascope or the bioscope. 

In i860, Czugafewicz invented a stereoscope 
with lenses so placed that the pictures could pass 
rapidly before them, and thus form a phena- 

The stereoscope and the heliochromoscope 
can also be combined. As is generally known* 

Fig. 55- 


the latter instrument was invented by Mr. F. E. 
Ives, to show photographs in colours, by the 
superposition of three pictures of different colours. 
It seems very easy then to join stereoscopically 
two heliochromoscopes, and thus obtain, at the 
same time, both colour and relief. 


EVERAL of the stereoscopes already described 

can be so simplified that anyone can make 

Thus that of Eliott & Waterston, and the 
one invented by Volpicelli can be made of a 
simple wooden or cardboard box. Wheatstone's 
stereoscope can also be made without great 

A stereoscope with a single prism can be 
made by fixing two sheets of glass having a 
surface of several square centimetres one on the 
other with a little modelling wax, and by intro- 
ducing between them some drops of water to form 
a liquid prism. 

There is, however, a much simpler way of 
examining stereographs mounted in the ordinary 
way. It is by looking at the pictures directly, 
without the interposition of any instrument what- 



We have seen that in natural vision the eyes 
looking at an object placed at the minimum of 
distinct vision converge at an angle of about 15 
degrees. In order to make each eye look at a 
separate object an artificial squinting must be 
forced, and that can be done without much 
difficulty after a few hours' practice. But, as at 
first the attempt results in great fatigue to the 
visual mechanism, it is better not to prolong it 
beyond a quarter of an hour each day. 

U LI 1 

A "B 

Fig. 56. 

The course of procedure is as follows : — 
Take two cards, G and D (two visiting cards 
would serve the purpose), and draw on the edge 
of each card a line, A B. Then, holding a card 
in each hand, bring them near together, as in Fig. 
56, so that the lines shall be only a few millimetres 
apart. Two lines will then be seen by first of all 
fixing an object a little further away so as to 
produce voluntary convergence. Under these 
conditions superposition will be produced without 



much effort, or to be more exact, three lines will 
be seen, each eye seeing two, the middle one being 
formed by the convergence of the other two. 
This experiment must be repeated several times, 
until superposition is obtained without difficulty. 
The two cards must then be separated by degrees, 
so as to accustom the eyes to a more and more 
feeble convergence, and so on until a separation 
of several centimetres has been attained. The 
two lines must then be replaced by two simple 
designs (for instance, the printed side of the 
visiting cards), and it will be found that convergence 
is easily obtained. Finally, take two stereoscopic 
pictures and cover one with the other, so as to 
place two similar points at the same distance, as 
in the previous experiment. The relief will appear 
at once, and the experiment must be continued 
by gradually separating the prints until the normal 
distance has been reached. 

It will be seen that if the prints are separated 
too quickly the eyes will immediately relapse to 
their normal convergence ; this would not matter 
very much, as the relief could be easily found 
again by slightly approaching the two pictures. 

This experiment illustrates in a striking manner 
the difference between stereoscopic and ordinary 
pictures. At each side of the central picture seen 
in splendid relief, the two flat pictures will readily 
be observed. 

7 6 


We must remark that in spite of the fatigue 
at first experienced, this way of looking at 
stereoscopic pictures entails no very abnormal 
effort on the muscles of the eyes, if the separation 
of the two prints be not greater than that of the 
A eyes. In looking at an 

object a long way off — a 
star, for instance — the eyes 
would be looking in parallel 
lines. If the conditions 
differ from ordinary vision 
to such an extent as to 
require a previous appren- 
ticeship, it is, that each time 
that we look at an object 
placed at a short distance, 
at the same time that 
natural convergence is pro- 
duced the crystalline lens 
accommodates itself instinc- 
Fig. 57 . tively to that distance. The 

two effects — convergence 
and accommodation — being connected, if the eyes 
converge as if they were looking at an object at 
the* distance O A, a little difficulty will be 
at first experienced in making them accommodate 
themselves to the distance O D. 

To look at prints without a stereoscope, Faye 
(1856) suggested the following method: — Take 


a sheet of paper in which two holes, 5 millimetres 
in diameter, are pierced at about the same distance 
as that of the eyes. Hold the sheet in one hand, 
and the picture in the other ; by degrees place the 
eyes to the sheet of paper, without ceasing to look 
through the holes. Soon the two holes will seem 

Fig. 58. 

to merge into one, and the picture in relief will 
then appear between the two flat pictures. 

Frick constructed for the same purpose the 
little instrument shown in Fig. 58. The prints are 
placed on G and D. The partition A B separates 
the two pictures, and prevents the sight of the flat 



ones on each side of the one in relief. The height 
A B corresponds to the minimum distance of 
distinct vision. 

Stereoscopic prints can, on the other hand, be 
examined by exaggerating the convergence of the 
eyes instead of diminishing it. The right picture 
must then be placed on the left, and vice versa. 
We shall not describe in detail this method of 
procedure, of which we have already spoken in 
connection with the stereoscopes of Eliott and 
Waterston and of Volpicelli. These instruments 
are by no means necessary for obtaining relief, 
and their only purpose is to relieve the observer 
of the embarrassment of seeing the two flat pictures 
at the same time as the one in relief. The second 
has the advantage over the first of allowing the 
use of pictures of larger dimensions ; on the other 
hand, it is not adapted for the use of prints 
mounted on cards as for the ordinary stereoscopes, 
because it necessitates the transposition of the 

Those of our readers who wish to try the 
experiment of looking at stereoscopic pictures may 
make use of the diagram on opposite page. It con- 
sists of a series of similar letters, with constantly 
increasing distances. First try to superpose the two 
Ss, then the two Ts, etc. When the two Es at the 
base, whose separation is about the same as that 
of the eyes, can be superposed, an ordinary stereo- 





graph can be examined without apparatus. We 
have stopped the table at the separation of the 
eyes, but, with practice, it is possible to go beyond. 
The optical angle then becomes negative, that is 
to say, the two visual axes appears to be behind 
the head ; but the sensation of relief and also that of 
distance can be equally perceived in the same way. 

When a stereoscopic picture is examined 
either by the naked eye, or by any of the above 
methods, or by means of a stereoscope, it is only 
possible to see at the same time a very small part 
of the print, and the optical angle has to be 
changed, when carrying the sight from one point 
to another, as in looking at a solid object. The 
reason is simple — two identical points b b, in the 
foreground of a sketch (Fig. 17) are at a shorter 
distance apart than a a, in the background. The 
optical angle must, therefore, be diminished, when 
the sight is carried from point a to point b. 


W TEREOSCOPES of projection are constructed 
^ for the purpose of allowing several persons 
to examine the same stereograph, so magnified 
that all its details may be seen at a distance. 

Three kinds of instruments have been con- 
structed of the above class up to the present. 
They are as follows* : — 

i st. Stereoscopes for coloured pictures ; 

2nd. Eclipse stereoscopes ; 

3rd. Stereoscopes for use with polarised 

None of these instruments come into practical 
consideration, though most of them — and especially 
the eclipse stereoscopes — give admirable effects ; 
but for such an apparatus to completely accom- 
plish its purpose, it should be easy of manipula- 
tion, so that the pictures can be rapidly changed ; 

* Duboscq proposed the use of total reflection stereo- 
scopes for examining pictures projected by a lantern. We 
simply mention this application, without describing it. 



the instrument put into the hands of the 
spectator should not be very costly ; and finally, 
no previous practice should be needed for the 
projections to be properly seen. It may be 
granted that the last condition is almost fulfilled, 
because the various instruments we are about to 
describe project the two pictures over each other, 
so that the eyes in looking at them converge as if 
they were looking at any ordinary object, and 
accommodate themselves to the distance corres- 
ponding to the convergence. The eyes, therefore, 
need make no abnormal effort. 

Before describing these numerous stereo- 
scopes, we will say something of the instrument 
which Claudet (1858) named the stereomonoscope, 
which utilises two pictures thrown on a screen, 
but which can only be used by one person. 

The two pictures right and left, thrown by 
two lanterns, are received on a screen of ground 
glass, to be viewed as transparencies. The 
position of the two lanterns is regulated so that 
the pictures may superpose each other as exactly 
as possible ; under these conditions, by standing 
before the picture (which appears single) and at a 
short distance from it, the stereoscopic relief will 
be perceived. The reason is that each eye does 
not see the two pictures with the same intensity ; 
but we shall allude to similar cases in another 


Stereoscopes for Coloured Pictures. — By 
the aid of two lanterns, two pictures are thrown 
on to the same screen, so that they superpose each 
other. A red glass is then placed before the 
condensor of one of the lanterns, and a green glass 
in front of that of the other. Each of the pictures 
thus takes a particular colour, but on the screen it 
is impossible to distinguish them by ordinary 
sight ; besides, the superposition not being always 
perfect, the mingled picture is rather grey and not 
very distinct. 

If looked at through green and red glasses, 
each of these glasses only allows the corresponding 
colour to penetrate, and each eye sees only one 
picture. Red and green being complementary 
colours, the resulting impression is a black and 
white picture seen in relief. In practice, it is very 
difficult to get a picture whose white parts are 
really pure, the glasses not allowing the rays to 
pass as absolute monochromes, and the two tints 
not being exactly complementary. 

For the experiment to be satisfactory, two 
powerful lights must be used, because of the great 
absorption produced by the coloured glasses. 

This method was mentioned by Rollman 
in 1853, and was rediscovered by D' Almeida, who, 
we believe, constructed the instrument for the 
first time in 1858. 



Eclipse Stereoscopes. — The principle of 
these instruments is due to D'Almeida (1858), and 
may be described in a few words : — 

Let us suppose that two optical lanterns L L 
are placed and regulated so as to project on the 
same screen E (fig. 59) two pictures, right and 
left superposed, and that a shutter R turning 

Fig. 59- 

rapidly before the two object glasses uncovers 
them successively. 

An observer looking at the screen will only 
see one picture without relief, formed in reality of 
a succession of right and left pictures. But if 
before the eyes O O', a shutter v turns with the 
same rapidity and simultaneously as the first R, 


the left eye will be uncovered each time that the 
left picture is thrown on the screen, and the right 
eye each time that the right picture is projected. 
Each eye can thus only see its corresponding 

D'Almeida invented a stereoscope of this 
kind, in which the two shutters R and r were 

Fig. 60. 

mechanically connected. He also suggested the 
use of electro magnets for obtaining the syn- 

A. Stroh presented to the Royal Society of 
London, in 1886, an eclipse stereoscope which is. 
represented by figure 60. 




The two lanterns are mounted side by side on 
one stand, which also holds the shutters. This 
instrument was made for two persons, but it could 
evidently be made for a greater number. The 
same mechanism, which actuates the shutters of 
the lantern, also move the ocular lids, mounted to 
the right and left of the lantern. The speed of 
rotation should be so great that the picture is 
continuous : for each eye from 30 to 40 flashes a 
second should be reckoned. As the discs used in 
Stroh's instrument produce two flashes each turn, 
their speed should be about 15 to 20 turns a 

We show below in detail, the form of the 
shutters, which should be so fixed that only one 
picture will be projected at once. The openings 
are cut in sections whose angle is a. 

If /3 be the angle formed by the extreme edges 
of the lenses (or of the holes) to be uncovered and 
a be the angle formed by two lines crossing the 
centres of these lenses, the part opened by the lid 
should be 

a=oc — /3 

and the part closed 


and as the entire disc forms two lids 
a+b=2 a =i8o° 



a condition easily secured in putting the apparatus 

In these kinds of stereoscopes, the eyes look 
directly at the picture, without the interposition of 
any prism or lens whatever. It is interesting to 

Fig. 61, 

notice that relief is produced, though the two 
pictures are not seen at the same time. 

Stereoscope for polarised light. — This 
stereoscope was invented by Mr. John Anderson. 



Two lanterns LL' project, as before, the two 
superposed pictures on one screen. Before each 
lantern a polariser PP' is introduced at a distance 
of 90 0 apart (in Mr. Anderson's apparatus these 
polarisers are made of blocks of glass). 

The picture received by the screen and 
appearing single, is thus formed of two polarised 


pictures, for instance one in a vertical, and the 
other in a horizontal position. 

If an observer look at this picture through two 
analysers a a placed before his eyes O O' and con- 
veniently fixed, each eye will see a different picture- 
in perfect stereoscopic relief. 

The analyser is formed of two nicols prisms 
mounted in a sort of opera-glass. Naturally, if 


the nicols be go° from their normal position, the 
right eye will see the left picture, and the left eye 
the right picture, in such a way that pseudoscopic 
relief will be produced. 


HE fact that a single picture drawn, 

painted, or photographed can give in 
different degrees the sensation of relief, has sug- 
gested the idea that it is possible to obtain a 
stereoscopic effect with one picture ; but it has 
been perfectly demonstrated that complete relief, 
as furnished by the stereoscope can only be 
secured by the two eyes looking at different 

There is one case where the picture, appearing 
single, can be seen in relief ; but a closer examina- 
tion of it, shows that it ought to be placed in the 
category of ordinary stereoscopic phenomena. 

In looking at an image on the ground glass of 
a camera it appears to be in relief. H. de la 
Blanchere made a series of experiments on this 
subject and demonstrated by the following 
deductions, that relief is due in reality to two 
pictures, furnished by the two edges of the object- 



glass, and that these pictures are received 
separately on each eye : 

ist. If one eye be closed, the stereoscopic 
effect disappears ; 

2nd. If the picture be looked at through a 
pseudoscope, the relief is reversed, as if the object 
itself were being operated on ; 

3rd. If the lens be well stopped down the 
picture will appear flat, but the relief will remain 
if the diaphragm has two holes pierced on the two 
extremities of its diameter, horizontal to the lens ; 

4th. If a blue glass be placed before one of the 
openings of the diaphragm, pierced in this way, 
and a yellow glass before the other one, by closing 
the eyes alternately, first a picture in the one 
colour and then in the other will be seen. 

It is easily explained how two pictures thrown 
at a different angle on to the same ground glass, can 
be seen separately by each eye. The intensity of 
the light diffused by the ground glass, decreases 
very rapidly, in accordance with its separation 
from the direction of the incident ray. (Thus, if 
a wide angle lens be employed it is necessary, in 
order to see the picture at the edge of the focus- 
sing screen, to look in the direction of the objective ; 
in looking normally at the glass the picture will 
appear very feeble). It is then understood that the 
rays emanating in the direction L'O will be 
scarcely visible to the eye O', while it will see the 



rays emanating in the direction L O'. Oiled or 
waxed paper presents under this condition the 
same properties as ground glass, but ordinary 
paper does not bring about the same effects, the 
intensity of the light diffused varying less rapidly 
than the angle of incidence. 

Claudet based on this same principle his 
stereomonoscope, which we have already mentioned 
(page 82). 



Fig- 63. 

It is possible for the same effect, to take place 
to a certain point when examining on a polished 
glass, transparencies taken by a lens with a large 
aperture, and for traces to be found in prints ; 
which would explain why the pictures proceeding 
from lenses with large openings present more relief 
than those obtained by the same lens stopped 
down.' 1 ' But there are also other reasons for it. 

* Gaudin (1851) and Norman (1855) proposed for 
obtaining prints in monocular relief, the use of a lens 
furnished with a diaphragm pierced by two holes 65 
millimetres apart. 



In reality, we know that the sensation of 
relief is not only furnished by the perception of 
two dissimilar pictures, but that the proportions 
of different parts of these pictures also contribute 
to it. Thus the perspective of a picture plays a 
very important part. A landscape with a fore- 
ground will appear more in relief than if it only 
contained subjects placed at a great distance. 

The relative distinctness of different per- 
spectives also influences the apparent relief. The 
eye, in fact, only sees distinctly one perspective ; 
and if a picture where the background is as 
distinct as the foreground (which is the case when 
obtained by a small opening) be presented to it, 
the whole will appear flat and uniform. Portrait 
photographers are always careful to bring their 
model forward, and to choose rather a light 
background. The perspective obtained with a 
wide angle lens is always more pronounced than 
that obtained with an ordinary one. This will be 
understood by examining Fig. 64, where A B, 
C D, represent two objects (supposed to be of the 
same size) placed in two different planes. The 

ab OC 

ratio — = of the sizes of these two objects 

ad OA 

is nearer to similarity as given by the lens O, than 
by O', of which the angle is greater. It follows, 
therefore, that the perspective can be augmented 


by the use of objectives of greater angles. There 
is, at the same time, a limit, which is, however, 
mentioned only in the artistic sense. It occasion- 
ally happens that with an ordinary rectilinear lens 
the admitted limit of perspective is passed, though 
the angle seen by this lens is much less than that 
of the eye. 

Landscape painters, however, are not in 
perfect accord on the question of the angle a 

Fig. 64. 

picture ought to embrace ; and' the distances of 
the eye from the object vary from 0*5 to 3 times the 
greatest dimensions of that object. F. Bossuet, 
who has studied the proportions of a great number 
of pictures by great masters, advises that it should 
be kept between 1 and 3 times. There are, 
however, few pictures where the distance is three 
times the height, and there is scarcely one where 
it is more .than that. 

9 6 


When a photograph is examined through a 
magnifying glass, it is generally found that the 
relief is accentuated. We consider that this effect 
is most obvious when the picture is isolated from 
surrounding objects, so that the presence of these 
objects can only serve as data to determine the 
perspective of the picture. A similar result can 
be obtained by isolating the picture with a card- 
board diaphragm having an opening of the same 
shape as the picture, and which is interposed at a 
short distance from the photograph (of course, 
looking at it with only one eye). 

Eye glasses or single glasses (often wrongly 
named monostereoscopes) have been made for the 
examination of ordinary photographs. 

We have already mentioned that similar 
effects can be secured by an iconoscope. It is 
strange that it should have been proposed (V. 
Eckhont, 1857) to use one sort °f telestereoscope 
to get at the same result ! We must draw attention 
to the fact that in these various instruments the 
form of the box limits the luminous rays, and 
therefore isolates the picture or photograph. 
Almost the same effect is realised by the use of 
opera glasses, or, more simply, by using two 
copper or cardboard tubes. 

It is evident that the result obtained by all 
these methods has nothing in common with 
stereoscopic relief. The reason we have described 


or alluded to them is because on one hand, the 
attempt to obtain " monocular relief'' has often 
been made, and on the other hand, because these 
different effects take place in the stereoscope itself, 
diminishing or increasing relief. 


j HE first stereoscopes were intended for the 
examination of drawings made by hand and 
reproduced by lithography ; but these drawings 
could only be obtained by means of a geometrical 
tracing, a long and very troublesome operation 
when the objects were of a complicated nature. 
Besides, only a small number could be found, 
which represented relatively simple subjects, such 
as geometrical figures, crystals, and architectural 

In 1845 Wheatstone conceived the idea of 
using Daguerreotypes in his stereoscope. This 
was the turning-point in the more important 
applications to which the stereoscope owes its 
position since 1850. It is curious to note that 
although the stereoscope was an English invention 
the French makers were the first to recognise its 
possibilities, to show the way in which it might be 
useful to artists, and to succeed in making a very 
incredulous public understand that photography 



was incomplete without it. These makers had, 
moreover, the satisfaction of seeing their ideas 
shortly put into practice in England itself. 

Since the rapid processes of photo-mechanical 
impression have permitted photographs to be sold 
at very low prices, it would have been thought 
that the stereoscope would have increased very 
materially in public favour. It is only recently 
that it has done so to any considerable extent, 
but its popularity has not grown in proportion to 
the facilities offered for enjoying its wonders. The 
photographic magazines are continually lamenting 
the fact, and lay the blame on the less conscientious 
photographers who supply to the trade so-called 
stereoscopic photographs which are, in reality, 
formed of two prints of the same picture. In our 
opinion the fault is rather in the stereoscopes 
themselves, which are generally lenticular instru- 
ments of fixed focus and of the same separation, 
i.e., they are only adapted to one sight. For one 
person who can use them with advantage, there 
are ten who will see two pictures, and who therefore 
think " when I close one eye I can see much 
better." It is therefore very necessary to have a 
stereoscope exactly adapted to the sight, or better 
still, one which can be adapted to all sights. 

In our opinion stereoscopic relief forms one 
of the most beautiful phenomena of natural 
philosophy. It is impossible not to be filled with 


admiration on first using the stereoscope. But 
apart from mere curiosity, it possesses another 
attraction : all that nature and art can offer for 
the enjoyment of the eye the stereoscope can, 
almost to colour, present with irreproachable 

On a stereoscopic print the artist finds clearly 
written very valuable information. Instantaneous 
pictures in particular, are to him very precious 

Landscapes, portraits, and figure studies 
become more realistic, more true to nature by 
use of the stereoscope. 

We have seen a catalogue of machinery, 
illustrated stereoscopically, and this application 
might with advantage be more extended than it 
is. In fact, by no other process can the exterior 
aspect of an object be so clearly shown. 

Apart from these direct applications, there 
are several special uses to which the stereoscope 
may be put. 

In order to measure strabism (squinting), 
Javal used a hinged reflecting stereoscope (Fig. 
65) constructed in the following manner : The 
two mirrors, G G', are united by a hinge, C C, 
and a divided semicircle fixed on C is used for 
measuring the angle made by them. A A' are 
two tablets fixed to the mirrors, and forming with 
them an angle of 45 0 . On each tablet there is a 




mark, P. When these two marks superpose each 
other stereoscopically, the angle of the two 
visual rays can be found by simply looking at the 
semicircle, C. 

Dove* (1859) discovered that each time two 
slightly dissimilar pictures are examined by a 
stereoscope certain parts appear in relief. If two 

Fig. 65. 

medals, struck from the same mould but in two 
different metals, be placed in the stereoscope, the 

* Dove, Ueber Amwendnng des Stereoskops um einen Druck 
von seinem Nachdriick, iiberhaupt ein Original von seiner Copie zn 

Monatsberichte der K., Preuss. Akademie der Wissens- 
chaften zu Berlin, 1859. P. 280-288. 

Poggendorf Annalen, CVL, 1859. P. 657-660. 


resulting picture will appear convex, because 
after the stroke of the beam, the two metals are 
unequally distended. Two medals, one of silver 
and the other of bronze, will give this idea very 

In the same way, if the same sentence be set 
up twice in type, and the proof be put into the 
the stereoscope, certain letters will appear to be 
detached from the others, and go before or behind. 
The reason is that the letters have not exactly the 
same space in the two proofs. The same effect 
would not be produced if the two phrases were 
printed twice from the same composition ; it is 
possible, however, in this case, for the whole to 
appear convex or oblique, on account of the 
unequal contraction of the paper. In this way a 
true bank-note may be distinguished from a forged 
one, or even two notes struck from two different 
plates, or two different editions of the same text. 
The equality of the divisions of a graduated scale 
may also be verified : it is only necessary to obtain 
stereoscopic pictures of two parts of it. 

We do not see why stereoscopic prints should 
not be used for the purpose of decoration, like 
ordinary photographs, and why they should not 
also have the honour of a frame. Duboscq has 
already used them for ornamenting a lamp shade ; 
there should be a great number of similar ways of 
making use of them. 




Finally, the stereoscope might become a 
valuable auxiliary of information, by facilitating 
the study of solid geometry, of analytic and 
descriptive geometry, and even of mineralogy and 
natural philosophy. 

Lissajous (1856) made a very curious applica- 
tion of it. The remarkable works of this learned 
man on the optical study of vibrating movements 
are well known. He showed that the even curve 
obtained by the composition of two rectangular 
vibrations, whose periods are in the proportion of 
two entire numbers, can be considered in the same 
way as the projection of a figure traced on a 
cylinder' 1 ', on a plane passing through the axis of 
the cylinder. 

Hence the different figures which correspond 
to the various different phases can be seen by the 
observer in moving round the cylinder, keeping 
his eye all the time at the level of the mean 
circumference. Two of these figures, taken at an 
anglet of from 10 to 12 degrees apart, and placed 

* This figure would be engendered by a point, turning 
with a uniform movement round the cylinder, at the same 
time that it oscillates according to the law of pendulums, 
from one part to another of the circumference traced on the 

f This angle represents precisely the difference of phase. 


in the stereoscope, give a resulting picture which 
is the generating curve. 

Marey (1885) in his studies of locomotion, 
made a remarkable application of stereoscopic 
photography, in order to register the movement 
described by one part of the body in walking or 
running. A man dressed in black, and carrying 
a bright light at the height of the sacrum, walks 

Fig. 68. 

on level ground in front of the stereoscopic camera. 
The two prints obtained (Figs. 66 and 67) examined 
in the stereoscope, reproduce in the form of a 
curve in space the trajectory described by the 
point of a pencil. The same author has analysed 
in the same way the actions of a horse, the motions 
of the body of a bird, etc. 



Stereoscopic Pictures by Drawing. — The 
use of stereoscopic pictures drawn by hand (Fig. 
68) has been almost abandoned ; their execution 
is, in fact, too troublesome, and they do not 
always give correct effect, because of the difficulty 
experienced in shading them properly. It is also 
almost impossible to draw anything but objects of 
well-defined geometrical form ; the proper execu- 
tion of a landscape or portrait may be looked upon 
as impracticable. 


T T is quite probable that without photography 
A the stereoscope would have been almost 
unknown outside the physics laboratory ; as the 
sensitive plate alone can furnish perfect stereo- 
scopic pictures, exact not only stereoscopically, 
but in perspective and light and shade. 

Two cameras placed at a small distance from 
each other can, in fact, look at an object as the two 
eyes do. We shall now examine in detail the 
conditions necessary to produce by photography 
the same effect as that given by a direct view of 
the object itself; in other words, how far apart 
the lenses of the two cameras should be, to secure 
exact relief. 

On this point authorities on stereoscopic 
matters have expressed radically different 
opinions, some saying that the distance should be 
that of the eyes ; others, that the optical angle 
should be the same as that made by the eyes, in 
looking at a picture in a stereoscope ; others 
again, assigning intermediate distances between 
these two extremes. 



The discord, however, is only apparent ; a 
stereograph is an artistic work, in which judg- 
ment and taste ought -to have precedence before 
calculation ; but the conditions of the desired 
relief once determined upon, it is no longer 
possible to hesitate over the separation of the 

Suppose that an observer standing before a 
landscape desires to have a stereoscopic picture of 
it with the same relief seen in nature ; it is very 
evident that the optical angle of the eyes, and that 
of the lenses should be the same, and therefore, 
that the lenses should be placed as far apart as 
the eyes are.' 1 ' If the landscape be at a great 
distance, relief will be extremely feeble, perhaps 
imperceptible, but it will be exact. 

Relief may be increased ; and to make it as 
pronounced as possible, it would be necessary to 
suppose the landscape reduced to a small scale, 
and placed at the minimum of distinct vision. 

To obtain two photographs which correspond 
in this case, the lenses must be separated, until 
the angle of convergence is equal to the optical 

* To avoid bringing perspective into the question, we 
are supposing that each lens is fixed at about the same angle 
as the eye. 


At the same time it is desirable to take into 
consideration the latitude affored by the range of 
the eyes. 

M. Cazes has, in fact, noticed that in looking 
at the different planes of an object, the optical 
angle and the focus of the eyes change at the same 
time, while in looking at a picture in the stereo- 
scope, the optical angle alone changes when the 
sight is carried from one plane to another. In 
order, then, to preserve the natural conditions, the 
photographs must be viewed at such a distance 
that the eye can see all the planes at the same 
time, without the necessity of varying the focus. 

M. Cazes admits that for a normal sight, at 
the distance v, two planes can be seen with the 


same adjustment at a distance — , so that if / be 


the united focus of the lens (the distance from the 
ground glass to the nodal points behind) and d the 
depth of the object, the minimum distance D from 
the lens to an average perspective of the object 
will be 

10 fd 

and the separation of the lenses must be chosen to 
correspond with the optical angle, probably 12 to 
15 degrees. 



If we reckon for v the value of 20 centimetres 
we have the following result : 

d 2 

That is to say, for an object glass of 10 centi- 
metres (four inches) focus, the distance of the 
object should be equal to, at least, five times its 

The most perfect relief is obtained by this 
method ; very vivid, but not exaggerated. At 
the same time its use is very limited, for on the 
one hand, the distance between the object-glass 
and the subject can rarely be measured ; and on 
the other, it is often difficult to find two con- 
venient points whose distance would correspond 
to the optical angle. Finally, it is more difficult 
still to assume a position at the distance D, 
indicated by the formula. It is seldom that a 
landscape can be photographed conveniently at 

* It is sometimes astonishing how much this method 
takes into account the distance v and also the stereoscope 
used. It is a fact, however, that relief depends on it. It 
may be accounted for by looking at the same stereograph at 
various distances, by one of the methods already mentioned 
(page 73). It will be easily found that relief is augmented 
when the distance between the eye and the print is greater. 
When a magnifying stereoscope is used, it is necessary to 
take for v, the distance at which the print would be from 
the eye, if it were seen directly and of the same size. 


two different distances, and when once the photo- 
grapher has found a point to suit him, he ought to 
be able to make his picture from there.'" 

At the same time this method, which can 
only be applied precisely in the case of detached 
objects (objects of art, crystals, etc.), furnishes 
some useful points when views of any kind are in 
question. Suppose that the desired position is 
too feeble by half according to the above formula ; 
the optical angle must be divided by two, and by 
bringing the lenses thus together the same cor- 
respondence will be found which ought to have 
been obtained by placing them twice as far apart 
with the normal angle. 

It is quite clear that it is by no means 
necessary for securing stereoscopic relief, to place 
the camera at exactly the desired distance : 
exaggeration of relief must be guarded against, 
and when there is a doubt it is better to diminish 
the angle than to augment it. But for all 
ordinary stereoscopic operations it is sufficient 
to estimate distances. 

* Pra&ically, for a given lens, the distance D is deter- 
mined by the size desired for the pi&ure. Therefore the 
most frequent problem is, not to find the distance D, which 
corresponds with the maximum separation of the positions, 
but to find the separation corresponding to a given 



But to sum up, the following are the 
principles to be followed with regard to the 
separation of the lenses. 

Calculate by the formula of M. Cazes given on 
page in the distance at which the camera should be 
fixed, and if the place, or the focus of the lenses prevent 
operating at that distance, bring the lenses together, so 
as to diminish the angle in the same proportion. 

Never exaggerate the separation ; rather be under 
than over the calculation. 

In some cases the manner of proceeding which 
we have mentioned would be of no use what- 
ever. Thus in photographing a panoramic view, 
or a landscape extending beyond the sight, a 
great mistake would be made if the depth were 
estimated. In such a case it would be better to 
approximate, after a proper trial, the angle to be 
given to the two lenses. The eye is satisfied with 
a stereoscopic picture, whatever its relief may be 
within very extended limits, always provided it 
be not exaggerated. The best proof of this is, that 
most photographers take landscapes exclusively 
with lenses of a fixed separation. 

We give a table showing to i6° the separa- 
tions which correspond to the different angles. 



















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To obtain with certainty the desired relief, we 
have used the following process, which is not 
based on calculation but on direct observation. 

The two lenses being properly focussed, we 
examined the images on the ground glass by means 
of the apparatus shown in Fig. 71, as if they were 
aCtually stereoscopic prints. 

This instrument is a sort of total reflection 
stereoscope, reduced in size as much as possible 
so as to occupy very little space. It consists of 

Fig. 71. 

two total reflection prisms, movable in a frame, 
work, which also allows their separation to be 
varied so that they may be adapted to different 

It is necessary to use an instrument of reflection 
because the pictures ought to be reversed. The 
pictures seen on the ground glass are thus reversed, 
and the right picture will, therefore, present the 


left perspective, so that in looking directly at them 
the pseudoscopic effect will be at once perceived. 

The right picture could also be presented to 
the left eye, and vice versa. For that a stereoscope 
similar to the one invented by Blanchere (page 37) 
should be used. This instrument might also be 
advantageously adopted when long-focus lenses 
are used, because it allows the pictures to be 
magnified. In fact, for considerable separations, 
the apparatus, (fig. 71) must be used for looking 
at pictures at a great distance (in looking at them 
near at hand they are too much deformed as 
regards breadth, and therefore as regards inclina- 
tion). In these conditions the relief of the picture 
is stronger than it would be at the minimum 
distance of distinct vision, and further, the details 
are less easy to grasp. Besides the methods 
mentioned above, which appear to us most 
rational, and which give good results, we shall 
state the principal rules given by various natural 
philosophers. As will be seen, these rules are far 
from agreeing with each other, and, with the 
same subject they give very different separations. 

Brewster was of opinion that the distance of 
the lenses should not be greater than that of the 

Wheatstone varied equally the optical angle 

and the distance, and suggested the following 

table. Given a distance D, corresponding to the 



various values a of the optical angle (the distance 
of the two eyes being e, the formula 

1 i 

D=- e cotg-a, 

2 2 

gives the figures of the succeeding table). 



(English inches 



71-5 in. 


357 ... 


23 8 „ 


178 „ 


13 2 




10 1 ,, 


8-8 „ 


7'8 „ 


7*0 m 


6-4 m 

2 4 ° 

5'8 „ 


5H » 


5'o „ 


4-6 M 

Sutton (1856) formulated the following rule : 
" The angle of convergence between the axes of 
the two lenses, directed towards the centre of an 
object, should be equal to the angle of con- 
vergence, between the axes of the eyes, directed 
towards the same centre, on the virtual picture 
seen in the stereoscope." 

As we have already seen, working by this 
rule will give true relief with objects of slight 
depth, but in most cases it will be exaggerated. 


Claudet* (1853) observed that "it is not 
necessary for the binocular angle to be greater 
than that which is sub- 
tended by a base of 2% 
inches, when looking at 
the object: at the nearest 
distance which permits 
all the picture to be 

He remarked also 
that it is necessary to 
have lenses of long focus, 
so that the foreground 
is not magnified more 
in proportion to the 
distance, than is the case 
in natural vision. 

He constructed an in- 
strument called the steveo- 
scopeometer, with which 
separations correspond- 
ing to various distances 
and angles can be rapidly 

This instrument (fig. 
72) is composed of a Fig. 72. 

section of cardboard, 20 degrees in size, divided 

* A. Claudet. Du stereoscope et de ses applications a la 



into degrees on the arc A B. A second arc C D 
indicates the corresponding separations, and the 
radius D O carries divisions which correspond to 
the distances of the object. By the points of 
division of this radius, arcs have been traced 
having the same centre as the section. At this 
centre is fixed a thread /. If the separation 
w,hich, at a given distance, corresponds to an 
angle of 4 degrees has to be found, the thread / 
is placed facing the division 4, and it is followed 
to its junction with the arc corresponding to the 
distance given : at this point of junction is found 
a parallel to the radius O D, which cuts the arc 
C D at a point which indicates the separation 
sought for. A certain number of parallels to O D 
have been traced on the apparatus, so that it can 
be followed directly without pencilling. 

There are various ways of obtaining a stereo- 
scopic view of an object or landscape. According 
to the appliances at our disposal, the negative 
can be obtained : 

1 st. By displacing the object ; 

2nd. By two successive exposures with one 
ordinary camera ; 

3rd. By a single exposure with a camera 
having two lenses, and a partition separating the 
bellows into two equal halves. 


I T is easy to understand that if, instead of 
taking two successive views, by moving 
round the object, the object itself can be moved 
before the fixed apparatus, the same result will be 

The most beautiful application of this method 
has been made by Warren de la Rue, who has 
obtained magnificent photographs of the planets 
(Saturn in particular), by photographing them at 
such intervals of time that they had turned round 
the desired angle. 

He also obtained photographs of the moon, 
by taking advantage of the movement of libra- 
tion. The relief obtained is magnificent. The 
maximum libration of the moon from east to west 
is 15 0 50', and therefore corresponds very nearly 
to the maximum optical angle. The libration from 
north to south is 15 0 34A 

The same method is applicable to photo- 
graphing objects of small dimensions, easily 
moved, such as scientific instruments, small 



pieces of machinery, etc. The object to be 
photographed is placed on a revolving stand (fig. 
73), the base of which is divided into degrees. 

When one plate is taken, the object is turned 
through the desired angle, and the second 
exposure made without moving the camera. 

Fig. 73- 

By continuing the rotation a third and fourth 
plate can be obtained and so on, so that each 
picture . shall be the left in reference to the 
preceding one and right in reference to the one 
following. By moving round the entire circum- 
ference, 10 degrees each time, 36 plates may be 
taken, which will give views of the object under 
all its aspects. 


It is convenient to fix prints of this sort on 
to a series of cards joined together by hinges of 
cloth, so that they will fold up like a screen (fig. 
75). The first print is fixed on i, the second on 
2, etc., so that they can be passed through the 
stereoscope from one end to the other, each print 
being right and left in turn according as it is 
coupled with the one preceding or the one following. 
At the end the first print must be repeated. 

Fig. 75- 

It is necessary to work with a plain back- 
ground. If the background has any details they 
are seen without relief and the effect is not 
satisfactory. If the whole be of such small 
dimensions, that the background may be turned at 
the same time a better effect is obtained. 

The fault of this method is that the light on 
the subject is not exactly the same during the two 
exposures, since the displacement is made in 



reference to the direction of the light. But the 
differences resulting from it are very small, and 
it is necessary to look very closely to discover them. 

Instead of moving the object by turning it on 
its own axis, it can be displaced — but only if a 
plain background be used — parallel to itself, and 

Fig. 76. 

the camera slightly turned (without displacing it). 
The distance A B of the two positions of the 
objecl will be the same as would be that between 
the two positions, if the camera were moved. 



This method, by displacing the object, is 
almost exclusively employed in stereoscopic 
photography, when medium or strong magnifying 
is in question. The object is fixed on to a stereo- 
scopic lever/'' This is an instrument for holding 
the object mounted between two fixed points (fig. 
77), whose axis passes through its centre, and 
which permits the object to be turned at a very 
small angle, to the right or left of the optical 

angle. This displacement is limited by two 
screws with ends. 

After having mounted the object on the lever 
(where it is held by supports or springs) it is 
focussed and its position rectified till the middle 
of the subject is in accordance with the optical 

By means of wedges or of regulating screws, 
the height of the object may also be varied, so 
that the axis of rotation is in accordance with the 

Fig. 77. 

* W. Seibert has also obtained good stereoscopic 
photomicrographs by the lateral displacement of the object. 



middle of the object. This regulation is necessary, 
because the plates which hold the object are not 
always of the same thickness. 

The angle to be given between the two 
positions of the levers cannot be calculated 
exactly, because of the uncertainty of the depth 
of the object, its distance from the lens, etc. It 
is also much better to find this out experimentally. 


Fig. 78. 

Meitessier (1866) suggested the angle of 12 
degrees for lenses of low power (Nachet's No. o), 
and 4 to 5 degrees for more powerful ones (No. 5). 

The operation is the same as in ordinary 
photomicrography, care being taken to verify, and, 
if necessary, to rectify the focussing between the 
two exposures. 


Fig. 77 represents an ordinary stereoscopic 
lever. Object P is fixed on the plate M, which is 
screwed on to the lever proper, B. By screwing 
this plate more or less, the height of the object is 
regulated. The screw F of the lever terminates 
in a socket, which fits into the obj eel: -holder of an 
ordinary microscope. 

Fig. 78 shows the side and underpart of 
Fritsch's lever. 

It is constructed as follows : a is the object- 
holder, on which the object is fixed by means of 
blocks m. This object-holder is movable round 
an axisj mounted at one of its extremities and 
resting on the lever proper c. The displacement 
round this axis controlled by the screw g f serves 
to regulate exactly the height of the object. The 
lever c is movable round the axis x, by the aid of a 
screw e, and the angular displacement is measured 
on a divided dial h. The screw-holder a can be 
displaced parallel to itself by the aid of the screws 
//, for the regulation of the position of the object 
before the camera. For this it is mounted on a 
guide b, fixed by two claws on the plate of the 


HIS method, by which one plate is exposed 

first, and then after moving the camera, a 
second of the same view, is not generally recom- 
mended, because of the following disadvantages : 

ist. The lighting of the subject may vary 
between thetwo exposures, but even if not, the times 
of exposure may not always be exactly measured ; 

2nd. In the case of a landscape lighted by 
the sun, the position of the shadows alters between 
the two exposures, and on the print the shadows 
often have the effect of black screens suspended 
in the air ; 

3rd. This method should only be applied to 
immovable objects ; it is not as convenient for a 

On the other hand, it permits the amateur 
photographer to work with his ordinary appli- 
ances, and produces a sensible reduction in cost 
of apparatus, as a special tripod top can be pur- 
chased — admitting of moving the camera — for a 
few shillings. 


Let us suppose that O is the subject to be 
photographed ; the separation G D of the 
positions must first be determined ; then, in the 
case of a landscape, the camera must be placed 
successively at G C, to make sure that all is 

/ \ 
/ \ 
i \ 
1 \ 

I v 

/ \ 
l \ 
l \ 

Fig. 79. 

satisfactory. (If this precaution be neglected, it 
sometimes happens that, after one plate has been 
taken, a branch of a tree is found in the way of 
the second.) The two plates are then taken, 
after carefully ascertaining when focussing, that 



the same parts of the landscape are within range. 
This operation is facilitated by tracing two diagonal 
lines on the ground glass, and focussing each time 
the same object at the crossing point. A camera 
furnished with a level is the best to use. 

The camera in the two cases should be, as 
far as possible, at the same height. 

There are two ways of proceeding : when the 
positions are very near, the stereoscopic plane- 
table may be used, and the stand of the camera 
need not be moved. But when, on the contrary, 

the distance between the positions is great, the 
apparatus must be mounted ordinarily on the 
stand, and the whole may be moved. 

The stereoscopic plane-table (fig. 80) is a sort 
of divided ruler, fixed by V on the stand of the 
camera, and having in its whole length, a groove 
in which the screw which fixes the camera can 


----- - -J 

Fig. 80. 




1 1 

slide. The divisions on the table permit the 
camera to be placed at the desired distance. 

When a plate has been taken, 
the table must be turned 180 
degrees, and the second taken in 
the same way, by turning the 
camera on its own axis. 

For convenience of carrying, 
the table is generally of the same 
length as the folded stand. The 
two views can therefore be taken 
at a separation a little less than 
twice this length. 

Often, too, the stereoscopic 
table is so fixed, that the camera 
may be mounted at each end 
without the table being moved 
(%. 8i). 

This method has also the 
advantage of allowing an easier 
regulation of the two positions of 
the camera. This regulation is 
effected by two set squares E E', 
against which the cameras are 
fixed. The two positions of the 
camera can thus, before operating 
be exactly determined, and easily 
found again during the operation, 
as the squares remain in their places, when the 


camera is removed. By this method the separation 
of the cameras can only be fixed at a length a little 
less than that of the table itself. 

If the separation of the two positions exceeds 
that distance, the camera is mounted on the stand 
in the ordinary way, a plumb-line being suspended 
between the legs. This serves to regulate the 
positions and to measure the separation (fig. 82). 

Fig. 82. Fig. 83. 

It is better still, if possible, to suspend the line 
from the lens. The cord should be tied in a slip- 
knot, which rapidly adapts the length to the 
height chosen (fig. 83). 

In default of a plumb-line, any object (a 
small stone, for instance) previously placed on the 



top of the lens, may be let fall. The place where 
it falls serves as a regulation point. 

Care must be taken to give exactly the same 
exposure to each plate. 

An intermediate method between this and the 
following, consists in employing a camera, in 
which the lens is movable horizontally. After 
marking the ground glass as in fig. 84, the lens is 
moved to the right, and the left of the range is 
covered by a card placed in the case behind the 
dark slide ; that done, the first (right) exposure is 

i / A V 


V. 2s. 

Fig. 84. 

made on half the plate ; the frame is then raised 
and the card put to the right, and the left portion 
exposed in the same way. It is evident that this 
method is limited in its application, since the 
maximum separation at which it can be fixed, is 
only that of the two extreme positions of the lens ; 
it is even less because each time the camera is 
turned slightly the subject must be focussed to the 
centre of half the plate. 


We have, however, found it very useful for 
photo-micrography with magnifying objectives of 
low power. 

M. Moitessier (1866) invented, also for photo- 
micrography, an interesting way of taking two 
plates without changing the position of either the 
camera or the object:. 

Fig. 85. 

For this purpose, he uses successively the 
two halves of the surface of the lens. 

The lens used has half its surface covered, 
and it is mounted by friction, in a tube in which 
it can be turned half-way round (fig. 85). 

A notch F to which a pin g is fixed, regulates 
exactly the rotation. 


HE most rational method of stereoscopic 

photography, is that by which two plates 
can be taken at the same moment. It is, moreover, 
the only one by which instantaneous work can be 
done ; and it also offers advantages in cases 
where the subject is in repose, as it makes the 
taking of the two plates by the same light, a 

In accordance with what we have already 
said, the separation of the lenses ought to vary 
with reference to the distance and depth of the 
subjecl:, if the maximum relief be desired. Therefore, 
for the realisation of this condition, two separate 
cameras, movable in accordance with each other 
should be used. 

But for various reasons, most photographers 
prefer to work with cameras of a fixed separation. 
The two instruments are then mounted together, 
and the operation does not differ materially from 
the taking of an ordinary plate. It is obvious 
that the relief must suffer from it, especially when 



the foreground is absent. Stereoscopic cameras 
of fixed separation should therefore be judiciously 
employed. Their use is recommended for the 
studio ; they are also useful for detectives, or for 
hand instruments ; and give very satisfactory 
pictures of a landscape, when there is a fore- 
ground at a short distance ; but they should not 
be used for distant subjects (panoramas, bird's 
eye views, etc. 

Fig. 86. 

Instruments of Fixed Separation. — These 
consist of a camera carrying two lenses as much 
alike as possible. The camera is divided into two 
by a screen, so that it makes two separate instru- 
ments, but each giving their pictures on one plate. 
Stereoscopic negatives are either made on half- 
plates or on the standard stereoscopic size, which 
is 6| X3J. 


The lenses are mounted ordinarily at the 
centres of each half of the plate. 

Fig. 87. 

The English manufacturers have lately given 
considerable attention to the manufacture of 

Fig. 83. 

stereoscopic cameras, and many excellent instru- 
ments can now be obtained. We are only able 


to illustrate and briefly allude to the principal 

Fig. 86 shows a stereoscopic camera of the 
ordinary kind, by Underwood. Fig. 87 shows 
Lancaster's Instantograph camera, fitted with 

Fig. 89. 

shutter. Fig. 88 illustrates Beck's Stereoscopic 
Hand Camera which holds six plates. In Fig. 
89 is shown Rouch's Hand Camera, which is fitted 
with a patent changing back, admitting of twelve 

Fig. 90. Fig. 91. 

successive exposures. The Eastman Co.'s Nos. 
5 and 6 Folding Kodaks are made for stereoscopic 
as well as for single pictures. Tylar's " Tit Bit " 
Stereoscopic Camera (figs. 90 and 91) is a very 
light hand instrument, weighing only two pounds. 


Lizar's " Challenge " Stereoscopic Camera (fig, 
92), but recently introduced, is very light and 
portable, and has an ingenious arrangement to 
obviate the use of a focussing cloth. 

In all stereoscopic cameras, the two caps 
should be made to work together, and to open and 

Fig. 92. 

shut at the same instant, or a double shutter 
should be used. 

One of the best stereoscopic shutters is the 
Thornton- Pickard (fig. 93). It is made to fit on 
the lens hoods, or in another form to work behind 
the lenses, so that it may be screwed to the camera 
front, and the lenses mounted on the front 

i 4 4 


of the shutter. The shutter has a detachable 
front panel, so that the pair of lenses may be 
removed and changed for another pair, or a panel 
carrying one of the lenses in the centre can be 
substituted for taking full sized pictures with the 
same camera. This panel can also be made with 
adjustable centres, so that the lenses may be 
separated to various distances. 

The idea of the binocular camera seems to 
have originated with Brewster (1849), who also 

Fig. 93- 

proposed to obtain lenses exacflly alike, by cutting 
in two an ordinary lens. This idea has never, 
however, been adopted : in fadt, it is of advantage 
to keep all the luminous powers of the lenses, and 
there is no difficulty in procuring them so 
accurately paired that the eye can perceive no 
difference in the focussing. Further, if there be a 
slight difference, it does not interfere with the 
perception of relief ; a difference of ^ passes 


Apparatus of Variable Separation. — These 
are the only cameras which correspond to all the 
needs of a stereoscopic photographer. 

They consist of two separate compartments 
mounted on a stereoscopic plane-table, divided 
into demicentimetres. 

The most convenient size for the dark com- 
partments is about 3 J x 3-J- inches ; however, as 
plates or films of this size are not easily procured, 
the size 4^X3^ is more frequently chosen, though 
it is rather more cumbrous ; on the other hand it 
does not require the same care in focussing the 
subject on the ground glass. 

The form of the dark compartment matters 
little : any camera which can be bought will serve. 
The lenses should be movable vertically ; the 
horizontal movement would be useless. The 
shutters should be worked by the same ball, by 
means of a X shaped pneumatic tube. 

A great many kinds of stereoscopic cameras 
of fixed separation may be bought ; but, as a rule, 
the photographer is obliged to make his own 
apparatus of variable separation. 

We hope the figure representing the appara- 
tus used by us, will be of service to our readers. 
The two cameras may be mounted either above 
or below the plane-table. We prefer the latter 
when the separation will allow it, because on one 
hand, the divisions of the table (which are above) 



are entirely uncovered ; and on the other, the frames 
are thus reversed, presenting the shutter side to 
the ground where the light can most easily get in. 
The lenses are furnished with pneumatic shutters, 
serving for either instantaneous or prolonged 
exposures. At each extremity of the table is a 

Fig- 94- 

hole large enough to allow the insertion of the 
button which fixes the camera. It is only neces- 
sary, therefore to unloose the button by a turn, in 
order to remove the camera. 



All the slides should fit either of the dark 
compartments. When the number of slides is 
uneven, care must be taken to alternate at the 
beginning two of the slides with a third, other- 
wise for the last exposure, two glasses would be 
left in one slide. For example, if there be three 
slides with plates numbered i — 2, 3 — 4, 5 — 6, the 
first exposure must be made with 1 and 3, the 
second with 2 and 5, and the last with 4 and 6. 

Fig- 95- 

We must mention one variety of stereoscopic 
plane-table which would be useful to cyclists ; it 
consists simply of fixing two dark compartments 
to the handle of a bicycle (fig 95). 

The ordinary stereoscopic plane-tables are 
fitted with a level ; it is, in fact, necessary that 
the optical centres of the two lenses should be on 



the same horizontal plane. At the same time, it 
is not necessary to be very precise, and the table 
may even be levelled by sight. It is obvious that 
if its length be horizontally level, it is not neces- 
sary for its width to be so also. Therefore, very 
high objects* may be photographed without in- 
convenience, or, on the other hand, views may be 
taken from a height by inclining the apparatus 
accordingly. The picture will evidently cease to 
be rectilinear. But its distortion becomes per- 
fectly admissible if, in examining the prints, the 
stereoscope be inclined in the same way, in which 
the camera was inclined. 

* Bulletin of the Photographic Society of the North of 
France, 1889, p. 53. 


TN 1881, M. F. Meheux, by reproducing 
Porta's camera, in which the picture is 
obtained simply by a pin hole, succeeded in gain- 
ing remarkable photographic pictures, with detail 
sufficient for certain applications. 

This process was followed by M. Colson, 
who by making the diameter of the hole in 
accordance with its distance from the plate, has 
succeeded in obtaining the maximum detail.* 

M. Meheux from the first expatiated on the 
advantages of this plan; on the one hand, showing 
the absolute rectitude of the picture ; and on the 
other, the possibility of varying the focus within 
certain limits. 

The process is also valuable in stereoscopic 
photography, the pictures obtained possessing 
perfect relief. An examination of figure 99, the 
reproduction of a stereograph in half-tone, gives 
assurance of this. 

* R. Colson. La Photo graphic sans objectif. 



By the aid of a stenope, an instrument illus- 
trated in figures 96, 97 and 98, simple or stereo- 
scopic views may be obtained as desired. The 
stenope is a disc, which is fixed on the camera in 
the place of the lens ; it is pierced by two series 
of holes, the two at the extremity of the same 
diameter being equal. Under this disc, which is 
movable round its centre, three larger holes are 
pierced, whose positions are indicated by the three 
lines traced on the margin. 

If the plate be turned, as shown in Fig. 96, 
i.e., if one of the lower holes be placed opposite 
one of the upper ones, the instrument gives a 
single picture ; if on the contrary, it be turned as 
in Fig. 97, two of the lower holes will face the two 
side upper holes, and by dividing the compart- 
ment, a stereoscopic negative may be obtained. 

Fig. 96.— 
Position for ordinary views. 

Fig. 97.— 
Position for stereoscopic views. 


For pin-hole stereoscopic work the diameter 
of the hole should be from 20 to T % 5 n of a milli- 
metre, and its distance from the plate from 5 to 
10 centimetres. M. Meheux, who gave us this 
information, pierces the holes with a bodkin in a 

Fig. 9 3. 

ferrotype plate or sheet of ebonite, so as to obtain 
a conical opening with sharp edges and without 

Openings stamped in the plate are very 
unsatisfactory, because of the reflections produced 
by the edges of even very thin plates. 


E have already insisted on the importance 

of obtaining the two halves of the stereo- 

scopic negative in conditions as nearly identical 
as possible, from the photographic point of view ; 
in other words, they should be taken at the same 
moment, and with the same length of exposure. 
There is, however, little difficulty in doing this 
ordinarily. But absolute simultaneity may be 
realised by the process of lighting the subject only 
during the time necessary for the exposure. 

This is only practicable by the use of an 
artificial light. 

The object O (fig. 100) must be focussed in 
the two cameras c c', and the two lenses be 
opened. The exposure is then made by means of 
a magnesium light ; if the face has to be lighted, 
this light must be placed at E. It is evident that 
under these conditions, the two plates receive the 
same light at the same moment. 


Fig. io i is a reproduction in half-tone of a 
stereograph thus obtained. 

We do not think it necessary to describe in 
detail the processes of lighting the sitter, and 
prefer to refer our readers to special treatises. 
All the sources of light employed by the ordinary 
photographer, may be also used in this case. 

Fig. ioo. 

Magnesium being generally used, we will, 
however, remark that the instruments in which a 
metallic powder is blown into a flame, cannot be 
used for instantaneous work, though they are the 
most convenient arrangements for ordinary 
exposures. The result of Dr. Eder's experiments 
demonstrates the facl that magnesium light thus 


produced has at least a duration of a third or a 
quarter of a second ; whilst special mixtures (in 
which magnesium is mixed with a combustible 
such as chlorate of potash) have for their duration 
only -q 1 ^ part of a second. 

In spite of the precautions necessary in the 
manipulation of these mixtures, we must always 
give them the preference when it is necessary to 
reduce to a minimum the time of exposure. 


E do not intend here to describe the details 

of photographic manipulation, Our 

book is addressed to photographers, who are 
already familiar with these operations. We will 
merely give formulae in ordinary use. One 
recommendation may prove useful : if the stereo- 
negative is on two plates, always treat both of 
them at the same time, develop them in the same 
bath, and intensify or reduce them together, etc. 




Potassium Oxalate 
Potassium Bromide 

32 ounces 
75 drachms 
15 grains 



Iron Sulphate 
Sulphuric Acid 

16 ounces 
50 drachms 
A few drops 

For use mix of : — 

Solution A 

4 parts 
1 part 

Solution B 

Pour B into A, and not A into B. 




Water 32 ounces 

Potassium Bromide i\ drachms 

Ammonia 4 J „ 


Water ounces 

Pyro 30 grains 

Equal parts of each. 


Water 32 ounces 

Sodium Sulphite 6 drachms 

Sodium Carbonate 6 

To make developer add pyrogallic acid to the 
above solution in the proportion of 50 or 60 grains 
io each bath of 32 ounces. 

* * 


Water 16 ounces 

Sodium Sulphite 6 drachms 

Hydroquinone 92 grains 

Sodium Carbonate 5 drachms 



Water 18 ounces 

Sodium Sulphite 25 drachms 

Eikonogen 5 

Potassium Carbonate 10 „ 


Water 48 ounces 

Hydroquinone 108 grains 

Eikonogen 185 „ 

Sodium Sulphite 38J drachms 

Potassium Carbonate 19 „ 


Water 32 ounces 

Sodium Sulphite 12 drachms 

Amidol 77 grains 

* * 


Water 10 ounces 

Sodium Sulphite 25J drachms 

Metol 154 grains 

Solution Soda Carbonate, 30% 10 ounces 

* * 


Water 3J ounces 

Hyposulphite of Soda 5^ drachms 

* * 


Water 3J ounces 

Mercury Bichloride 30 drachms 


Water 3J ounces 

Ammonia 2J drachms 



Avoid over-development, as hard prints pro- 
duce even stronger contrasts under the stereoscope 
than when viewed in the ordinary way. The 
curious effect of " snow," seen in many stereoscopic 
views, arises from lack of uniformity in the 
development of the negatives. 



TEREOSCOPIC positives may be printed on 

paper or on glass. As before, we will leave 
the details of manipulation, and content ourselves 
with recalling the formulae. 

There is a great variety of sensitized papers 
upon which prints may be made, such as 
albumenized, salted, ferro-prussiate, platinum, 
the various gelatine papers (aristotype, citrate of 
silver, etc.), and lastly, gelatino-bromide papers. 

For transparencies, either ordinary gelatino- 
bromide plates, or plates prepared specially for 
positives (gelatine-chloride, etc.), may be used. 
But, in preference, we advise the use of dry 
collodion, which gives transparencies more delicate 
and of a more agreeable tone. 



Sodium Acetate 
Gold Chloride 

32 ounces 
y\ drachms 
15 grains 




Water 32 ounces 

Hyposulphite of Soda 7J drachms 

* * 


Water 25J ounces 

Hyposulphite of Soda 6j ,, 

Ammonium Sulphocyanide 5 drachms 

Sodium Acetate 4 

Solution of Saturated Alum 14 

Pui into the bottle a few paper clippings or a 

little chloride of silver to saturate the bath. In 
three or four days filter and add : — 

Water 6 J ounces 

Gold Chloride 15 grains 

Ammonium Chloride 30 

■* * 



Water 17 ounces 

Sodium Hyposulphite 6J 

Ammonium Sulphocyanide drachms 

Alum 7J 

Solution of Acetate of Lead 1 J ounces 

Filter when cold. 


Distilled Water 3 ounces 

Gold Chloride . . . 15 grains 


The bath is formed of: — 

Water ... 
Solution A 
Solution B 

3^ ounces 

H „ 

2J drachms 




Amidol 2J drachms 

Potassium Metabisulphite 15J 

Water 2 J 

Solution A 4J drachms 

Water 22 ,, 

Solution of Potassium Carbonate 10% 2 
Ammon. Bromide % 8 


Water 21 ounces 

Sodium Sulphite 1 

Amidol 1 drachm 

When the two plates are separate it is con- 
venient to put them near together on one glass, 
taking care, in order to avoid a space between the 
two prints, to cut from the right of one plate and 
the left of the other, all that part which extends 
beyond the print itself. The two plates are then 
fixed together with bands of gummed paper, 
after being put at exactly the same height. The 
positions may be tested by printing a trial proof; 

1 68 


and looking at it in the stereoscope. The whole 
of the double glass must then be enclosed in a 
frame, which masking the print will serve to 
regulate the cutting of it. 

When the printing is less important, it is 
sufficient to place the plates side by side in one 
frame. Thus the two proofs are printed on the 
same sheet of paper, which makes it sure that 
both proofs will be subsequently operated on at 
the same time. They are afterwards cut apart 
and mounted on the same card. 

The size of stereoscopic cards is 6Jx 3i- 
When the negatives are on separate plates it 
is better to put the two plates in the frame, with 
the tops at opposite ends, so that in opening only 
one of the sides, the lower part of one of the prints 
and the upper part of the other are seen. 

For this purpose, a frame opening across its 
width (fig. 102) is very useful. If negatives 

Fig. 102. 


obtained with a binocular camera have to be 
dealt with, they can be cut for operating as 
before ; but it is better to keep them together, and 
so print the positives directly for subsequent 
cutting and to transpose them. This trans- 
position is necessary, for the double picture seen 
on the ground glass presents itself as under : — 

The positive will therefore be seen in the same 
way, and after turning them right side up, they 
will be : — 

so that it is necessary to transpose the prints. 

In order to avoid this transposition, the use 
of a paper twice the length of the plate has been 
suggested ; it should be folded as shown in Fig. 
103. The negative is first placed on the part 
A B, then on the part C D ; afterwards the paper 
is cut across M M' and two prints are obtained 



showing the pictures in their true positions. 
Another method is to cover half the plate and 
print the proofs successively so that they may be 
put at the proper sides ; but all these methods do 
not appear of great value to us ; in fact, great care 
has to be taken to put the prints exactly in place, 
and also the two halves of the stereograph being 
printed at different times, stand a great chance of 
not being equally dense. This is only of 

Fig. 103. 

importance as regards the general aspect of the 
prints, as the stereoscopic effect does not suffer. 

The printing of transparent positives by 
contact can be done on a single glass, when the 
two plates have been placed sufficiently near 
together. In other cases, the two positives are 
printed separately, and are fixed together on one 
glass, which serves at the same time to protect 
them. It is better to use very thin glasses for 


both the positive and the covering, so that the 
thickness of both together does not exceed two 

For the positive printed by contact to be 
seen correctly, the him side must be looked at ; 
therefore the glass which covers it must not be 
flashed. This is no inconvenience, as many 
stereoscopes are already fitted with flashed glass. 
It would also be easy to remedy it by placing a 
piece of flashed glass in the stereoscope with each 

To replace this dull glass, the grain of which 
is often disagreeable, Woodbury proposed to cover 
the stereoscopic positive with an emulsion of zinc 
oxide in gelatine. 

Transparencies may be printed on the same 
glass, even from plates obtained by binocular 
cameras, by taking them at two separate ex- 
posures, or better still by using a copying camera. 

To work by contact by two separate exposures, 
a special frame is used (fig. 104), which is half as 
long again as the binocular negative ; the opening 
part of the surface is only the size of a single 
picture. The negative being placed D G on the 
left end of the frame. The plate is placed at P ; 
it is then exposed to the light for an impression 
of the right picture to be made. The negative is 
then moved to the right, and the glass to the left, 
for the left impression to be made. The opening 



part of the frame is covered by a shutter V, which 
is opened for arranging the position. 

It is not an absolute necessity to have a 
special frame for this operation. M. A. Buguet 
has fixed in an ordinary frame a sheet of black 
paper, which brings about the same result. 

Fig. 104. 

In mounting transparencies it is better to 
introduce between the two glasses a mask of black 
paper, which marks clearly the picture and 
improves the general aspect, by hiding the 
irregularities always present at the edges of the 


To obtain a print from a binocular negative 
with the copying camera, the negative is placed 
before a stereoscopic camera of long focus. This 
camera has two lenses, and is also furnished with 
an interior partition ; this partition may be fixed 
outside if desired, between the two lenses and the 

The two positives obtained, seen as in nature, 
will be in their proper places, the left picture to 
the left and the right one to the right. The 
positive may be looked at either from the glass or 
the film side, according as the face or back has 
been turned to the camera. 

M. Donnadieu has specially invented for this 
application, a camera with two compartments, by 
which the work is more easily done. The part 
which receives the negative is formed of two 
bellows, prolonged by a partitioned box. The 
plate holder is movable so that an exact centre 
may be found. 

Stereoscopic positives may be coloured : we 
always find, however, that they lose their exacti- 
tude very perceptibly after colouring, because it 
is very difficult to limit the colour properly. This 
is particularly noticeable in the foreground. It is 
well-known that all deformation of the contours 
tends to falsify the relief. All the sunken parts, 
the masses of verdure, the details which have 
such a pretty effect in the stereoscope, lose their 



delicacy. Therefore it is better to use only very 
light tints and with all possible precision. 

The same remarks also apply to transparent 
positives, which can be coloured in the same way. 
M. L. Vidal advises the use of transparent aniline 
colours, diluted with a rather thick varnish, having 
a basis of gum-lac. The painting is done on a 
retouching desk. Care must be taken not to 
breathe on the transparency, or it will take an opal 
tint : this can be remedied, however, by warming it. 

All the processes of photo-mechanical impres- 
sions, either sunk or in relief, may be used for the 
rapid printing of stereoscopic pictures. Half-tone 
constitutes an excellent means of stereoscopic 
illustration, as figures may be introduced into the 
text. We have, moreover, made use of it in this 
book. Photogravure appears to us the most 
perfect means of obtaining at a cheap rate, 
collections of artistically valuable stereoscopic 


I N the preceding chapters we have mentioned, 
as far as possible with the dates of invention, 
the various improvements pertaining to the stereo- 
scope or to stereoscopic work. To complete this 
account, we propose to review the principal ideas 
having connection with binocular vision, held 
before Wheatstone, by inventing the first stereo- 
scope, gave material proof of their exactitude. 

It was long a well-known fact that the two 
eyes saw with a different perspective the same 
object placed at a short distance. 

Leonardo de Vinci in Trattata della Pictura 
Scultura ed Architettuva, Milan, 1584) remarked that 
the two pictures furnished by the eyes, intercepted 
on the background two different parts ; and also 
added, that this was the reason why no painting 
could have a relief, equal to that given by a direct 
view, if the object be not placed at too great a 



According to the researches of Brewster, J. B. 
Porta gave, in 1593, a drawing so complete of the 
two pictures, as seen by the two eyes, that not 
only the principle, but the construction of the 
stereoscope also is readily recognised. At the 
same time there is no proof that before Wheat- 
stone,* anyone ever drew right and left pictures. 

In 1613, the Jesuit, Aguilonius, in his essay 
on Optics, also advocated the idea that pictures 
seen by the two eyes were different. 

In 1775, Harris plainly said that there were 
no other means of distinguishing relief but those 
by which we distinguish distance, light or shade, 
and that owing to the separation of the eyes, we 
can see the two sides of an object placed 

* In 1859, Messrs. A. Crum Brown and John Brown, 
visiting the Wicar Museum at Lille, noticed two drawings, 
one by the pen and the other in water colours (Nos. 215, 216), 
representing a young man seated on a bank, the work of 
Jacopo Chimenti da Empoli, painter of the Florentine 
School (1554-1640). Brewster was of opinion that these two 
pictures, taken from two rather different points of view 
might be united stereoscopically, so as to give a pidlure in 
relief. But Mr. Bingham presented to the French Photo- 
graphic Society, a short time after this observation, photo- 
graphic reproductions of these drawings. It was found that 
superposition could be produced, but that there was no 



sufficiently near, and smaller than that separa- 
tion ; so that a certain amount of relief is the 

Mayo was the first to plainly set forth in 
1833 (Outlines of Human Physiology) the principle 
on which the stereoscope is based : — " A solid 
object, being so placed as to be regarded by both 
eyes, projects a different perspective figure on 
each retina ; now if these two perspectives 
be actually copied on paper, and presented one to 
each eye, so as to fall on corresponding parts, the 
original solid figures will be apparently reproduced 
in such a manner, that no effort of the imagination 
can make it appear as a representation on a plane 

In the winter of 1832 he had his first stereo- 
scopes made by Newman, but it was not till 1838 
that he published his first account of the subject. 

Contributions to the Physiology of Vision. 

(Part I. — On some remarkable and hitherto 
unobserved phenomena of binocular vision.) 

Philosophical Transactions, 1838. Pp. 371-394. 
Annates de Chimie II., 1841. Pp. 330-370. 
Peggendorf's Annalen LI., 1842. Pp. 1-48. 
Part II.— Id.— 

Philosophical Transactions, 1852. Pp. 1-18. 

i 7 8 


Philosophical Magazine III., 1852. Pp. 241-267, 

From 1845, Wheatstone used photographs for 
his stereoscope of reflection. 

The subject was again taken up by Sir David 
Brewster, who has published a great many 
memoranda on the subject : 

On the law of middle position in single and 
binocular vision, and on the representation of solid 
figures, by the union of dissimilar plan pictures on the 
retina (1843). 

Transactions of the Royal Society of Edinburgh 
XV., 1844. Pp. 349-368. 

Philosophical Magazine, XXIV., 1844. Pp. 

356-365, 439-455- 

On the knowledge of distance given by binocular 

Transactions of the Royal Society of Edinburgh, 
XV., 1848. Pp. 663-675. 

Philosophical Magazine, XXX., 1847. Pp. 305- 


An account of a new stereoscope. 

Reports of the British Association for the Advance 
of Science, 1849. Pp. 6-7. 

Description of several new and simple stereoscopes 
for exhibiting as solids one or more representations of the 
solid on a plane. 


Transactions - of the Royal Society of Arts, III., 
1851. Pp. 247-258. 

Philosophical Magazine, III ., 1852. Pp. 16-26.* 

It was in 1849 that Duboscq began to make 
stereoscopes with lenses, and binocular daguerreo- 
types, and delivered to the trade instruments 
which are still the most practical stereoscopes, 
and whose forms have only been modified in 

* Memoranda on the modifications and improvements of the 
stereoscope, by Sir David Brewster. 

The End. 


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