From the French of F. Dronin .
TRANSLATED BY MATTHEW SURFACE,
Editor of The Practical Photographer.
Percy Lund &
Co., The Country Press, Bradford;
Hall, Ludgate Circus, London.
PERCY LUND AND CO.,
PRINTERS AND PUBLISHERS,
THE COUNTRY PRESS, BRADFORD;
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.
TABLE OF CONTENTS.
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
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
THE PERCEPTION OF RELIEF.
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
AND STEREOSCOPIC PHOTOGRAPHY.
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
AND STEREOSCOPIC PHOTOGRAPHY. g
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 1 1
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.
THE PURPOSE OF JUDGMENT IN
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
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
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 1 5
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.
AND STEREOSCOPIC PHOTOGRAPHY. 17
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
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.
THE TELESTEREOSCOPE AND
I HE perception of relief obtained by binocular
vision proceeds from the difference in the
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 STEREOSCOPIC PHOTOGRAPHY. 21
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 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
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
AND STEREOSCOPIC PHOTOGRAPHY.
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>
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 25
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
26 THE STEREOSCOPE
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
AND STEREOSCOPIC PHOTOGRAPHY. 2J
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
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 3 1
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 33
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 35
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 '
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
AND STEREOSCOPIC PHOTOGRAPHY.
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
AND STEREOSCOPIC PHOTOGRAPHY. 39
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.
Refractive Stereoscopes. — These instru-
ments are those almost exclusively employed at
the present time. They allow the use of prints
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
stereoscopic relief will
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
* The name stereograph is often given to the right and
left pi&ures thus mounted side by side.
AND STEREOSCOPIC PHOTOGRAPHY. 41
drawn one over the other for greater clearness.)
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
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).
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
Fig. 25, so as
to make them
r e ct angu 1 ar,
and fix them to-
gether by their
should be about
wide (3£in.), the
(2% in.) The
sharp edges of
the two half len-
ses can be left
and inserted in
(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
AND STEREOSCOPIC PHOTOGRAPHY. 43
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-
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
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
44 THE STEREOSCOPE
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
the side (Fig. 28). It follows therefore that in
varying the position of the eye, with reference to
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
AND STEREOSCOPIC PHOTOGRAPHY.
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
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.
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.
AND STEREOSCOPIC PHOTOGRAPHY.
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
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
the hand (Fig. 33) or mounted on a stand.
AND STEREOSCOPIC PHOTOGRAPHY. 49
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 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.
AND STEREOSCOPIC PHOTOGRAPHY.
This frame can be moved backwards and forwards
on the baseboard for the purpose of focussing.
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
pattern is very low, about half-a-crown, or even
two shillings for the cheaper makes, is the usual
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 53
(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.
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
AND STEREOSCOPIC PHOTOGRAPHY. 55
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
56 THE STEREOSCOPE
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 STEREOSCOPIC PHOTOGRAPHY.
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,
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.
AND STEREOSCOPIC PHOTOGRAPHY. 59
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 6l
practical, are still very interesting, and worthy of
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
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
62 THE STEREOSCOPE
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
AND STEREOSCOPIC PHOTOGRAPHY. 63
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
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 65
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
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
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 67
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
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
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
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*
AND STEREOSCOPIC PHOTOGRAPHY. 71
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.
THE EXAMINATION OF
STEREOSCOPIC PICTURES WITHOUT
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
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
AND STEREOSCOPIC PHOTOGRAPHY.
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 77
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
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
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-
AND STEREOSCOPIC PHOTOGRAPHY.
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.
STEREOSCOPES OF PROJECTION.
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
AND STEREOSCOPIC PHOTOGRAPHY. 83
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
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
rapidly before the two object glasses uncovers
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,
AND STEREOSCOPIC PHOTOGRAPHY. 85
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
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
a=oc — /3
and the part closed
and as the entire disc forms two lids
a+b=2 a =i8o°
AND STEREOSCOPIC PHOTOGRAPHY. 87
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 89
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.
OBTAINING RELIEF BY A SINGLE
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
AND STEREOSCOPIC PHOTOGRAPHY.
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
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
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
ratio — = of the sizes of these two objects
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
AND STEREOSCOPIC PHOTOGRAPHY. 95
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
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.
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
AND STEREOSCOPIC PHOTOGRAPHY. 97
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.
OF THE STEREOSCOPE.
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
AND STEREOSCOPIC PHOTOGRAPHY. IOI
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
Dove* (1859) discovered that each time two
slightly dissimilar pictures are examined by a
stereoscope certain parts appear in relief. If two
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.
AND STEREOSCOPIC PHOTOGRAPHY. IO3
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
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
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.
AND STEREOSCOPIC PHOTOGRAPHY. IO7
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
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
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
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.
AND STEREOSCOPIC PHOTOGRAPHY. Ill
At the same time it is desirable to take into
consideration the latitude affored by the range of
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
and the separation of the lenses must be chosen to
correspond with the optical angle, probably 12 to
If we reckon for v the value of 20 centimetres
we have the following result :
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.
AND STEREOSCOPIC PHOTOGRAPHY.
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.
AND STEREOSCOPIC PHOTOGRAPHY. II5
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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
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
AND STEREOSCOPIC PHOTOGRAPHY. II7
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
D=- e cotg-a,
gives the figures of the succeeding table).
23 8 „
10 1 ,,
2 4 °
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.
AND STEREOSCOPIC PHOTOGRAPHY. ng
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
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.
STEREOSCOPIC PHOTOGRAPHY BY
DISPLACING THE OBJECT.
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.
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.
AND STEREOSCOPIC PHOTOGRAPHY. 125
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.
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
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.
AND STEREOSCOPIC PHOTOGRAPHY.
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
* 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.
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
AND STEREOSCOPIC PHOTOGRAPHY. I2Q,
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
Fig. 78 shows the side and underpart of
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
STEREOSCOPIC PHOTOGRAPHY BY
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
132 THE STEREOSCOPE
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
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
AND STEREOSCOPIC PHOTOGRAPHY.
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
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
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
AND STEREOSCOPIC PHOTOGRAPHY . I35
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
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.
AND STEREOSCOPIC PHOTOGRAPHY. 137
We have, however, found it very useful for
photo-micrography with magnifying objectives of
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:.
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.
STEREOSCOPIC PHOTOGRAPHY BY
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.
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.
AND STEREOSCOPIC PHOTOGRAPHY. 141
The lenses are mounted ordinarily at the
centres of each half of the plate.
The English manufacturers have lately given
considerable attention to the manufacture of
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
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.
AND STEREOSCOPIC PHOTOGRAPHY. I43
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
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
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
AND STEREOSCOPIC PHOTOGRAPHY. I45
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
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
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.
AND STEREOSCOPIC PHOTOGRAPHY.
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.
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
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.
Position for ordinary views.
Position for stereoscopic views.
AND STEREOSCOPIC PHOTOGRAPHY. 151
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.
STEREOSCOPIC PHOTOGRAPHY BY
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
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.
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
AND STEREOSCOPIC PHOTOGRAPHY. 1 59
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.
FERROUS OXALATE DEVELOPMENT.
A few drops
For use mix of : —
Pour B into A, and not A into B.
l62 THE STEREOSCOPE
PYRO AND AMMONIA.
Water 32 ounces
Potassium Bromide i\ drachms
Ammonia 4 J „
Pyro 30 grains
Equal parts of each.
PYRO AND SODA.
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
Potassium Carbonate 10 „
AND STEREOSCOPIC PHOTOGRAPHY. I
HYDROgUINONE AND EIKONOGEN.
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.
Water 32 ounces
Hyposulphite of Soda 7J drachms
TONING AND FIXING FOR ARISTOTYPE.
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
COMBINED TONING AND FIXING BATH.
Water 17 ounces
Sodium Hyposulphite 6J
Ammonium Sulphocyanide drachms
Solution of Acetate of Lead 1 J ounces
Filter when cold.
Distilled Water 3 ounces
Gold Chloride . . . 15 grains
AND STEREOSCOPIC PHOTOGRAPHY. 167
The bath is formed of: —
TRANSPARENT POSITIVES GELATINO- BROMIDE .
Amidol 2J drachms
Potassium Metabisulphite 15J
Water 2 J
Solution A 4J drachms
Water 22 ,,
Solution of Potassium Carbonate 10% 2
Ammon. Bromide % 8
DEVELOPER FOR GELATINO -CHLORIDE TRANSPARENCIES.
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;
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
AND STEREOSCOPIC PHOTOGRAPHY. 169
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
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
AND STEREOSCOPIC PHOTOGRAPHY. 171
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.
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
AND STEREOSCOPIC PHOTOGRAPHY. 173
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
A FEW WORDS OF HISTORY.
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
AND STEREOSCOPIC PHOTOGRAPHY.
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
Transactions of the Royal Society of Edinburgh
XV., 1844. Pp. 349-368.
Philosophical Magazine, XXIV., 1844. Pp.
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.
AND STEREOSCOPIC PHOTOGRAPHY. 179
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.
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AND STEREOSCOPIC PHOTOGRAPHY.
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AND STEREOSCOPIC PHOTOGRAPHY.
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