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PROCEEDINGS 


OF THE 


AMERICAN ACADEMY 


OF 


ARTS AND SCIENCES. 


Vout. LVII. 


FROM MAY 1921, TO MAY 1922. 


BOSTON: 
PUBLISHED BY THE ACADEMY. 
1922. 





Tse Cosmos PREss 
CAMBRIDGE, MASS. 


CWA . (Lr ¢ i 4 ‘ i} g A ha ,. = 
Sy, , 
= 


Ly Ifans 











II. 


ITT. 


IV. 


VI. 


Vil. 


VIII. 


IX. 


XI. 


XII. 
XIII. 


XIV. 


XV. 


XVI. 


CONTENTS. 


Paae. 


The Grid Structure in Echelon — Lines. By N. A. Kent 
AND L. B. Taytor . , 


The General Conditions of _— a the Principe 1 Le Chatelier. 
By A. J. LoTKa . 

The Effect of Tension on the Electrical Resistance of Certain Ab- 
normal Metals. By P. W. BripGMan PCr ae 

Notes on the Early Evolution of the Reflector. By Louis Bre. . 

The Effect of Pressure on the Thermal —- - Metals. By 
P. W. BRIDGMAN 

The Failure of Ohm’s Law in Gold and Silver at — Current 
Densities. By P. W. BRIDGMAN 


A Table and Method of Computation of Electric Wave eiieaiaatais 
Transmission Line Phenomena, Optical Refraction, and Inverse 
Hyperbolic Functions of a Complex Variable. By G. W. 
PIERCE . Fe a eae aS ae ES AS oe TT 

Artificial Electric Lines with Mutual Inductance between — 
Series Elements. By G. W. Pierce pes R 


The Parasitic Worms of the Animals i Bermuda. I. Trematodes. 
By F. D. BARKER , ‘ ; 
Additions to the H _— Fauna af the Bermudas. By Rvupo.Fr 
BENNITT ‘ j ; 
Some Hymenopterous Parasites of Lignicolous Itonididae. By 
C. T. Bruges Peay es eterna ote 8, es os eae 
A Revision of the Endogoneae. By RoLAND THAXTER ‘ 
The Echinoderms ¢ the ibrar Bank, Bermuda. By H. L. 
CLARK re age anger ag 
Atmospheric Attenuation - Ultra-Violet — By E. R. 
SCHAEFFER We Pee er eal SO 
The Ratio of the Calorie at 73° to that at 20°. By ARNOLD Rom- 
BERG pegs rata ee ae ig eS a 
Studies on Insect Spermatogenesis. IV. The Phenomenon of Poly- 
megaly in the _— Cells — the gees Pentatomide. By R. 
H. BowEn . , A ke apos” 


421175 


1 


19 


39 
67 


75 


129 


173 


193 


213 


239 


261 
289 


351 


363 


375 


388 





CONTENTS. 


XVII. Note on Two Remarkable Ascomycetes. By RoLaAnp THAXTER. 


XVIII. Recorps or MEETINGS 

BIOGRAPHICAL NOTICES 

OFFICERS AND COMMITTEES FOR 1922-23 

List OF FELLOWS AND FoREIGN HONORARY MEMBERS 
STATUTES AND STANDING VOTES . 

RUMFORD PREMIUM 


INDEX 








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ik ae 
ae 


pray os 


Pee 





57 —1 


Proceedings of the American Academy of Arts and Sciences. 


“Vou. 57. No. 1.— Decemser, 1921. 





THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 


By Norton A. KEntT AND Lucien B. TAYLor. 


INVESTIGATIONS ON Licnr AND HEAT MADE AND PUBLISHED WITH AID FROM THE 
RumFrorp Funp. 





(Continued from page 3 of cover. ) 


VOLUME 57. 


i. Kent, Norton A. and Taytor, Lucren B.— The Grid Structure in Echelon Spectrum 
Lines. pp. 1-18. December, 1921. $.75. 





bi 


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eke 


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er Ma Fe 








Proceedings of the American Academy of Arts and Sciences. 


Vou. 57. No. 1.— Drecemser, 1921. 





THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 


By Norton A. Kent anp Lucien B. Taytor. 


INVESTIGATIONS ON LIGHT AND HEAT MADE AND PUBLISHED WITH AID FROM THE 
Rumrerp Funp. 














THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 


Norton A. KEntT AND LuciEN B. TAyLor. 


Received July 7, 1921. Presented October 19, 1921. 


SoME years ago Nutting?! noted a peculiar, complex structure, 
termed by him the “fluting” or “grid,” which appeared in many 
echelon spectrum lines, and consisted of several fine components of 
different and often changing intensity. Later one of us ? independ- 
ently noted this structure. Nutting crossed the 12’ Lummer plate 
of the Bureau of Standards with his echelon and was apparently 
forced to the conclusion that the structure was real — that is, that it 
indicated an actual discontinuity of emission in the source. 

Proceeding on the assumption of reality, the writers attempted a 
solution of the problem using Lid 6104 which, although known to be a 
spectroscopic doublet, offered peculiar advantages in that the grid 
was extremely brilliant, well-marked and persistent. 


APPARATUS. 


The apparatus used consisted of :— 

Two echelons: No. 1, made by Porter, 30 plates, each 14.76 mm. 
thick, step 1 mm., aperture 31.0 by 33.0 mm.; No. 2, made by Petit- 
didier, 30 plates, each 23.29 mm. thick, step 1 mm., aperture 31.0 by 
35.5 mm. | 

The Bureau of Standards 12’ Lummer plate kindly loaned by Dr. 
Stratton. 

A Hilger Lummer plate — length 131 mm., width 14.5 mm., depth 
4.827 mm. 

A Hilger constant deviation prism spectroscope combined with an 
echelon as in Figure la; also a separate Hilger spectroscope with 
another echelon spectroscope as in Figure 1b. The achromatic lenses 
of both echelon spectroscopes are of about 50 cm. focal length and 5 cm. 





1 Astrophys. Jour. 23, pp. 64 and 220. 1906. 
2 Kent, Proc. Am. Acad. XLVIII, No. 5. Aug. 1912. 








4 


KENT AND TAYLOR. 


aperture; each echelon bed rotates on an axis at its center; the Hilger 
micrometer is fitted with one fixed and two movable cross-hairs as 
shown in Figure 2. 











Fic. la. 








vt 
°} 








va 








m 











LSo 


Fia. lib. 


, L 
s 
ee 
s & 


/ ; 











\ 


mM M 


® 


Ss 
Fic. 2. 





FicurREs laand lb. S, slit; L, L, lenses; E, echelon; P, prism; 
O, ocular. 

Figure 2. SS’, fixed crosshair; MM’, MM’ movable system; 
LL’, spectrum line. 


A Littrow mount spectroscope consisting of a Petitdidier 
achromat — focal length 30 feet, aperture 6”; and an An- 
derson grating — aperture 33” vertical by 5” horizontal, 
15,000 lines per inch, used in the third order. 

A5K. W. transformer, 110 to 30,000 volts ratio of trans- 
formation, fed by a 60 cycle Holtzer-Cabot 4.5 K. W. gen- 
erator. Various large induction coils capable of giving 6” 
sparks and operated by a rotary mercury break and an 
electrolytic interrupter, had proven insufficient. 

A vacuum arc of construction as indicated in Figure 3. 








THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. ~ 5 
























































a 
D 
O 
x | 
SS S 
S008:7 
»_ A™® JAN 
Nex * AA 
NZ) 
N 
Ni 
N 
N 
N 
SS . 
\ N 
\ 
\ T 
A, 
e 4 
° , 
‘ 
W Ww 
1 
‘ \ 
e \ 
| ‘ 
‘ 
~ 
ae 
\ 
Ni 
NI 
J N 
NJ 
NJ 
N 
N 
: 











OCR EE OROR EE Ee 

















==G@ 


Iz 


Ficure 3. One fourth original size. D, D, fibre disks; O, O, water outlet; 
P, P, fibre plugs; A, A, asbestos; J, water jacket; W,W, W, windows; 
I, I, I, water inlet. ‘ 








6 ; KENT AND TAYLOR. 
This was also adapted to pressures of several atmospheres as the glass 


windows and fibre plugs were held in place by threaded rings. 
Quartz vacuum tubes — even pyrex glass having proven unsuit- 


Water intake 


yo 


To vacuum pump 


Water intake 
| 

















R R 
| . e Water outlet § HF 
(ZZ TET 


Vater outlet 
Figure 4. C,C, cork stoppers; R, R, rubber sponges; T, T, terminals. 







































































| ~ 
Ay Abr ~dh- nan 
f NP ‘ F wee, 
Jo vacuum 
| pumps 








To H, rator 
- ky M 





drying apparatus 
Ficure 5. TT, tube; M, manometer; B, bulb. 
able — of various forms, the most successful of which, for salts such as 


lithium chloride, proved to be that shown in Figure 4 in which fine 
brass wire, often in helical form, was fitted into brass caps, 6 mm. in 








THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 7 


diameter, and sealed in with De Khotinsky cement, each joint being 
cooled by a water jacket. The salt is shoved into the capillary by a 
wire and the tube will run many hours without refilling. It may be 
used end on as well as side on. The capillaries varied from 2 to 0.5 
mm. 

Auxiliary apparatus as shown schematically in Figare 5. The bulb 
B, prevented too rapid changes in pressure. The system was washed 
out with hydrogen from a Kipp generator, dried by sulphuric acid and 
a calcium chloride tower. The mercury manometer, M, indicated the 
pressure — generally from 8 cm. to a fraction of a millimeter. 


PROCEDURE AND CERTAIN RESULTs. 


Both Lummer plates were each in succession crossed with echelon 
No. 1. In each case, with a carbon are soaked with lithium chloride, 
both at atmospheric pressure and in a moderate vacuum, there . 
appeared a pattern which, at this stage of the investigation, seemed 
to indicate that the grid was real. The following facts, (1) to (6), are, 
however, clearly not in accord with this conclusion, and prove con- 
clusively that this curious structure is due to the phenomenon of 
“secondary maxima” observed by Stansfield * and resulting from 
successive reflections from the surfaces of the echelon plates, producing 
a Fabry and Perot system in the region of the primary light of the 
echelon. (1) to (3) deal with some of the criteria of echelon secondary 
maxima given by Stansfield. These criteria are, in essence, indicated 
below by italics. 

(1) The width of Lid 6104, given by an open carbon arc at atmos- 
pheric pressure, as seen in the Littrow grating, using a narrow slit, 
was found to be about 0.25 t. m. when echelon No. 2 showed the grid 
plainly. The suspicion, therefore, was confirmed that the line was 
too wide for the echelon, the difference between the adjacent orders, 
being about 0.26 t. m. In the case of Janicki’s observation * of 
Hg. \ 5461, Nutting’s work on lines of many elements, and the work 
of one of us on the zinc lines as given by arc and spark, the indications 
are that with all lines for which the echelon shows the grid, their 
breadth is so great that the use of this instrument is not at all justifi- 
able. 

The writers then proceeded to study the structure from this new 





3 Phil. Mag. (6) 18. 383. 1909. 
4 An. der Phys. Vol. XIX, p. 36. 1906. 











8 KENT AND TAYLOR. 


standpoint, considering the primary line of width approximately 
0.25 t. m., and not as formerly, one of the grid components itself. 

These components are indeed, in this sense, each narrower than the 
primary maximum — 0.25 t. m.— the grid components, all of them 
now regarded as secondary maxima, being only about 0.05 t. m. in 
width in echelon No. 2. 

(2) The curvature of one of the mercury yellow lines was compared 
with that of a grid component in A6104. By stopping down the 
echelon spectroscope slit, a line of definite length was observed, and 
by setting the stationary cross-hair of the filar micrometer upon 
the ends of the image, and the movable system upon its center, the 
horizontal distance, d, Figure 2, from the ends of each line to its 
center were measured. It was found that the curvature of the com- 
ponent is about 25%, greater than that of the primary line. 

(3) With a small mirror, set at 45°, over the lower half of the echelon 
spectroscope slit an argon vacuum tube and the lithium are were 
observed at the same time. The relative motion of the grid compo- 
nents in \6104 and a nearby argon line were then studied as the 
echelon was rotated. The primary argon line moves about one-half as 
fast as the grid components. 

Quantitative measurements of the relative displacements were later 
made with Zn 44810. A quartz vacuum tube was fitted with coiled 
brass wire leads and brass terminals, exhausted, filled with hydrogen 
to 10 or more cm. pressure and then gradually exhausted to 1 mm. or 
less. The zinc lines given by the brass wire leads appeared very 
sharp, steady and brilliant. With \4810, as thus produced, was com- 
pared the “gridded” line of a cored carbon arc at atmospheric pres- 
sure, in which small pieces of zinc had been placed, the small mirror 
arrangement allowing simultaneous observation of both sources. 
Upon rotation of the echelon the grid components rushed by the 
narrow tube line. To measure the relative speed a plane mirror was 
attached to a side of the echelon case. The image of an illuminated 
slit in a piece of cardboard was formed by a lens upon a distant scale 
after reflection from the mirror. The echelon was set near the 6 = 0 
position. A reading of the position of the slit image on the scale was 
taken when the tube line lay upon the fixed hair of the filar micrometer. 
The echelon was then rotated until the slit image moved about 2 cm. 
The displacement of the tube line was then measured by the movable 
cross-hair system. A similar series was then taken with a grid line. 
The ratio of the displacements was 3.6 : 6.4 or about 1 : 2. 

(4) The echelon was removed and the ocular focussed on the prism 














THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 9 


image of a line. Replacing the echelon shortened the focus for a true 
narrow echelon image by about 0.6 mm. The focus for the grid com- 
ponents of the same line was 0.7 mm. shorter yet — the light forming 
the grid had traversed the echelon plates more than once. 

(5) Although the grid components are generally very well defined 
(the minimum being “deep”’), it is a difficult matter, with a fluctuating 
source such as an open arc, to obtain accurate measurements. The 
grid spacings appear to vary slightly at different stages. When the 
grid is complete the spacing is regular and, within the limits of error of 
measurement is equal to one fifth the distance between the orders. 
This was proven as follows:— A quartz tube having merely coils of 
fine brass wire as terminals gave extremely fine zinc lines. Echelon 
No. 2 was set in double order condition and Ao, the difference between 
the two orders, measured for \4810 (see Table I). Then the grid was 
measured as given by a 3 ampere open carbon arc. Three distinct 
series of readings were taken. Then the tube was again used. ‘The 
accuracy of an individual setting was about 0.2% in Aoand about 5% 
in Ag. It thus appears that in this region, at least within 2%, Ao = 
5Ag. The focus of the instrument was, of course, not changed, the 
difference between that for primary and secondary maxima being so 
slight that distances between the components of the grid are not 
appreciably affected. | 





TABLE I. 


Distances are measured in divisions of the{micrometer head. Each Ao die. 
tance given is the mean as calculated from four settings; each Ag from two. 
Settings were made on the six centrally situated grid components. 











Ao for Zn \ 4810 | Ag for six grid components. 
22.60 1-2 | 2-3 | 3-4 | 45 | 5-6 |Mean/| Mean 
Mean Motes 
4.3 4.8 4.6 4.8 4.7 4.6 


to 
bo 
qr 
QO 


4.8 | 4.4 | 46 | 48 | 4.1 |] 4.5 | 46 
22.50 4.3.] 5.0 | 5.0.|.4.3 | 4.7 | 4.6 





























22.58 +4.6=4.9 or Ao = 4.9 dg 











A similar series for Zn \6362. gave Ao = 27.65 and Ag = 5.6, 5.9, 
5.5, 5.6, 5.4: mean = 5.5. Hence Ao = 5.0 Ag. | 

















10 KENT AND TAYLOR. 


(6) The structure given by both echelons is the same. That is, 
there are five secondary maxima for every primary maximum. Ag for 
Hydrogen 6563 is 0.061 t. m. for instrument No. 2, while for No. 1 
it is about 0.096 t: m., which again is another fact fatally inconsistent 
with the existence of a definite discontinuous emission in the source. 
With this evidence at hand the writers then attempted to clear up 
the results of the crossed dispersions. The source previously used was 
hardly adequate. By removing the soft core of the lower carbon it 
was possible to feed copiously into the are a strong LiCl] solution. 
Greater brilliancy and steadiness were obtained. The results were 
unmistakably in accord with the facts given in (1) to (6) above. 
Figures 6a and 6b indicate the structure observed. These will be 








% 





















































Fic. 6a. Fic. 6b. 


Ficure 6a. Li 46104 with crossed Lummer plate and echelon. Grid not 
indicated. , and dz in single and double order condition respectively. 

Ficure 6b. Li \6104 as in Fig. 6a. Grid shown. A, and Az both between 
single and double order condition. 


discussed in full below (see page 16). When one component of the 
spectroscopic doublet is in double and the other in single order condi- 
tion three lines appear; when both are in a condition between single 

















THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 11 


and double order there are four lines. It is probable that these four 
lines, under conditions of inferior illumination, were interpreted as 
four separate and true lines. It is unfortunate that at first the only 
line available for study was a doublet. With this latter and better 
source a zinc chloride solution gave 4810 sufficiently strong. The 
crossed dispersions prove it to be a simple, though broad, single line 
when the echelon alone shows the grid. 


FuRTHER RESULTS: CHARACTERISTICS OF THE GRID. 


(a) From numerous observations upon Li \6104 and Zn 4810, as 
developed by various sources, such as vacuum tubes and arcs (on 110 
and 220 volt D.C. circuits and from 1 to 20 amperes) under high, 
normal and low pressure, in which the cross-hairs of the filar micro- 
meter were set successively upon the true, narrow, lines given by the 
tube and the grid components given by the are, it is quite certain that 
the grid is built up approximately as follows:— Suppose that in a 
hypothetical grating of resolving power and dispersion equal to that 
of the echelon, a line which is at first very narrow, e.g., 0.025 t. m., 
gradually becomes less monochromatic, owing to changing conditions 
in the source, and appears as represented diagrammatically by the 
small letters a to e, Figure 7. Four cases must be discussed as shown 
in Figures 7 to 10, respectively. 

CasE I:— The echelon in double order condition gives successively 
images A to E. When the line is very narrow the echelon shows it as 
such, in A. Similarly for a line of width, Ag — the width of a grid 
component or an intergrid distance — it is shown as in B. When of 
width 3Ag, the echelon shows no change, C appearing as B; for, at m, 
the primary and secondary action together give a decided minimum. 
When the line has a width, as in d, the echelon shows a triplet, D, and 
when of width as in e, or greater, the grid is complete — five grid 
maxima, 1 to 5 and 6 to 10, for each maximum, such as 3 and 8, which 
a narrow line would give; four maxima, 4 to 7, between the double 
order positions, 3 and 8, of such a narrow line. For a given position 
of the echelon these grid components do not, in forming, move very 
much, if at all: they come up in situ. There exists an apparent 
motion, in and out, which is probably due to the changing width of 
the primary line, which may not at all times be such as to complete 
the entire width of a grid component. 

CasE II:— When the position of the echelon, its temperature and 
the wave length of the line observed, result in the central grid minimum 









































KENT AND TAYLOR. 











(23945678 90N 

















224756789 











Fic. 8. Fic. 9. 


Ficure 7. Case I: Echelon in double order 
condition and a grid maximum coincident with 
the primary maximum. 

FicureE 8. Case II: Echelon in double order 
condition and a grid minimum coincident with the 
primary maximum. 

Ficure 9. Case III: Echelon in single order 
condition and a grid maximum coincident with 
the primary maximum. 











THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 13 


occurring in the position of the narrow tube line, as in Figure 8, for a 
double order condition of the echelon, there are nine or even eleven 
components when the grid is strong. Note that the grid components 
2, 3, 7, 8, which at first are very brilliant when the grid is “young,” 
grow weaker, 2 and 8 often being so faint that it is difficult to make 
accurate micrometer settings upon them. 

CasE III:— The treatment is the same for a single order condition 
of the echelon, as in Figure 9 which shows a triplet, quintuplet, or, 
with neighboring parts of adjacent orders, even as many as eleven 
components. 

Cast IV:—Here a grid minimum coincides 
with the primary maximum and the grid com- 
ponents are as shown in Figure 10. 

The above statements explain why an origi- 
nally narrow line, as its width increases, may ap- 
pear, as it actually does, a triplet or quintuplet, as | 





in Figures 7 and 9, or may, as it were, “reverse” 
and then quadruple, asin Figures8 and 10. Actual 
reversal as shown by the grating probably occurs 


much later in the history of the line. (See page 
15.) 
Further, if a line be intrinsically unsymmetri- | 


cal, shading off to the red for instance, the sec- 
ondary action masks an early stage of broadening, 
and the left grid line, 2, forms as in A, Figure 11. | 
Line 3, as in A’, then comes up as 2 strengthens. 

(b) The grid begins to disappear and the line 
gradually becomes broad and structureless when 
the primary line exceeds 2Ao in width, Ao being ie 
the distance between two adjacent orders. This 
was determined as follows:— Using 


as narrow a slit as possible, a low 
power ocular and a mm. scale, an 
A i eye estimate was made of the 


breadths of various portions of an’ 





$23456789H 








arc line shown by the grating. 


| | These were reduced tot. m. The Fic. 10. 





same source was viewed simultane- 





a46 





Figure 10. Case IV: Echelon in single order condition and 
Fig. 11, a grid minimum coincident with the primary maximum. 











14 KENT AND TAYLOR. 


ously by echelon No. 2. For Zn \4810 three components of the grid 
exist when the grating shows a line 0.12 t. m. broad. Ao for \4810 = 
0.155 t. m. $ X 0.155 = 0.09 t. m. which compares favorably with 0.12 
t.m. The complete grid exists when the line is 0.3 t. m. or 2Ao 
broad and the image begins to pass into a structureless line at 3Ao. 
Similarly for Li \6104 a full and well-marked grid exists at a line 
width about 0.2 to0.5 t.m. or Ao to 2Ao (as here Ao = 0.25t. m.). 
The grid is poorly marked above about 2Ao and is gone at 3Ao. 

(c) Numerous lines in the spectra of Na, Hg, Fe, Mg, Cd, Ca, Sn, 
Pb, and Bi, developed by an open carbon arc, show the grid whenever 
the line is sufficiently broad — rendered so by introducing more of 
the substance or increasing the current; also by increasing the capac- 
ity in the case of a spark. 

(d) Li AA6708 and 6104, Zn \d4810, 4722 and 4680, also Hg \5461 
(mercury being fed into the lower cored carbon) show by their behavior 
that a line which is too broad will appear structureless in the echelon, 
that the center of the core of an arc may show the grid complete while 
light from the wings of the image gives a simple structure of but one 
to three components. With a sufficient amount of vapor the com- 
plete grid may be obtained even at low pressure. 

(e) A study of Zn 44810, from an arc in the vacuum or pressure tank, 
at pressures from 2 cm. of mercury to about three atmospheres, 
showed that moderate changes of pressure do not produce measurable 
displacements in the grid components, but merely alter somewhat 
their relative intensities, shifting the maximum over one or two com- 
ponents or even bringing up new ones. This of course means that, 
as long as a grid exists, the components do not change appreciably 
their position with changes of wavelength as small as 0.015 or 0.020 
t.m.° Their position is affected more strongly by the position of the 
echelon and its temperature. Similarly, the grid components of the 
spectroscopic doublets Li AA6708 and 6104 developed in vacuum tubes 
show intensity shifts with changes of pressure over the range of one 
atmosphere. 

(f) The “end on”’ position of a vacuum tube will generally show a 
more complete grid than that “side on.”’ 

(g) If a line broaden unsymmetrically with increase of current the 
maximum of intensity will shift. Those components which are just 
being formed show an apparent motion outward as the number of 





5 According to Humphrey’s and Mohler’s results for Zn, the pressure shift 
reduced to 4000 is 0.057 t. m. for twelve atmospheres. 








THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 15 


components increases, the first step resembling a narrow reversal as 
in Figures 8 and 10 or a central fixed line with two moving wings as in 
Figures 7 and 9. But the writers feel that this apparent motion is 
due to the fact that each grid component is not formed 2n toto at once: 
the part which lies nearest the center of the system is formed first. 
Certain it is that this apparent motion ceases abruptly when the 
component has reached a position which is one grid distance from 
its neighbor. If the source be an arc, many rapid fluctuations in 
intensity occur. 

(h) Although the resolving power of the grating (225,000 in the third 
order) is far below that of the echelon (about 750,000 for \6100 for 
echelon No. 2) it is hard to reconcile the images given by the two 
instruments on any other assumption than that the grid is due to 
secondary action. 

To throw further light on the problem, Li \6104, given by a vertical 
carbon arc soaked with LiCl, was viewed simultaneously by echelon 
and grating. Table II gives a summary obtained from various ar- 
‘rangements. 


TABLE II. 


|| || indicates the grid; § a broad structureless line; | a narrow unreversed 
line, or one very slightly reversed; jj a broad and strongly reversed line. 








Arrangement when oF ree me . Echelon shows Grating shows 
upper pole with solution 

1 + + At + pole |||| | 

: cee, oe eg iT 

2 + ” a | 
eo. «@ 4 il 

3 ai Be oe. 2 s il 
“—" B | 

4 = + ae B il 
a ae | 























Therefore which pole is soaked makes no difference, nor does it 
matter which pole is above. The region near the + pole generally 
shows the grid in the echelon, that near the — pole a broad structure- 
less line. The grating always gives a narrow unreversed line or one 
very slightly reversed where the echelon shows the grid, and a strongly 
reversed line where the echelon shows no structure. Thus the grid 
does not result from conditions which produce a reversed grating line. 











16 KENT AND TAYLOR. 


With Li \6708, which usually appears widely reversed in the grat- 
ing, the grid is more difficult to obtain in the echelon, while with 
Na \4972 — given as an unreversed line by the grating at either edge 
or centre of the arc image — the echelon shows the grid at both edge 
and centre. 

(i) We are now in a position to discuss in detail Figures 6a and 6b. 
These were obtained with the 131 mm. Lummer plate set between the 
collimator and prism of Figure 1b and crossed with echelon No. 2. 
The source was that described on page 10: the are current being from 
10 to 25 amps. The plate dispersed vertically, the echelon horizon- 
tally. Both figures are drawings based on visual filar micrometer 
measurements, a single cross hair being moved successively along the 
axes, vv’ (vertical), hh’ (horizontal), aa’ (across the structure) pp’ 
(parallel to it), as shown below the two figures. 

Two Lummer plate orders are shown in each figure, the primes 
distinguishing these. ‘The numerals indicate the two components of 
the spectroscopic doublet, the breadth along axis aa’ their approximate 
relative intensity. A, is the weaker line, A». the stronger in both 
figures — dz being the component of longer wavelength. 

In Figure 6a d2 is in double order condition; in 6b both A; and dz 
are between double and single order. The echelon grid structure is not 
indicated in Figure 6a: in 6b its approximate position is shown. It 
was difficult to observe at the ends of the lines and so is not there 
indicated: it is slanted at an angle of about 2.4° (see gg’ in Figure 6b) 
with the vertical. The slant of the lines themselves as well as that of 
the grid changes with the positions of both plate and echelon: further, 
the grid slant is not due to the curvature of the echelon image. This 
may throw some light on the disappearance of the grid at a breadth 
of line greater than 2Ao. For, as the echelon action alone is given by 
the projection, on the pp’ axis, of the grids of the lines \; and dg, it is 
evident that lack of coincidence owing to slant would tend to obliter- 
ate the grid altogether, this indicating that two broad lines, the centers 
of which lie as far as 0.1 t. m. apart (the Ad of the two components of 
Li \6104), may not give coincident grid structures; or, in other words, 
the grid maxima do not (for any one position and temperature of the 
echelon) necessarily fall together. This is not inconsistent with shift 
of intensity for small changes of wavelength (0.015 to 0.020 t. m.) as 
noted on page 14. Shift of intensity and position probably both enter 
with change of wavelength of the center of gravity of a primary echelon 
image. 

These two figures show that the grid is unquestionably a secondary 














® 


THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 17 


echelon action. Otherwise the regions between lines 1 and 2 would 
have been filled in with a structure along axis aa’ similar to that 
along pp’. 

With an echelon alone we have obtained only the weaker component 
of Li \6104 as a single narrow line. We plan to cool the tube with 
liquid air, thus sharpening the stronger component so that it will 
no longer suffer the secondary action, to which the small satellite is 
probably due. 

(j) We have no record of having observed in either echelon any 
ungridded line of width greater than Ag. Either there exists (1) a 
very narrow line, (2) an irregular series of such, as, for instance, in the 
yellow mercury lines, (3) a line of width Ag, (4) a series of such (the 
grid more or less complete) or (5) a broad, structureless image cover- 
ing between one and two orders. And it appears extremely probable 
that the “reversal” of the main component of Hg \5461, noted under 
certain conditions by several observers and often noticed by us, may 
be modified by the entrance of secondary action due to the excessive 
breadth of this component. 

(k) The retardation producing the primary maxima of a narrow line 
is proportional to n—1, while that of the light undergoing secondary 
action is proportional to 3n—1. Thus the difference in retardation 


in case of the two actions bears the ratio to the retardation of the 
2n 





n—l1 


varies from 5.50 for \6563 to 5.37 for \4341 in echelon No. 2; and from 
5.48 to 5.35 respectively in No.1. Since echelons are generally made of 
substantially the same kind of glass, any two having equal separation 
of primary orders will have equal separation of secondary maxima, 
because this separation is the same fractional part of the separation of 
the orders; but the values of Ag in t. m., varying with the dispersion, 
will, of course, differ in different instruments. 

We cannot state just why Ao = 5Ag. The measurements given 
above indicate that this is so within the limits of experimental error 
for both the violet and red regions. 

It would be interesting to assemble an echelon under water, press 
the plates together and allow the superfluous water to drain off. This 
process might vastly reduce the secondary action. If successful 
Canada balsam might be substituted for water thus producing a 
more permanent instrument. We plan to try this experiment shortly. 


2n 
primary of —y which is a function of nalone. The value of 











KENT AND TAYLOR. 


CONCLUSION. 


Summarizing the above results, we may state that the evidence is 
entirely against the existence of a discontinuity of emission in the 
source. The grid is due to a secondary action of the echelon which 
enters when the line under investigation is not sufficiently monochro- 
matic. This means that the previous work of one of us ® must be 
considered as of small value and also that an explanation of the 
apparent complexity of structure obtained by Nutting’ can be 
found in secondary action. 

The results obtained emphasize the fact that when an echelon is 
used to measure small wavelength differences, great care must be taken 
to obtain the lines so narrow that their width is less than 3 Ao, else 
secondary action may enter to cut off an edge of a line and thus give 
a false intensity-maximum position. 

We must record our appreciation of the help rendered by various 
student assistants, especially Messrs. Greenleaf and Risga. We are 
also indebted to Dr. Lucy Wilson for her skilful aid during part of this 
research and to our assistants, Miss Pearson for mathematical work 
in connection with the calculation of the constants of the echelons, 
and Mr. Gilman for making the sketches accompanying this article. 

We wish also to thank sincerely the Rumford Committee of the 
American Academy for numerous grants which made possible the. 
purchase of the main pieces of apparatus used in this investigation. 





6 Proc. Am. Acad., Vol. XLVIII, No. 5. Aug. 1912. 
7 Astrophys. Jour., XXIII, pp. 64 and 220. 1906. 


PuysicaL Lasoratory, Boston UNIVERSITY, 
May, 1921.