Horace Scudder Uhler.

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H. S. UHLER and R. W. WOOD


Published by the Carnegie Institution of Washington
May, 1907








In spite of the very large amount of work which has been done on ab-
sorption spectra, there exists practically no collection of photographed spectra
from which one can pick out the media most suitable for any particular line
of investigation. The greater part of the published records are drawings
made from visual observations, and give no information regarding the optical
properties of the media in the ultra-violet. It seems desirable therefore to
compile a set of photographic records which are free from the errors liable
to enter into observations made by visual methods, and to arrange them in
such a way that the medium or media necessary to secure a desired result
could be readily found from a mere inspection of the plates.

A great deal of experimental work was necessary before satisfactory
photographic records were obtained. The details of the spectrograph and
the refinements of the method have been worked out very skilfully by
Dr. Uhler, who has done practically all of the experimental work. It was
our original plan to include the colored salts of metals, and to examine a
large number of colorless substances for peculiarities in the ultra-violet. The
solutions of the inorganic compounds could not, however, be investigated
in precisely the same manner, owing to their less powerful absorption. Much
thicker absorbing wedges were required, and these gave trouble, even when
compensated, as a result of dispersion. It was therefore decided to limit
the present work chiefly to a study of the aniline dyes, which are used to a
much greater extent than the metallic salts, in the preparation of absorbing
screens. The absorption spectra of a number of metallic salts, however,
have been photographed, as it is believed that many of them will be useful
in the preparation of ray filters ; some of them are far more transparent in
the ultra-violet than the aniline dyes. Even such substances as the salts
of erbium, neodymium, and praseodymium are useful in special cases where
it is desired to suppress one or more isolated spectral lines. For example,
a solution of neodymium has a very narrow and intense band coincident with
the D lines, and has therefore the property of cutting out the sodium radia-
tion from a given source, transmitting at the same time nearly the whole of
the remainder of the spectrum. The same salt can be used to advantage
when working with the new cadmium and zinc arc lamps in quartz tubes,



made by Heraeus. The fact that the absorption of each substance in the
ultra-violet is recorded, makes the plates of especial value to any engaged
in the preparation of screens for spectroscopic or photographic purposes.

For the removal or transmission of one or more isolated lines some
other arrangement is often more useful than an absorbing screen. A
spectroscope with a slit placed in the focus of the observing telescope
(monochromatic illuminator) is frequently all that is necessary. But if more
light is required, the following device may be used. A block of quartz from
2 to 4 cm. in thickness, cut perpendicular to the optic axis, is mounted
between two Nicol prisms. The transmitted spectrum is crossed by black
bands, which result from the rotatory power of the quartz. By adjusting
the nicols and varying the thickness of the quartz it is often possible to get
rid of the spectrum lines which are not desired, and at the same time to
utilize the whole area of the source, which can never be done with the spec-
troscope. In this way, with a quartz plate 45 mm. thick, the line 4809 of
the zinc arc in quartz can be completely removed and the two lines 4721
and 4679 transmitted. This method is especially useful in the study of the
fluorescence excited in various bodies by monochromatic light.

If it is necessary to separate radiations of very nearly the same wave-
length, for example if we wish to work with the light of one of the two
sodium lines, the following arrangement can be used : A quartz plate about
2 cm. in thickness, cut parallel to the axis, is mounted between crossed nicols,
with its axis making an angle of 45 with the principal planes of the polar-
izing prisms. The source is placed behind a vertical slit 2 or 3 mm. in
width, and the light, after traversing the polarizing system is brought to a
focus by a lens. A number of concentric maxima and minima will be
formed, the light of Dj and D 2 being found in adjacent maxima. The
wave-length which is not desired can be stopped by a screen of suitable
dimensions placed at the focus of the lens. In this way it is possible to
obtain a source of Di or Dz radiation of sufficient intensity to show distinct
fringes in a Michelson interferometer. By a curious coincidence this method
occurred independently to the writer and to Professor Michelson on the same
day. It has been found to give excellent satisfaction. The thickness of
the quartz plates used in either of the above cases depends upon the close-
ness of the lines which it is desired to separate.

We are under great obligation to the Actiengesellschaft fiir Anilinfabrika-
tion and to Meister, Lucius & Briining, both of which firms presented the
Johns Hopkins University with a large collection of aniline dyes.



If we look over the literature of the subject of absorption of light we
fail to find a collection of absorption spectra presented in such a manner as
to enable the observer to select at a glance a substance which produces
either general or selective absorption in any specified part of the visible or
ultra-violet spectrum. The wave-lengths of the absorption bands and other
characteristics of the absorption exhibited by innumerable natural and arti-
ficial compounds and mixtures, both inorganic and organic, may be found
in a great many books, journals, memoirs, and dissertations. If all of these
results were reproduced and catalogued in one volume they would not satis-
factorily fulfil the requirements just mentioned, because the different experi-
menters have had various objects in view and hence they have worked in
various and limited parts of the spectrum, have used different numerical
dispersions, have employed optical systems of unlike dispersion curves, have
not made it possible even to reduce their results to graphical form much less
to a common basis of wave-lengths and normal dispersion, etc. The nearest
approach to a work of the kind under consideration is made by the publica-
tions of J. Formanek, especially the two volumes entitled respectively "Die
Qualitative Spektralanalyse anorganischer Korper" and "Spektralanaly-
tischer Nachweis kiinstlicher organischer Farbstoffe ; " Berlin, 1900. For-
manek's investigations are very extensive and complete from the point of
view explicitly stated in the preface to the last-named volume. It was his
aim to develop a practical spectroscopic method of procedure by which any
given organic coloring matter could be unambiguously identified. He says:

" Das Princip des hier beschreibenen neuen Verfahrens beruht auf der Kombina-
tion der spektralanalytischen Beobachtung und der chemischen Untersuchung ; dieses
Verfahren liefert nicht nur sichere Resultate, sondern sein Vortheil liegt auch darin,
dass man mit Hilfe desselben alle einzelnen Farbstoffe von einander unterscheiden

Formanek, in order to obtain his results, varied the concentrations of
his solutions until each absorption band of a given substance became in suc-
cession as well defined as possible, so that the wave-lengths of their maxima
might be read off with precision. This method is preeminently adapted to
locating maxima, but it gives very little, if any, information relative to the
absorption between and beyond the maxima, for bodies exhibiting marked



selective absorption, and it tells even less about substances presenting
weak, general absorption. Another important respect in which Formanek's
diagrams fail to give the data required by the first sentence of this section is
that he confined his measurements to eye observations, unaided by phos-
phorescent screens, and hence he omitted the entire ultra-violet region.
In fact, his wave-lengths have the limits 420^ and 74 1*, i. e. , from
"above" the G line to a little " below" the a line. Formanek used a prism
spectroscope to the dispersion of which he gives no clue.

To fill in this gap in the then existing collections of absorption spectra
the present research was begun in the spring of 1903. Its chief object is to
furnish graphical representations, on a normal scale of wave-lengths, of the
absorption spectra, both in the visible and in the ultra-violet regions, of a reason-
ably large number of compounds.

The most obvious use to which such a collection can be put is the pro-
duction of color screens either for photographic work or for removing higher
orders of spectra from the first order, in the case of diffraction gratings. It
also makes possible the selection of such solutions as will transmit relatively
narrow, and hence roughly monochromatic, regions of the spectrum. Such
solutions are often convenient substitutes for somewhat elaborate pieces of
apparatus which first disperse the light by a prism (or grating) and then
permit any desired portion of the resulting spectrum alone to continue unin-
terrupted by means of a suitable slit and screens. Other directions in which
the data given below may be of practical value need not be pointed out here.


That a great deal of time was consumed in constructing apparatus and
in performing preliminary experiments is shown by the fact that, although
the investigation was entered upon in the spring of 1903, it was not until
July, 1904, that the first really satisfactory negative was obtained. Only
aqueous solutions of the aniline dyes have been investigated up to the present
time. As is well known, the position of an absorption band may be shifted
within wide limits by varying the solvent ; * moreover, many aniline dyes
are insoluble, or nearly so, in water. On this account it would have been
desirable to have made use of the alcohols, benzol, and other organic com-
pounds as solvents for the media under investigation. But difficulties were
met with which were not overcome until the study of the dyes was completed.
Chief among them may be mentioned the rapid evaporation of the fluid held
between the quartz plates. Attempts were made to obviate the difficulty by
painting the edges of the wedge with melted paraffin, but the heat of the
spark was sufficient to drive off the greater part of the fluid before the

*See Nos. 158 and 165.


exposure was finished. Water is, however, the solvent generally used, and
the easiest one to manage. It is moreover free from ultra-violet absorp-
tion, which is not true of the majority of the other solvents available, and
all dyes which can be dissolved in water can be used for staining gelatin
films. The gelatin can be dissolved in the solution of the dye and clean
glass plates flowed with the warm solution, or an unexposed photographic
plate, after preliminary treatment with thiosulphate of soda and thorough
washing, may be stained with the solution of the dye. It is probable that
the position of the absorption bands is the same in gelatin as in water, for
the indices of refraction of the two media are very nearly the same.


Because of their great variety, strong selective absorption, and general
interest, aniline dyes and their related organic compounds were selected as
best suited for the study contemplated.


In order to obtain reasonably normal spectra a spherical, concave,
speculum grating, whose radius of curvature was 98.3 cm., was used. For
the first order spectra and for short photographic exposures the astigmatism
of the reflector did not produce deleterious effects. This was determined
by actual measurements. The length of one line of the ruling was 1.96 cm.,
and the assemblage of lines covered 5.36 cm. The spectroscopic resolving
power was 21,250 (2.125 inches with 10,000 lines per inch). The incon-
venience of superposed higher orders will be mentioned later on. To obtain
a general idea of the normality of the spectrograms and of the linear dis-
persion it may be stated that, by calculation one millimeter the center of
which was at 214.7^, or 399. 4w, or 656.3;^, covered 25.77, 2 5-84, and
25.71 A. U., respectively, for the spectrum was designed to be normal at
the air line 399. 4w.


Because of the short radius of curvature of the focal surface (about 49
cm.) celluloid films were employed in most cases. The films used through-
out were M. A. Seed's "L-ortho cut negative films," size 5 by 7 inches.
The emulsion is by no means equally sensitive over the field of wave-lengths
studied, i. e., from 0.2^ to 0.637*. The chief maximum of sensitiveness is
in the yellow, about o. 56//.. A much weaker maximum is near 0.49/4. The
middle of the less sensitive intervening region is very roughly 0.52/4.

For the short exposures given throughout, these films are not appreciably
influenced by wave-lengths longer than about 0.61/1. The resultant effect of
the Nernst glower and the Seed emulsion is best understood by referring to
fig. 102, plate 26, for which the times of exposure were, in order, 2 seconds,
5 seconds, 15 seconds, 30 seconds, i minute, 2 minutes, and 3 minutes.


Various schemes to make the resultant action more uniform were tried
and other makes of films were tested, but no improvement on the simple
combination of the Seed emulsion and the Nernst glower resulted, therefore
they were used almost exclusively. The Seed films are good in the ultra-
violet as is shown by the fact that with an exposure of 5 minutes the alumin-
ium line at 185^ was clearly recorded. To see if appreciable shifts in the
apparent positions of the absorption bands were produced by the yellow
maximum and the green minimum of the Seed films, negatives of the same
absorbing medium, under exactly the same conditions, were taken on sev-
eral different makes of films and plates which did not exhibit maxima and
minima of sensitiveness for the same wave-lengths. Also, other and inde-
pendent tests of this possible source of error were made. The conclusion
was that no noticeable displacements of the bands were caused. However,
in the cases of brown and other visibly colored solutions, exhibiting weak,
general absorption, the observer of the appended positives must be careful to
distinguish between true absorption and the spurious effects in the vicinity

of O. 52/1.

In photographing bands in the orange and red, Cramer "Trichromatic"
plates were found to be the best and hence they were used. The plates
being plane they had to occupy a mean position with respect to the focal
surface of the grating. Since only a comparatively small region of wave-
lengths was thus recorded, no measurable errors were introduced. In fact,
in the region considered, the second order ultra-violet of a discontinuous
spectrum taken on a film and on a plate could be superposed line for line.

The developer used was a simple hydrochinone solution made up
according to Jewell's formula.*


For wave-lengths from "above" 0.65/1 to "below" 0.326/1, and for
exposures of about one minute, the Nernst glower was found to be the most
satisfactory. Prevailing circumstances made desirable the use of 104 volt
glowers on a circuit carrying about 133 cycles. The emissivity of the Nernst
lamp varies so very greatly with the e. m. f. impressed upon its terminals
that it was obligatory to keep in series with the glower a Thomson A. C.
ammeter having a range from zero to two amperes and graduated directly
to 0.02 ampere. Fluctuations of more than 0.02 ampere invariably resulted
in a 'spoiled photograph, consequently boxes containing variable metallic
resistance were maintained in series with the ammeter and thus, in spite of
large changes in the load on the dynamo, due to other experimental circuits,
it was possible to prevent the effective current in the filament from changing

*L. E. Jewell. Astrophys. Jour., v. xi, 1900, pp. 240-243.



by more than o.oi ampere. The current was usually 0.8 ampere or a little
less. The ammeter was appreciably more sensitive to small changes in the
terminal voltage than a comparably graduated Thomson A. C. voltmeter,
because the current shunted through the voltmeter was not negligible in
comparison with the current which fed the glower. Among other sources the
electric arc was given a fair trial and discarded for two reasons, first, because
of the intensity of the carbon and cyanogen bands, and second, because of
the inconveniences resulting from its unsteadiness and great emission of heat.

For wave-lengths between the strong ultra-violet of the Nernst glower
and o. 2/4 a spark discharge in air of about i cm. length was used. In obtain-
ing the greater number of the negatives one electrode was composed of an
alloy of equal parts by weight of cadmium and zinc and the other was made
of sheet brass. The alloy wore away so rapidly that the brass electrode was
employed to reduce the labor attendant upon sharpening the terminals.
The electrodes were given a form apparently not
described before. As is very well known, many
spectral lines, both weak and strong, produced
by sparks between metallic surfaces extend only

a short distance beyond the metal and hence do $|f||||lflf Sparks
not offer a continuous source of light! across the
entire spark gap. In order to obtain a back-
ground of uniform intensity from edge to edge
of the negatives it was necessary to use some Fig. 1. Flat and edge view.
scheme to nullify the effects of the non-uniformity of emission in the spark.
One way of accomplishing this is to rapidly translate the electrodes (main-
tained at a fixed distance apart) back and forth parallel to the length of
the slit of the spectrograph by some mechanical device.

The reciprocating action associated with this plan shakes the camera
and grating to such an extent as to demand greater rigidity in the apparatus
than it usually has. Therefore the electrodes were made in the shape of
wedges or chisels with the sharp edges parallel to the slit. The well-known
distribution of a rapidly alternating current in a conductor necessitated
curving the edges of the electrodes, as is shown in fig. i, which is j| natural
size. Due to the tearing away of the metal, and to various other causes,
the innumerable thread-like sparks changed the positions of their ends so
rapidly that the integrating action of the photographic film recorded a
perfectly uniform negative for exposures of 15 seconds or more. The
exposures generally lasted 75 seconds. The electrodes had to be kept sharp
and smooth, for, when this was neglected, the elementary sparks persisted
much longer in one position than in another and consequently caused streaks
of varying intensity to run along the negatives parallel to their length, as
can be seen in some of the positives reproduced in the appended plates, e. g. ,
fig. 99, plate 25.


The current for the spark was obtained in the following manner: An
alternating e. m. f. of about 106 volts (133 cycles) was impressed on the
terminals of an induction coil of unknown ratio of turns. Eight or nine
amperes commonly flowed in the primary. The interrupter of the coil was
thrown out of circuit and the coil therefore performed the functions of a
transformer. In parallel with the secondary was placed a Leyden jar about
1 8 inches high and of unmeasured capacity. No auxiliary spark was intro-
duced. The system could spark about 2.5 cm. in air between metallic

The great intensity of some of the lines characteristic of all the common
metals tried (Al, Cd, Cu, Fe, Pb, Zn, etc.) made these metals undesirable
for the present work. Cadmium and zinc were selected only because of the
strong continuous background to which they give rise. Uranium, its salts or
its earths were not used in this work because they are unmanageable.
Naturally the pure metal in air burns to oxide at once; pitchblende can not
be worked into a suitable shape (at least, for such specimens as we have
been able to obtain); and, pitchblende is so very heterogeneous that the
position of the spark can not be depended upon for an instant. To have
employed a neutral atmosphere in conjunction with a reciprocating mechanism
would have consumed, obviously, too much time and would have demanded
too complicated, cumbersome and inconvenient an assemblage of apparatus.


In order to show the variations in the absorption spectrum of a given
substance when the thickness of the absorbing layer changed linearly, a
wedge-shaped cell was constructed. Vessels made on this principle have
been designed and used often before, notably by Angstrom, Gladstone, Govi,
Gibbs, Tumlirz, Hodgkinson, F. Melde, Hartley, and others.* Neverthe-
less, because the precise form of the cell is supposedly new and certainly
useful it may not be superfluous to enter into a detailed description of it
here. This little piece of apparatus was designed so that the relative posi-
tions of the quartz surfaces through which the light entered into, and emerged
from, the absorbing liquid could be varied at will, within certain limits. In
other words, matters were so arranged that the liquid could be in the form
either of a wedge, of variable angle, with zero thickness at the refracting
edge, or of a prism of variable angle and finite depth throughout, or of a
plane-parallel layer of changeable thickness. To satisfy these conditions it
was convenient to rely upon gravitation to preserve certain parts of the cell
in mutual contact. This in turn necessitated both the horizontal position of

* See H. Kayser, " Handbuch der Spectroscopie, " v. HI, pp. 58, 59.


the bottom of the cell and (because it was desirable to reduce the number of
reflecting surfaces to a minimum) a vertical type of spectrograph.

The cell comprised five separable parts, as follows: (i) A brass frame-
work upon which the other parts rested; (2) a transparent tray, without a
lid, which confined the liquid in proper bounds; (3) a transparent boxlike
system which gave the upper surface of the liquid the desired position ; (4) a
vulcanite framework to hold the last mentioned box in place ; and (5) four
mahogany pins or pegs to fasten the box to its framework.

(i) A side view of this framework is presented in figure 2. There
were three micrometer screws, all of the same pitch, viz : i turn =. in.

T =0.053 cm - The heads

I |~ ~ i of the screws were grad-

IJ_ L uated, on their upper sur-

i v '

faces, in ten equal parts.

Fig. 2. Four-fifths natural size. The screw T was in the

medial plane of the cell while the remaining screws (T' only is shown) were at
the other end of the system, were equidistant from this plane, and were as far
apart as possible. The micrometer screws called for vertical scales on the
adjacent brass-work to count whole turns. The handle is denoted by HH.
A black fiducial mark, F, on a white ground, enabled the experimenter to
tell what position the cell occupied with reference to the length of the slit of
the spectrograph. The lower end of F moved over a scale parallel to the
slit and in the plane of the jaws of the latter. The flange at the bottom of
the framework was made of brass only 0.014 cm. thick so that the absorb-
ing medium might be as near the slit as possible.

(2) An accurately ground, plane-parallel plate of quartz 40 mm. long,
18.5 mm. wide, and 2 mm. thick had cemented to its periphery four
rectangular sheets of thin glass 8 mm. high. Hence, the greatest depth of
liquid which could be studied by the aid of this cell was 6 mm.

(3) In figure 3, a, b, c, and d designate the vertices KT ^ ^

of the section of a quartz plate, made by a plane per-
pendicular to the plane the trace of which is the line

1 3 4 5 6 7 8 9

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