166 
and covered it with a dye, that in the spectrum, where the 
dye absorbed, there a photographic action, although beyond 
the usual boundary of photographic action, would be seen. 
We followed up this very carefully, hoping to find some dye by 
which we might be able to photograph the ultra-red rays of 
the spectrum. Had I known as much then as I do now I should 
not have followed any such chimerical idea. But what Vogel 
stated was perfectly correct, viz. that in that regions of the 
spectrum which certain dyes absorbed, a photographic action 
would take’ place. Suppose I take a plate prepared with some 
silver salt, and flow over it a dye, and then expose it to the spec- 
trum ; I find where the dye absorbed there a photographic image 
was formed, What was the meaning of this? This required in- 
yestigation as well, The first dye that was taken up was that of 
cyanin blue. I have here a plate covered with cyanin blue, and 
when this plate was placed in the spectrum it was found that it 
bleached in the yellow, No. If. Fig. 5. Now what was the 
meaning of that bleaching in the yellow? Let us consult the 
absorption spectrum of cyanin blue, to see whether it absorbs in 
that particular part of the spectrum; for if it absorbs in that 
particular part there work must be done as I have already shown 
you. I will throw the spectrum on the screen, and then intro- 
duce a solution of cyanin blue in front of the slit of the lantern, 
No. I. Fig. 5. We do this, and it will be seen that there is 
great absorption in the yellow, so that that particular portion 
of spectrum bleached the cyanin blue which the cyanin blue 
cut off. So that work and absorption went hand in hand : 
when the action was investigated more closely, it was found 
I VIOLET. 
Fic. 5.—I., absorption spectrum of cyanin blue; IT., bleaching effect of 
spectrum on paper stained with cyanin blue; III., bleaching effect of 
spectrum on a silver bromide film stained with cyanin blue ; IV., photo- 
graphic impression of spectrum prepared asin III, The shaded parts 
show metallic silver on development. 
that the work performed was an oxidation ; in other words, 
that bleaching took place through the effect of oxidation. So 
much for cyanin blue, fer se; when, however, we took a plate 
cevered with bromo-iodide of silver in collodion, and dyed 
with cyanin blue, exposing it for some time to the spectrum, 
it was found not only that the cyanin was bleached in the 
yellow, but it was also bleached in the blue, No. III. Fig. 5. 
It should be remembered that the only factor of difference was 
in the first case we had cyanin blue, in the collodion by itself, 
and in this last the dye and iodide and bromide of silver. What 
then was the explanation of this? That required a further 
investigation, and I think, perhaps, I shall be able to show you 
what really did happen. I will take that same cell of cyanin 
blue used before, and place it in front of the slit of the lantern, 
and we have the cyanin absorption spectrum on the screen. 
Now I have told you that when bromide of silver or iodide of 
silver is exposed to light, one atom of iodine or bromine is given 
off, and if the exposure be prolonged the amount is measurable, 
therefore it is possible that the bleaching action in the blue might 
be due to the action of bromine, and if so, bromine ought to be 
able to bleach cyanin blue. If we take bromine water and drop 
it into the cell, I think you will find ‘that the whole spec- 
tram will appear again in its usual brilliancy; we drop the 
bromine water in, and the whole spectrum does appear on the 
screen, Our question then is answered. The bromine liberated 
from the bromide of silver by the action of light when the dyed 
film was placed in the spectrum, was able to bleach it in the 
blue part of the spectrum in the same way that the oxygen in 
the air was able to bleach it under the influence of the yellow 
riys. 
Now I will show you what the action of oxidised matter on 
silver is. Here we have a glass plate on which was written 
“May 25th”’ with an oxidised solution of albumen. This was 
coated with a collodion film containing bromide and iodide of 
NATURE 
[Dec. 15, 1881 
silver, and developed in the usual way. You will see that where 
the oxidisable matter is placed, there we have a deposition of 
silver upon those particular portions. Apply this to the spec- 
trum developed ona plate stained with cyanin blue; where it 
is bleached in the yellow, the oxidised dye will cause a deposit 
of silver to be formed,! whilst where the blue rays have acted 
we shall have a deposition of silver due to ordinary develop- 
ment, as already explained. I throw upon the screen a spectruin 
showing this. The film of collodion containing the silver salts 
was dyed, and then the bromide of silver dissolved away. You see 
we have a bleaching in the yellow and also a bleaching in the 
blue, one being due to the oxidising action of the yellow rays 
on the plate, the other due to the action of the bromine upon the 
dye itself, Next I will show you a photograph (No. IV. Fig. 5) 
of the spectrum taken on such a dyed plate. We have the part 
impressed by the blue rays, and a deposition of silver as before, 
and alo we have the yellow where there is another strong depo- 
sition of silver, For convenience’ sake I have photographed the 
absorption spectrum of cyanine and placed it below the spectrum 
photographed on the silver stained with cyanin, You will thus 
see that the band in the yellow impressed on the latter plate 
corresponds exactly with the absorption of the cyanin blue 
itself, 
Carrying the investigation a little further, it was found that the 
same took place with eosin. I have an eosin solution here, and 
here is an absorption-spectrum of eosin which cuts off a great 
deal of the green—we have the yellow, but the green is cut out 
ang the blue is damped. The green is the principal portion 
which is absorbed ; in other words, the work which has to be 
done on the dye will be done in that part of the spectrum. 
In the photograph of the spectrum of eosin taken with bromide 
of silver dyed with eosin, you see asa result that we have the 
plate impressed by the blue rays, and also the plate impressed 
by the green, The deposition of silver on the two parts is due 
to difterent causes: that in the green is due to the work done 
on the dye; the work was not done on the silver directly, but 
on the dye first. That on the blue was due to the work done 
on the silver bromide itself. I may say that all dyes which I 
have found useful in the photographic sense are what we call 
fugitive dyes ; in other words, dyes which fade in the light. 
Ladies are perfectly well acquainted with the fact that some dyes 
will not stand well ; those which fade most rapidly give the best 
results in spectrum photography. 
(Zo be continued.) 
UNIVERSITY AND EDUCATIONAL 
INTELLIGENCE 
CAMBRIDGE.—The Clothworkers’ Exhibition for proficiency 
in Physical Science, tenable for three years by an unattached 
student at Oxford or Cambridge, has been awarded to J. Davies. 
The next Clothworkers’ Exhibition will be awarded for Physical 
Science by means of the examination for certificates to be held 
next July by the Oxford and Cambridge Schools Examination 
Board. Candidates must be non-collegiate students of one 
term’s standing at Oxford or Cambridge, or non-residents who 
are prepared to enter as such. 
The oral and practical examinations in the second part of the 
Natural Sciences Tripos concluded on Monday last (12th). 
Prof. Stuart has now thirty-eight pupils in mechanism and 
engineering, and more space and new machinery are needed to 
meet their growing requirements. A new room mea-uring 
thirty-six by twenty-five feet is asked for, with motive power, a 
heavy lathe, a slotting machine and larger forge. Messrs. 
Greenwood and Batley will present a slotting machine when 
there is a place to put itin. The building will cost 2257. 
Mr. D. MacAlister, Fellow and Medical Lecturer of St. John’s 
College, will lecture on Methods of Physical Diagnosis three 
times a week next term, beginning February 2. 
SOCIETIES AND ACADEMIES 
LONDON 
Linnean Society, December 3.—Sir John Lubbock, Bart., 
president, in the chair.—Mr. J. Harris Stone exhibited specimens 
of the dried plant and made remarks on Lychmis viscaria as a 
1 It has been objected by Dr. Vogel that the bleaching action requires 
time to effect it, and that the phenomenon is visible after a short exposure. 
The simple answer to the question is, When does the bleaching commence ? 
The merest trace of reduced dye would act as a nucleus for development, as 
does the merest immeasurable trace of subiodide or bromide. 
