Dec. 15, 1881} 
NATURE 
165: 
low these the lines in the red A and Bin the red are shown; but 
still lower down there are three lines delineated, which he calls 
a, B, y. NRecollect the way he proceeded. He exposed the 
daguerrotype to light, and he allowed the spectrum to fall upon 
it. What was the meaning of that? The meaning of that was 
he altered the iodide of silver to begin with, I have here a piece 
of iodide of silver paper. One half has been exposed to light, 
and the other has not been exposed to light at all, and you will 
see the difference in colour. One hasa greenish brown tint, and 
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the other a decided yellow. The yellow you saw cut off the 
blue ; the brown tint, if you put that in the spectrum, would allow 
the yellow to pass, but would cut off not only the blue but also 
some of the redtoo, How then do we explain this action? It is 
true he found that these lines, A, B, a, 8 and y, had impressed 
themselves, but he found they were what we call reversed lines, 
that is to say, what ought to be black before were white, 
and what ought to be white before were black. How can this 
be explained? The spectrum was the same, the iodide of silver 
HK on 
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Fic. 3- 
plate was the same. All was the same except the previous ex- 
posure to light. Now, this remained unexplained for a long 
time. It was supposed there was a certain antagonism between 
the rays, that is to say, that the red rays were able to undo the 
work which the blue had done, and that the yellow remained 
neutral, But was this an explanation? ‘There can be no such 
thing as antagonism of energy in this matter, and therefore it 
required some further explanation, 
In investigating the subject it fell to my lot to try experi- 
ments on the constitution of the photographic image, and my 
experiments led me to find that what was supposed to be 
antagonism of rays in the red part of the spectrum to those of 
the blue was nothing else but another chemical action which was 
called into play. 
I have here a negative of a line drawing. I place a collodion 
plate containing iodide of silver in contact with it, but before I 
develop it I will place half of it in a solution of peroxide of 
hydrogen, which is an oxidising solution, and on developing I find 
only half the image has appeared, viz. the part which was not 
placed in the oxygen peroxide ; to adopt the old explanation, there 
is anantagonism of the peroxide of hydrogen to the action of light! 
This gave a clue to Dr. Draper’s reversal of the image which 
he got in the red rays of the spectrum. I will show you what 
it occurred to me did happen. Dr. Draper’s plate after it had 
been exposed to light contained molecules of sub-iodide of silver, 
and when he allowed the speetrum to play upon it the sub-iodide 
of silver was anxious to obtain anything it could to satisfy itself, 
and so took up oxygen from the air and formed an oxy-iodide of 
silyer, Ag"I, combined with oxygen, and that oxide of silver was 
totally incapable of development. Why? Simply because its 
energy of attraction was, satisfied ; there was nothing to attract 
the mercury c ndensed from the mercury vapour by which his 
image was developed. This then might be an explanation of 
Draper’s photograph which should be capable of proof, and in 
order to prove it I will show the way I proceeded. I willtakea 
plate of iodide of silver and expose it to light before you ; then 
I propose to immerse it in a cell containing an oxidising solution 
of very delicate peroxide of hydrogen, which, as we know, is a very 
strong oxidiser, in fact one which will give up oxygen very freely to 
anything brought in contact with it. I have an arrangement by 
which I can do that, consisting of a dark slide in which I can 
put the cell and the silver plate. eve you see the prepared 
plate in its normal state. The electric light is allowed to act 
upon it, and of course the iodide of silver will be reduced to sub- 
iodide. I shall next allow the spectrum to play on the plate 
whilst immersed in this oxidising solution, and see if we cannot 
get the same results that Draper did in his reversed spectrum, If 
the theory that I give of the production of Draper’s oxidising 
photograph is correct, then the red light ought to aid the oxida- 
tion of the photograph, or rather of the subiodide, forming that 
oxyiodide of silver which I mentioned to you. I will leave the 
bichromate cell in front of the slit in order that you may see the 
blue rays have nothing at all to do with the matter. I will give 
rather a longer exposure than I did before, and in that way I 
think I shall be able to get a result. I will give it to my 
assistant to develop, and then throw it on the screen as soon as 
he has finished with it. Now what shall we expect to find in 
this photograph? If the red rays help the oxidation of the 
iodide we shall expect to find that where the red rays are active 
no development whatever will have taken place, and as a fact 
that is really what we find. This method of Draper’s, of photo- 
graphing the ultra-red part of the spectrum, is exceedingly in- 
convenient, as it requires a long exposure, and, I may say, is 
unsatisfactory, because it gives very bad definition. You have 
to use a very open slit in order to get it. 
Fic. 4. 
T now throw onthe screen a portion of the so/ar spectrum taken 
in the way I have just carried out before you. In it you will see 
a facsimile almost of Draper’s photograph. The question arises, 
what would happen suppose the plate was immersed in a de- 
oxidising solution such as potassium nitrite? We have the 
answer at hand in the shape of a photograph so exposed. The 
reversal, you see, is entirely absent. 
Now the question comes, Is it possible to show the existence 
of the ultra-red rays of the spectrum by visible means? in other 
words, by the exposure of any surface to the light? I think I 
can show you that it is possible by Balmain’s phosphorescent 
paint. If I expose this to the light of the electric lamp we 
shall find that of course it will become very luminous indeed. 
Now when I expose this luminous surface to the action of the 
spectrum something ought to happen, perhaps which will 
give us an idea of Draper's photographs. I will try, and then 
I must pass round this luminous plate, because you will not 
be able to see it at a distance. I bring the light to a focus 
on the slit of the spectroscope, and place the surface of the 
paint, which is still luminous, in the spectrum for a short period, 
and now I will pass it round, and you will be able to see the 
phenomena ; first of all a bright patch, and then a black patch 
beyond, The bright patch is caused by the blue rays; the black 
patch beyond is caused by the ultra-red rays, the red rays and 
the yellow rays ; in other words, these rays have the property of 
quenching the vibrations of the phosphorescent particles, so that 
you see we have a means of showing visibly the existence of the 
ultra-red rays of the spectrum. 
Our knowledge of the value of the photography of the spec- 
trum, as regards its most refrangible portion, was very limited 
indeed until Mr. Lockyer took up the subject of spectrum photo- 
graphy with earnestness. In the year 1872 at Chatham we 
also began our researches in this matter, and we hoped that 
what we had found to be so immensely valuable in the violet 
and the blue regions of the spectrum—we might also be able 
to accomplish for the red and ultra-red rays of the spectrum 
as well. In the year 1872 Vogel made an important announce- 
ment, which, if it had proved everything one could haye wished, 
would have left no need for further experimentation. He said 
if you took a bromide of silver plate or iodide of silver plate 
