_ _ reconverted into the red bromide. 
188 
NATURE 
[ Dec. 22, 1881 
nitrate of silver together with nitric acid, particular particles of 
bromide of silver are dissolved by the nitrate of silver, and 
then are re-deposited, built up, as it were, into bigger and bigger 
molecules, until finally we find we have a bromide of silver 
which literally is green when placed before the lantern, These 
two plates are respectively coated with the two kinds of 
bromide ; first we have the ordinary bromide; and second, the 
bromide modified in molecular structure in the way I have 
described. The light from the lantern traverses the two films 
placed side by side, and you will see that they are eminently dif- 
- ferent in every way : the one being of an orange tint, absorbs the 
blue rays, the other being of a greenish-blue tint, absorbs the red. 
Now to show you that those two states are identical as far as the 
chemical composition of the molecules is concerned, I will take 
the green bromide of silver film which I had just now, and rub it 
with my finger; you will find that the blue bromide is once more 
It has been scratched a little 
8! 
SS 
' 
C@@AZX® 
Prismatic Solar Spectrum. 
Measured from a Fholograph . 
Fic. 7.— Diagram of the least refrangible portion of the solar spectrum. 
in the rubbing, and you will see, where the scratching has taken 
place, that it is of a fine orange tint. Here then we have a solution 
of the problem, the production of a bromide which will absorb 
in the red as well as one which can absorb in the blue, which is 
the ordinary state. 
Now no sooner was this bromide obtained than, of course, 
there was great interest displayed to test its effect as regards its 
being a photographic salt. To do this we naturally appealed to 
the spectrum. But here I have sometbing which will be perhaps 
more efficacious in showing you what that salt can do than any- 
thing else. This is a photograph of the poles of an electric 
light taken through this sheet of black ebonite. It is perfectly 
opaque to all visible radiation, but through it those rays which 
are below the red can penetrate to a certain extent. You see 
then we are able to photograph with the dark rays, absolutely 
without using the visible rays at all. Another example of tbis 
might probably be interesting for you to see, and that is this: it 
is an experiment I carried out to-day, so that it is fresh in my 
memory. Here I have a card pierced with a few: holes 
That card was taken and laid very nearly, but not quite, in 
contact with this blue bromide film, and over it was placed a 
blackened kettle of boiling water, If those dark rays had any 
effect on the blue bromide, the radiations from the kettle of 
boiling water ought to alter the salt. Let us see whether 
it did so. The photograph is rough, but still I daresay it is 
specific enough to show you the result. You will see that the 
images of these holes are exactly reproduced, and the source of 
illumination, if it may be so called, was the kettle of boiling 
water, the radiations of which sufficed to cause an alteration in 
the silver salt. I have been able once—I have not tried to repeat 
the experiment—to photograph a kettle of boiling water by its 
own radiation ; that is to say, it became a source of light. 
We will next appeal to the spectrum to see whether it is 
sensitive to all the radiations, and I think you will find that it will 
answer our expectations to the highest degree. I have on the 
screen the first photograph of the prismatic spectrum 
which was taken with this salt, You will be able to 
note the position of the spectrum with regard to the 
8 blue, the green, the yellow, the red. Below the im- 
pression made by the latter we have the famous A 
line, and below this again we have an impression made 
by the infra-red rays. What we next attempted was 
of course to get better photographs than the one I 
have already shown you; and next to draw a map of 
the prismatic spectrum. 
In the following diagram we have the results of 
the measurements of these photographs. You see 
to what an enormous extent the solar spectrum ex- 
tends below the limit of the visible spectram—the A 
line is seen with great difficulty in the spectroscope 
(Fig. 7). The last band in the photograph that I last 
showed you was the band marked 7, but below that 
there are other bands which I was subsequently able 
to obtain. It is very rarely that tkese bands can 
be photographed at all, not because the plate is 
not sensitive to those radiations, but simply because 
of the atmospheric absorption which cuts off these 
particular radiations and prevents them from reaching 
our earth. I may say that the theoretical limit of 
the prismatic spectrum is very nearly reached here— 
not quite, but nearly. Cauchet showed that if you 
set up along the length of the spectrum, as we have 
it here, the inverse square of the wave-length of any two 
lines, say the inverse square of the wave-length of the B 
line, and erected a perjendicular line of a length repre- 
senting that particular number, and also of the wave- 
length of the F line sayin the same way, and then joined 
of the points thus obtained, we should geta line on which 
the inverse squares of the wave-lengths of HGCD 
would lie, and also theoretically the wave-lengths of 
lines below the red. Thus if we took and joined two 
points, all the other inverse squares of the wave-lengths 
would lie along that line, very nearly. In that way 
a theoretical limit of the prismatic spectrum can be 
obtained ; in other words, the prismatic spectrum 
must stop where the wave-length is infinity. You 
will see that in this diagram we very nearly reach 
the theoretical limit. Where there is no atmosphere 
to interfere with the radiation, it would be easy to 
reach it. Since the spectrum we photographed is 
the solar spectrum, between the slit and the source of radiation 
many miles of atmosphere with more or less aqueous vapour 
intervene, which prevent us obtaining the limit; but with the 
electric light the absolute limit can be reached on some 
occasions, though with some difficulty. It may be asked 
if we can assume that there is a practical as well as theoretical 
limit of the prismatic spectrum; and in answer to this I may say 
that the measurements made from other photographs, to which 
reference will be made, will demonstrate that one is fully justified 
in adopting the theory. 
The disadvantage of using the prismatic spectrum for mea- 
surement is this: you will notice the waves are very much 
compressed as you get down towards the red. The ordinates to 
the curve (Fig. 7) represent the wave-length ; andif you calculate 
the wave-lengths from the inverse squares given by the nearly 
straight line it forms a curve like the above. 
Now owing to the compression of the ultra-red it was very 
difficult to decipher the full meaning of the impressions obtained 
10000 
6090 
