CHEMICAL ACTION OF THE SOLAR RADIATIONS. » 145 
allowed to continue its action, there was observed to come on suddenly a new 
and much more intense impression of darkness confined in length to the blue 
and violet rays, and, what is most remarkable, confined in breadth to the mid- 
dle of the sun’s image, so far at least as to leave a border of a lead-coloured 
spectrum, traceable not only round the clear and well-defined convexity of the 
dark interior spectrum, at the least refrangible end, but also laterally along 
both its edges.” 
At the same time, ignorant of these refined researches of Sir John Herschel, 
I observed similar results upon a Daguerreotype plate: the record of these 
observations will be found in the Philosophical Magazine (vol. xvi. 3rd Series, 
p. 267), the same number containing the abstract of the Memoir of Sir John 
Herschel, read before the Royal Society, which first made me acquainted with 
his observations. It was most distinctly stated that there was “ἃ real differ- 
ence between the chemical agencies of those rays which issue from the cen- 
tral portion of the sun’s disc, and those which emanating from its borders have 
undergone the absorptive action of a much greater depth of its atmosphere.” 
Therefore the first observation cf this is not due to M. Arago, who has only 
very recently noticed the fact in his ‘ Memoirs on Photometry.’ It must not, 
however, from any evidence yet afforded us, be supposed that the peculiar 
protecting influence of the extreme red ray, and the simular influences of the 
lateral edges of the spectrum, are of precisely the same order. It would rather 
appear that the least refrangible rays have a function arising from a combina- 
tion of chemical and calorific power which is distinct from anything exhibited 
by the other radiations. 
We have now to consider the remarkable fact, that nearly all bodies sus- 
ceptible of receiving any impression from the ordinary red rays assume more 
or less a red colour, This was noticed very early by Daguerre and Talbot, 
and it has been confirmed by every subsequent experimentalist. The cause of 
this production of colour is not very evident; but we must regard it as due 
to the new molecular arrangement produced by the chemical changes effected 
by these radiations. From time to time we hear of the productions of co- 
loured images of prismatic spectra, and lately M. Edmund Becquerel has 
created some sensation by exhibiting such images, and also copies of highly 
coloured drawings. 
This is not a novelty in photographic phenomena. Herschel, in 1839, 
obtained a coloured spectrum upon a paper prepared with two washes of a 
solution of nitrate of silver and a wash of muriate of soda applied between each. 
This was described as ““ coloured with sombre, but unequivocal tints imitating 
those of the spectrum itself.” In the same year I found that papers prepared 
~ with muriate of barytes and nitrate of silver, would, after having been allowed 
to darken, if placed under different coloured media, assume, to a certain extent, 
the eclours of the rays permeating them. ‘“‘ After a week’s exposure to dif- 
fused light, it became bright red under the red glass, a dirty yellow under the 
yellow, a dark green under the green, and a light olive under the blue*.” 
Again, in 1844, I was fortunate enough to obtain very decided evidences of 
feet upon papers prepared with the fluoride of potassium and nitrate of 
silver f. 
M. Edmund Becquerel, investigating the conditions of the spectrum with 
particular reference to its influences on the Daguerreotype plate, was led to 
_ regard the spectrum as consisting of two remarkable divisions, which he calls 
rayons excitatewrs and rayons continuateurs ; the least refrangible rays being 
supposed to continue the action set up by the chemical or most refrangible 
* Philosophical Transactions for 1840, pt. }, p. 43, + Researches on Light, p. 106. 
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