CHEMICAL ACTION OF THE SOLAR RADIATIONS. 143 
we should regard the spectrum, from the extreme red ray at one end of the 
spectrum to the lavender or gray ray at the other*, as representing the extent 
of luminous power, which has its maximum in the yellow ray and its minima 
at the outer limits of the extreme red and the lavender rays, since we have not 
been enabled to trace any luminous effects beyond these points. It is conve- 
nient to employ these coloured rays to mark points of action and of inaction ; 
but it appears important that we should dispossess our minds of the idea that 
chemical change, or the contrary, takes place by virtue of any function of a 
particular coloured ray. 
It had been already noticed by Berard, that the mean luminous rays, even 
when condensed by a lens, produced no chemical change on chloride of silver, 
Sir John Herschel was the first to observe that the rays at the red end of the 
spectrum protected chloride of silver from that change which is induced even 
beyond the spectrum by the diffused light which always accompanies the pris- 
matic image; that whereas papers prepared for photographic purposes were 
darkened more or less over every other part; over the space ‘‘ on which the 
full red of the spectrum had fallen, there was an appearance of whiteness, a 
sort of white prolongation of, or appendage to, the dark photographic impres- 
sion.” It is thus proved that the red end of the spectrum is not inactive; it 
is not, that any sort of polarity exists in the spectrum: we have a positive 
evidence, an action as energetic as that of the blue end of the spectrum, but 
exerted in an opposite direction. If paper is blackened by exposure to the 
_violet end of the spectrum, or by the influence of diffused light, and subse- 
quently exposed to the action of the red rays, it becomes of “‘a full and fiery 
red” over the entire space upon which those rays fall. A similar result, but 
not so decided, is produced by the radiations which permeate a ruby glass 
coloured with oxide of gold. 
It has lately been shown by M. Claudet that on the Daguerreotype plate 
these red rays restored the sensibility of the iodized silver after it had been 
acted upon by the more refrangible chemical rayst. The powerful action of 
the red end of the spectrum was further proved by Sir John Herschel, who 
employed two prisms and threw the red rays of one spectrum upon the 
violet rays of another. ‘‘ The blackening power of the more refrangible rays 
“ΠΝ It is important that the condition of the prismatic spectrum should be distinctly com- 
prehended : for a complete examination of the subject I must refer to the Transactions of the 
Royal Society of Edinburgh for 1822; and to Sir John Herschel’s Treatise on Light, Ency- 
clopedia Metropolitana; Sir David Brewster’s Optics, Lardner’s Cyclopedia; and numerous 
memoirs in the Transactions of the Royal Society of Edinburgh. To state in brief the facts 
as presented to us, upon examining the Newtonian spectrum of seven colours—red, orange, 
yellow, green, blue, indigo and violet,—we must at once perceive that those seven are com- 
. pounds of but three colours—red, blue and yellow. By the combination of these all the 
others can be produced, but not by any combination of the other rays. By examining the 
spectrum with coloured media we can detect extensions of these primary colours, and fair 
eyidence is afforded that these colorific bands overlap each other. Now, if we look at the 
spectrum through ἃ cobalt blue glass, a colour unseen by the naked eye becomes visible, the 
extreme red ray. This is evidently the result of a mixture of blue and red. By throwing 
the spectrum upon turmeric paper a prolongation of the luminous portion beyond the violet 
is seen, to which Sir John Herschel gave the name of the lavender ray. Thus nine instead 
of seven bands present themselves. Now, if we examine all the conditions of these colours, 
we shall find that the yellow ray blends with the blue, and produces green ; then, that the blue 
becomes more and more decided, passing on to blackness in the indigo; but that red reap- 
pearing at that, the most refrangible end, produces by mixing with blue ἐλ violet, and yellow 
blending with the violet produces the neutral davender or gray ray. On the other side, yellow 
mixing with red produces orange, and then the red growing in intensity and purity, again 
blends with blue at the least refrangible end of the spectrum to produce the crimson ray or 
extreme red. 
T Philosophical Magazine, 1850. 
