146 REPORT—1850. 
rays. For example, a Daguerreotype plate being impressed in the camera 
with its dormant, invisible image, is placed under the influence of radiations 
which are deprived of their actinism, and yet the image is slowly and steadily 
developed. On this curious question the papers of M. Claudet should also 
be consulted. M.E. Becquerel’s classification of the effects observed, which 
is as follows, is good :— 
_ Ist series. Bodies exhibiting a physical modification without any change 
in composition. 
2nd series. Elements combined under solar influence. 
8rd series. Combination destroyed in part or entirely by the influence of 
solar rays. 
If a careful examination is made of spectra chemically formed, it will 
be found that scarcely one of those impressed upon papers prepared with 
inorganic matter exhibit any influence over the space covered by the yel- 
low ray; that is, the most luminous portion of the prismatic spectrum pro- 
duces no chemical change upon them. This is only a confirmation of the 
observation of Berard previously mentioned. As the sensibility of the pho- 
tographic preparation is increased, we find the resulting chemical impression 
considerably lengthened : it is not only extended to a greater distance beyond 
the utmost extent of the luminous image, but chemical change becomes evi- 
dent more nearly up to the centre of the yellow ray,—the point of maximum 
illuminating power. In no case however has any decided effect been observed 
up to this point. I have been disposed to refer this to a power of light an- 
tagonistic to that of chemical action. But it must not be disguised, that the 
phenomenon appears to be explicable also upon some view of interference, 
although this is by no means reducible to any satisfactory condition in the 
present state of our knowledge. It has been proved by experiments with 
coloured media, which have been employed to analyse the prismatic spectrum, 
that every luminous ray may be made to protect chloride of silver from chemical 
change. ‘Thus lines of blue, yellow and red rays, with their interblending 
tints (after having been filtered by a glass stained with oxide of silver), have 
been thrown upon highly sensitive photographic papers, which have been at 
the same time under the influence of diffused light; and it has been found, 
that although every part of the paper, except that portion covered by the spec- 
trum, has been deeply darkened, the whole of this line has been protected 
and preserved perfectly white*. We have usually been accustomed to speak 
of the chemical agency of the solar radiations, as belonging in their varieties 
to some particular coloured ray. Thus the yellow ray has been regarded as 
the least chemical, and the blue as the mast energetically so. Evidence how- 
ever has been afforded to show that the blue ray may be deprived of its 
chemical power, and we shall presently see that some forms of chemical 
change are in a peculiar manner determined by the rays emanating from the 
yellow band. Therefore, without in any way interfering with any theory of 
luminous action, we can no longer regard the colour of a particular ray as 
an indication of its power to produce chemical change. Colour is a peculiar 
function of light, not directly connected with any chemical phenomena. 
It becomes important to ascertain the effects of transparent media on these 
chemical radiations. It was shown by Malagutti that certain colourless 
transparent media possessed a power, in virtue of which the chemical action 
of the rays permeating them was very frequently exalted}. This subject has 
also been investigated by M. Biot and M. Edmund Becquerel, who have 
* British Association Reports, 1848, Swansea. Lecture by R. Hunt, Royal Institution. 
Atheneum, 1849, No. 1122, p. 438. 
+ Annal, de Chimie, vol. ]xxii, 5, 
