The ferrotype and how to make it - The Grian Press

POSITIVE PHOTOGRAPHY, 19
ploy agents still more sensible, he would have
observed analogous effects. To show plainly the
great disproportion which exists in this respect
between the. energies of different colored rays, M.
Berard, concentrated by means of a lens, all that
part of a spectrum which extends from the green
to the extreme violet; he also concentrated by
another lens all that portion which extended
from the green to the extremity of the red ; this
last pencil formed a white so brilliant that the
eyes were scarcely able to endure it, yet the ni-
trate of silver remained exposed more than two
hours to this brilliant point of light without un-
dergoing any sensible alteration. On the other
hand, when exposed to the other rays, which were
much less bright and less hot, it was blackened in
less than six minutes. After some further re-
marks on the importance of M. Berard's experi-
ments they proceed as follows :
'* If we consider
solar light as composed of three distinct sub-
stances, one of which occasions light, the other
heat, and the third chemical combinations, it will
follow that each of these substances is separable
by the prism into the infinity of different modi-
fications like light itself; since we find by ex-
periment that each of these properties is s})read,
though unequally, over a certain extent of the
spectrum, and we must suppose on that hypothe-
sis, that there exists three spectrums, one above
the other ; namely, a Calorific, a Colorific, and a
Chemical Spectrum. We must likewise admit