below that at which the paper chars, the salt is decomposed.
Where the heat is greatest, the silver is revived,
and immediately around it, the paper becomes a deep blue;
beyond this a pretty decided green color results,
and beyond the green, a yellow or yellow brown stain is made.
This exhibits a remarkable analogy between heat and light,--
before spoken of in chap. II--and is of some practical
importance in the preparation of the paper.

PRISMATIC ANALYSIS.--The method of accomplishing the prismatic decomposition
of rays of light by the spectrum has already been described on pages 22
and 23. The color of the impressed spectrum, on paper washed with nitrate
of silver, is at first, a pale brown, which passes slowly into a deeper shade;
that portion corresponding with the blue rays becoming a blue brown;
and under the violet of a peculiar pinkey shade, a very decided green tint,
on the point which corresponds with the least refrangible blue rays,
may be observed, its limits of action being near the centre of the yellow ray,
and its maximum about the centre of the blue, although the action up to
the edge of the violet ray is continued with very little diminution of effect;
beyond this point the action is very feeble.

When the spectrum is made to act on paper which has been
previously darkened, by exposure to sunshine under cupro-sulphate
of ammonia, the phenomena are materially different.
The photographic spectrum is lengthened out on the red or negative
side by a faint but very visible red portion, which extends
fully up to the end of the red rays, as seen by the naked eye.
The tint of the general spectrum, too, instead of brown is
dark grey, passing, however, at its most refracted or positive
end into a ruddy brown.