These rings must be regarded as films of the ioduret of silver,
varying, not only in thickness, but in the more or less perfect
states of combination in which the iodine and metal are.
The exterior circle is an ioduret in a very loose state
of chemical agregation; the attractive forces increase as we
proceed towards the centre, where a well formed ioduret,
or probably a true iodide of silver, is formed, which is acted
upon by sunlight with difficulty. The exterior and most
sensitive film constitutes the surface of Daguerreotype plates.
The changes which these colored rings undergo are remarkable;
by a few minutes exposure to sunlight, an inversion of nearly
all the colors takes place, the two first rings becoming a deep
olive green; and a deep blue inclining to black.

The nature of the change which the ioduret of silver undergoes
on Daguerreotype plates, through the action of light,
Mr. Hunt considers to be a decided case of decomposition,
and cites several circumstances in proof of his position.
These with other facts given by Mr. Hunt in his great work
on the Photographic art, but to volumnious to include in a
volume of the size to which I am obliged to cofine myself,
should be thoroughly studied by all Daguerreotypists.

PRISMATIC ANALYSIS.--The most refrangible portion of the spectrum,
(on a Daguerreotype plate) appears, after the plate has been
exposed to the vapor of mercury, to have impressed its colors;
the light and delicate film of mercury, which covers that portion,
assuming a fine blue tint about the central parts, which are
gradually shaded off into a pale grey; and this is again surrounded
by a very delicate rose hue, which is lost in a band of pure white.