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 voluminous to include in
a volume of the size to which I am obliged to confine 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. Beyond this a protecting influence is
powerfully exerted; and notwithstanding the action of the dispersed
light, which is very evident over the plate, a line is left, perfectly
free from mercurial vapor, and which, consequently, when viewed by a
side light, appears quite dark. The green rays are represented by a
line of a corresponding tint, considerably less in size than the
luminous green rays. The yellow rays appear to be without action, or
to act negatively, the space upon which they fall being protected from
the mercurial vapor; and it consequently is seen as a dark band. A
white line of vapor marks the place of the orange rays. The red rays
effect the sensitive surface in a peculiar manner; and we have the
mercurial vapor, assuming a molecular arrangement which gives to it a
fine rose hue; this tint is surrounded by a line of white vapor, shaded
at the lowest extremity with a very soft