experienced in copying bright green foliage, or warmly colored
portraits; a large portion of the yellow and red rays entering into the
composition of both--and the imperfections of a Daguerreotype portrait
of a person with a freckled face depends upon the same cause.

A yellow, hazy atmosphere, even when the light is very bright, will
effectually prevent any good photographic result--and in the height of
summer, with the most sensative process, it not unfrequently happens
that the most annoying failures arise from this agency of a yellow
medium. A building painted of a yellow color, which may reflect the
sun's rays directly into the operator's room will have the same effect.
Daguerreotypists, being ignorant of these facts, are very apt to charge
their want of success to the plates, or chemicals, or any thing but the
real cause; and it would be well to bear these facts constantly in mind
and as far as possible avoid them. This, may be accomplished, in a
measure, by a choice of location or by having the glass of your windows
tinged with blue; or a screen of thin blue paper may be interposed
between the light and sitter. In selecting subjects, all striking
contrasts in color should be avoided, and sitters for portraits should
be cautioned not to wear anything that may produce the effect spoken
of--dark dresses always being the best.

The action of light both combines and decomposes bodies. For instance,
chlorine and hydrogen will remain in a glass vessel without alteration
if kept in the dark; but if exposed to the rays of the sun, they
immediately enter into combination, and produce hydrochloric acid. On
the other hand, if colorless nitric acid be exposed to the sun, it
becomes yellow, then changes to red, and oxygen is liberated by the
partial decomposition effected by the solar rays.

Of the organic substances none are more readily acted upon by light
than the various combinations of silver.

Of these some are more, and others less sensitive. If Chloride of
silver, which is a white precipitate formed by adding chloride of
sodium (common salt) to a solution of nitrate of silver, be exposed to
diffused light, it speedily assumes a violet tint, and ultimately
becomes nearly black. With iodide of silver, bromide of silver,
ammonio-nitrate of silver, and other salts of this metal, the result
will be much the same.

Some bodies, which under the influence of light, undergo chemical
changes, have the power of resto