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Suppose we wish to copy by the Daguerreotype, or Calotype process,
any objects highly colored--blue, red and yellow, for instance predominating--
the last of course reflects the most light, the blue the least;
but the rays from the blue surface will make the most intense impression,
whilst the red radiations are working very slowly, and the yellow
remains entirely inactive. This accounts for the difficulty 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 anoying 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--