om--Proportions of Light, Heat, and Actinism composing a
Sunbeam--Refraction--Reflection--Lenses--Copying Spherical
Aberration--Chromatic Aberration.

It is advisable that persons engaging in the Daguerreotype art should
have at least a little knowledge of the general principles of light and
optics. It is not the author's design here to give a full treatise on
these subjects, but he only briefly refers to the matter, giving a few
facts.

It has been well observed by an able writer, that it is impossible to
trace the path of a sunbeam through our atmosphere without feeling a
desire to know its nature, by what power it traverses the immensity of
space, and the various modifications it undergoes at the surfaces and
interior of terrestrial substances.

Light is white and colorless, as long as it does not come in contact
with matter. When in apposition with any body, it suffers variable
degrees of decomposition, resulting in color, as by reflection,
dispersion, refraction, and unequal absorption.

To Sir I. Newton the world is indebted for proving the compound nature
of a ray of white light emitted from the sun. The object of this work
is not to engage in an extended theory upon the subject of light, but
to recur only to some points of more particular interest to the
photographic operator.

The decomposition of a beam of light can be noticed by exposing it to a
prism. If, in a dark room, a beam of light be admitted through a small
hole in a shutter, it will form a white round spot upon the place where
it falls. If a triangular prism of glass be placed on the inside of
the dark room, so that the beam of light falls upon it, it no longer
has the same direction, nor does it form a round spot, but an oblong
painted image of seven colors--red, orange, yellow, green, blue,
indigo, and violet. This is called the solar spectrum, and will be
readily understood by reference to the accompanying diagram, Fig. 1.

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To those who are unacquainted with the theory of light (and for their
benefit this chapter is given), it may be a matter of wonder how a beam
of light can be divided.

[Illustration: Fig. 1 (amdg_1.gif)]

This can be understood when I say, that white light is a bundle of
colored rays united together, and when so incorporated, they are
colorless; but in passing through the prism the bond of union is
severed, and the colored rays come out singly and separately, because
each ray has a certain amount of r