CHAPTER IV.
Light--Optics--Solar Spectrum--Decomposition of Light--Light, Heat, and Actinism--Blue Paper and Color for the Walls of the Operating Room--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.