Philosophers on this head, but content myself with that of Sir Isaac Newton;
who supposed rays of light to consist of minute particles of matter,
which are constantly emanating from luminous bodies and cause vision,
as odoriferous particles, proceeding from certain bodies, cause smelling.

The effects of light upon other bodies, and how light is effected
by them, involve some of the most important principles, which if
properly understood by Daguerreotypists would enable them to improve
and correct many of the practical operations in their art.
These effects we shall exhibit in this and the following chapters.
Before we enter on this subject it will be necessary to become
familiar with the

DEFINITIONS of some of the terms used in the science of optics.

Luminous bodies are of two kinds; those which shine by their own light,
and those which shine by reflected light.

Transparent bodies are such as permit rays of light to pass through them.

Translucent bodies permit light to pass faintly, but without
representing the figure of objects seen through them.

Opaque bodies permit no light to pass through them, but reflect light.

A ray is a line of light.

A beam is a collection of parallel rays.

A pencil is a collection of converging, or diverging rays.