NERGIA.

THE NATURE of Light is not wholly known, but it is generally believed
to be matter, as in its motions it obeys the laws regulating matter.
So closely is it connected with heat and electricity that there can be
little doubt of their all being but different modifications of the same
substance. I will not, however, enter into a statement of the various
theories of 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.

A medium is any space through which light passes.

Incident rays are those which fall upon the surface of a body.

Reflected rays are those which are thrown off from a body.

Parallel rays are such as proceed equally distant from each other
through their whole course.

Converging rays are such as approach and tend to unite at any one
point, as at b. Fig. 3.

Diverging rays are those which continue to recede from each other, as
at e. Fig. 3.

A Focus is that point at which converging rays meet.

MOTION OF LIGHT--Rays of light are thrown off from luminous bodies in
every direction, but always in straight lines, which cross each other
at every point; but the particles of which each ray consists are so
minute that the rays do not appear to be impeded by each other. A ray
of light pas