sing through an aperture into a dark room, proceeds in a
straight line; a fact of which any one may be convinced by going into a
darkened room and admitting light only through a small aperture.

[Illustration: Fig. 1 (hipho_1.gif)]

Light also moves with great velocity, but becomes fainter as it recedes
from the source from which it eminates; in other words, diverging rays
of light diminish in intensity as the square of the distance increases.
For instance let a fig. 1, represent the luminous body from which light
proceeds, and suppose three square boards, b. c. d. severally one,
four and sixteen square inches in size be placed; b one foot, c two
feet, and d four feet from a, it will be perceived that the smallest
board b will throw c into shadow; that is, obstruct all rays of light
that would otherwise fall on c, and if b were removed c would in like
manner hide the light from d--Now, if b recieve as much light as would
fall on c whose surface is four times as large, the light must be four
times as powerful and sixteen times as powerful as that which would
fall on the second and third boards, because the same quantity of light
is diffused over a space four and sixteen times greater. These same
rays may be collected and their intensity again increased.

Rays of light are reflected from one surface to another; Refracted, or
bent, as they pass from the surface of one transparent medium to
another; and Inflected, or turned from their course, by the attraction
of opaque bodies. From the first we derive the principles on which
mirrors are constructed; to the second we are indebted for the power of
the lenses, and the blessings of sight,--for the light acts upon the
retina of the eye in the same manner as on the lens of a camera. The
latter has no important bearing upon our subject.

When a ray of light falls perpendicularly upon an opaque body, it is
reflected bark in the same line in which it proceeds; in this case the
reflected ray returns in the same path the incident ray traversed; but
when a ray falls obliquely, it is reflected obliquely, that is, it is
thrown off in opposite direction, and as far from the perpendicular as
was the incident ray, as shown at Fig. 2; a representing the incident
ray and b the reflected. The point, or angle c made by the incident
ray, at the surface of the reflector e f, with a line c d,
perpendicular to that surface, is called the angle of incidence, while
the angle formed by the ref