For instance let a fig. 1, represent the luminous body from
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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,