History and Practice of the Art of Photography | Page 9

ray consists are so
minute that the rays do not appear to be impeded by each other. A ray
of light passing 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 admiting light only through a small aperture.
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 [hipho_1.gif]
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 [hipho_2.gif] 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 reflected ray b and the perpendicular line d is called the angle of
reflection, and these angles are always equal.
It is by this reflection of light that objects are made visible; but unless
light falls directly upon the eye they are invisible, and are not sensibly
felt until after a certain series of operations upon the various coverings
and humors of the eye. Smooth and polished surfaces reflect light most
powerfully, and send to the eye the images of the objects from which
the light proceeded before reflection. Glass, which is transparent--
transmitting light--would be of no use to us as a mirror, were it not first
coated on one side with a metalic amalgam, which interrupts the rays in
their passage from the glass into the air, and throws them either directly
in the incident line, or in an oblique direction. The reason why trees,
rocks and animals are not all mirrors, reflecting other forms instead of
their own, is, that their surfaces are uneven, and rays of light reflected
from an uneven surface are diffused in all directions.
Parallel rays falling obliquely upon a plane mirror are reflected parallel;
converging rays, with the same degree of convergence; and diverging
rays equally divergent.
Stand before a mirror and your image is formed therein, and appears to
be as far behind the glass as you are before it, making the angle of

reflection equal to that of incidence, as before stated. The incident ray
and the reflected ray form, together, what is called the passage of
reflection, and this will therefore make the actual distance of an image
to appear as far again from the eye as it really is. Any object which
reflects light is called a radiant. The point behind a reflecting surface,
from which they appear to diverge, is called the virtual focus.
Rays of light being reflected at the same angle at which they fall upon a
mirror, two persons can stand in such a position that each can