g and his setting. The rectilineal propagation of light may be
illustrated by permitting the solar light to enter, through a small
aperture in a window-shutter, a dark room in which a little smoke has
been diffused. In pure _air_ you cannot see the beam, but in smoky air
you can, because the light, which passes unseen through the air, is
scattered and revealed by the smoke particles, among which the beam
pursues a straight course.

The following instructive experiment depends on the rectilineal
propagation of light. Make a small hole in a closed window-shutter,
before which stands a house or a tree, and place within the darkened
room a white screen at some distance from the orifice. Every straight
ray proceeding from the house, or tree, stamps its colour upon the
screen, and the sum of all the rays will, therefore, be an image of
the object. But, as the rays cross each other at the orifice, the
image is inverted. At present we may illustrate and expand the
subject thus: In front of our camera is a large opening (L, fig. 2),
from which the lens has been removed, and which is closed at present
by a sheet of tin-foil. Pricking by means of a common sewing-needle a
small aperture in the tin-foil, an inverted image of the carbon-points
starts forth upon the screen. A dozen apertures will give a dozen
images, a hundred a hundred, a thousand a thousand. But, as the
apertures come closer to each other, that is to say, as the tin-foil
between the apertures vanishes, the images overlap more and more.
Removing the tin-foil altogether, the screen becomes uniformly
illuminated. Hence the light upon the screen may be regarded as the
overlapping of innumerable images of the carbon-points. In like manner
the light upon every white wall, on a cloudless day, may be regarded
as produced by the superposition of innumerable images of the sun.

[Illustration: Fig. 2.]

The law that the angle of incidence is equal to the angle of
reflection has a bearing upon theory, to be subsequently mentioned,
which renders its simple illustration here desirable. A straight lath
(pointing to the figure 5 on the arc in fig. 3) is fixed as an index
perpendicular to a small looking-glass (M), capable of rotation. We
begin by receiving a beam of light upon the glass which is reflected
back along the line of its incidence. The index being then turned, the
mirror turns with it, and at each side of the index the incident and
the reflected beams (L _o_, _o_ R) t