throw half a dozen bright spots on the opposite wall.

[Illustration: Fig. 10.--Manifold Reflections.]

In Fig. 10 the sunbeam enters at A, and, striking the mirror _m_
at _a_, is partly reflected to 1 on the wall, and partly enters
the glass, passes through to the silvered back at B, and is totally
reflected to _b_, where it again divides, some of it going to the
wall at 2, and the rest, continuing to make the same reflections
and divisions, causes spots 3, 4, 5, etc. The brightest spot is
at No.2, because the silvered glass at B is the best reflector
and has the most light.

When the discovery of the moons of Mars was announced in 1877,
it was also widely published that they could be seen by a mirror.
Of course this is impossible. The point of light mistaken for the
moon in this secondary reflection was caused by holding the mirror
in an oblique position.

Take a small piece of mirror, say an inch in surface, and putting
under it three little pellets of wax, putty, or clay, set it on
the wrist, with one of the pellets on the pulse. Hold the mirror
steadily in the beam of light, and the frequency and prominence of
each pulse-beat will be indicated by the tossing spot of light on
the wall. If the operator becomes excited the fact will be evident
to all observers.

[Illustration: Fig. 11.]

Place a coin in a basin (Fig. 11), and set it so that the rim will
conceal the coin from the eye. Pour in water, and the coin will
[Page 40] appear to rise into sight. When light passes from a medium
of one density to a medium of another, its direction is changed.
Thus a stick in water seems bent. Ships below the horizon are
sometimes seen above, because of the different density of the layers
of air.

Thus light coming from the interstellar spaces, and entering our
atmosphere, is bent down more and more by its increasing density.
The effect is greatest when the sun or star is near the horizon,
none at all in the zenith. This brings the object into view before
it is risen. Allowance for this displacement is made in all delicate
astronomical observations.

[Illustration: Fig. 12.--Atmospherical Refraction.]

Notice on the floor the shadow of the window-frames. The glass
of almost every window is so bent as to turn the sunlight aside
enough to obliterate some of the shadows or increase their thickness.

DECOMPOSITION OF LIGHT.

Admit the sunbeam through a slit one inch long and one-twentieth
of an inch wide. Pass it throu