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