apter we shall also see that light
is due to some kind of periodic wave motion in the Aether. So that not
only heat, but light also, it would appear, is due to certain periodic
wave motions that are set up in the Aether by the vibrations of hot or
luminous bodies. The question therefore arises, how many wave motions
are there in the Aether? Are there different wave motions which in one
case produce light, and in the other case produce heat, or are light and
heat both produced by the same set of aetherial waves? The identity of
light waves with heat waves is manifested by the fact that wherever we
get light we get heat, as can be proved in many ways. One of the
simplest proofs is found in the common lens or burning-glass, by which
the light waves are brought to a focus, and as a result, heat is
manifested. Although there is this close identity between light and heat
waves, yet there must be some distinction between the heat and light
waves, because while light waves affect the eye, heat waves do not.
There is actually a difference between the two kinds of waves, and that
difference is one of period or length. It must not, however, be thought
that there are really two classes or sets of waves in the Aether, one of
which could be called light waves, and the other heat waves, but rather
the same wave may be manifested in two different forms because of its
different wave lengths. In one case the waves may affect the eye, and we
have the sensation of sight, but in the other case they affect the body,
and we experience the sensation of warmth. An analogy from the waves of
sound may make these facts much clearer. We know that sound travels
about 1100 feet per second. If, therefore, we have a bell which vibrates
about 1100 times per second, we should have a wave one foot long. If it
vibrated 100 times per second the waves would be 11 feet long, while if
it vibrated only 11 times per second, the waves would be 100 feet long.
Now the impression made upon the ear depends upon the number of
vibrations the bell makes per second, and from the rate of vibration we
get the idea of pitch. If the vibrations are very rapid, then we get a
note of high pitch, and if the vibrations are slow, then we get a note
of low pitch. A note of high pitch, therefore, will correspond to waves
of short length, while a low note will correspond to waves of a greater
length; so that the greater the rapidity with which a sounding bell
vibrates, the shorter will