be the length of the sound waves which it
generates, and _vice versa_. The range of the ear however for sound
waves is limited, so that if the vibrations be too rapid or too slow,
the ear may not be able to respond to the vibrations, and so no distinct
impression of the sound will be conveyed to the brain. It need hardly be
pointed out, that both the very short and long waves are of exactly the
same character as those of a medium length, which the ear can detect,
the only difference being one of rapidity. We do not therefore suggest
that in the case of sound, where the vibrations lie outside the compass
of the ear, those which lie outside are not sound waves, or that they
are different from those which lie within the compass of the ear, and
which the ear can detect. Whether the sound waves are long or short,
whether they can be detected by the ear or not, we still say that all
are sound waves, and that all are due to the vibrations of the sounding
body, which vibrations are transmitted through the air, in waves, that
fall upon the tympanum or drum of the ear, and set that vibrating, which
vibrations are transmitted to the auditory nerve and so give rise to the
sensation of hearing. In a similar manner, every atom and every particle
of matter, every planet, every sun and star, is constantly in a state of
vibration, sending off aetherial waves on every side. Nothing in Nature
is absolutely cold, nothing is absolutely still. Therefore all matter,
whether in the atomic form, or in the planetary or solar world, is
constantly generating aetherial waves, which travel from their source or
origin with the velocity of light. If these aetherial waves so generated
fall within certain limits, then they affect the eye, and we get the
sensation of sight. To do this they must vibrate 5000 billion times per
second, and if they fail to do this, they fail to give rise to the
sensation of sight. If the aetherial waves fall below this limit, then
they affect the body, and give rise to the sensation of heat. For it
must be remembered, that as the ear has a certain compass for sound
waves, which may vary in different individuals, so the eye has also a
certain compass for aetherial waves, with the result that some waves may
be too slow or too rapid to affect the eye, and consequently fail to
give rise to the sensation of sight. When that is so, the sensation of
warmth helps us to detect these longer waves, so that the longer waves
would warm us