ion of the
Aether, and it can be demonstrated that the various colours of light are
due to different wave lengths. Colour is to light what pitch is to
sound. As has been shown in Art. 62, the pitch of a note depends upon
the number of air waves which strike upon the tympanum of the ear in a
given time. The more rapid the vibration, the higher the note. The more
rapidly a sounding body vibrates, the shorter will be the length of each
wave. If a violinist wants to produce a note of higher pitch, he presses
his finger on the string, thereby shortening it, and by so doing
increases the rapidity of vibration, and raises the pitch of the note.
Now the colours of the spectrum are to the eye what the notes are to the
ear. The aetherial waves which produce the red colour are slower in
their vibrations, and are longer than those which produce the orange
colour. Those which produce the orange colour are of slower vibrations,
and longer than those which produce the yellow colour, and so on through
all the other colours; until we get to the violet and to the
ultra-violet, or invisible violet rays, which are the most rapid in
their vibrations, and consequently their wave lengths are the shortest
of the whole group. It has been ascertained that it takes about 39,000
waves of red light to measure an inch if placed end to end. Now light
has a velocity of 186,000 miles per second. If this is reduced to
inches, we find that there are 11,784,960,000 in that distance. Let us
therefore multiply this number by 39,000, and we shall then find how
many waves of red light must enter the eye to produce the sensation of
red colour. That number is 459,613,440,000,000, so that all these waves
enter the eye in one second of time, and must strike the retina of the
eye in order to produce the sensation of redness. In the same way, the
number of waves that must strike the retina of the eye to produce the
sensation of violet can be determined. It takes about 57,500 waves of
violet to measure an inch, so that a violet wave is only 1/57000 part of
an inch in length. All the other colours of the spectrum which lie
between the violet and the red waves gradually get longer and longer in
their wave lengths, and slower and slower in their vibrations, until at
the red end of the spectrum and beyond it we have the longest waves,
which are from 1/39000 part of an inch in length to 1/10000 part of an
inch.

The seven colours seen in the spectrum are called the Visib