In the left-hand photograph an electric current is passing through the
coil, thus producing a magnetic field and transforming the poker into a
magnet. The poker is then able to support a pair of scissors. As soon as
the electric current is broken off, as in the second photograph, the
ether disturbance ceases. The poker loses its magnetism, and the
scissors fall.]
We have said that light consists of waves. The ether may be considered
as resembling, in some respects, a jelly. It can transmit vibrations.
The waves of light are really excessively small ripples, measuring from
crest to crest. The distance from crest to crest of the ripples in a
pond is sometimes no more than an inch or two. This distance is
enormously great compared to the longest of the wave-lengths that
constitute light. We say the longest, for the waves of light differ in
length; the colour depends upon the length of the light. Red light has
the longest waves and violet the shortest. The longest waves, the waves
of deep-red light, are seven two hundred and fifty thousandths of an
inch in length (7/250,000 inch). This is nearly twice the length of
deep-violet light-waves, which are 1/67,000 inch. But light-waves, the
waves that affect the eye, are not the only waves carried by the ether.
Waves too short to affect the eye can affect the photographic plate, and
we can discover in this way the existence of waves only half the length
of the deep-violet waves. Still shorter waves can be discovered, until
we come to those excessively minute rays, the X-rays.