all his experiments on interference were accounted for; on the
assumption that light was flying particles, nothing was explained. In
the time of Huyghens and Euler a medium had been assumed for the
transmission of the waves of light; but Newton raised the objection
that, if light consisted of the waves of such a medium, shadows could
not exist. The waves, he contended, would bend round opaque bodies and
produce the motion of light behind them, as sound turns a corner, or
as waves of water wash round a rock. It was proved that the bending
round referred to by Newton actually occurs, but that the inflected
waves abolish each other by their mutual interference. Young also
discerned a fundamental difference between the waves of light and
those of sound. Could you see the air through which sound-waves are
passing, you would observe every individual particle of air
oscillating to and fro, _in the direction of propagation_. Could you
see the luminiferous ether, you would also find every individual
particle making a small excursion to and fro; but here the motion,
like that assigned to the water-particles above referred to, would be
_across_ the line of propagation. The vibrations of the air are
_longitudinal_, those of the ether _transversal_.

The most familiar illustration of the interference of sound-waves is
furnished by the _beats_ produced by two musical sounds slightly out
of unison. When two tuning-forks in perfect unison are agitated
together the two sounds flow without roughness, as if they were but
one. But, by attaching with wax to one of the forks a little weight,
we cause it to vibrate more slowly than its neighbour. Suppose that
one of them performs 101 vibrations in the time required by the other
to perform 100, and suppose that at starting the condensations and
rarefactions of both forks coincide. At the 101st vibration of the
quicker fork they will again coincide, that fork at this point having