by a series of such waves, and you can imagine a second
series sent through the same air, and so related to the first that
condensation coincides with condensation and rarefaction with
rarefaction. The consequence of this coincidence would be a louder
sound than that produced by either system of waves taken singly. But
you can also imagine a state of things where the condensations of the
one system fall upon the rarefactions of the other system. In this
case (other things being equal) the two systems would completely
neutralize each other. Each of them taken singly produces sound; both
of them taken together produce no sound. Thus by adding sound to sound
we produce silence, as Grimaldi, in his experiment, produced darkness
by adding light to light.

Through his investigations on sound, which were fruitful and profound,
Young approached the study of light. He put meaning into the
observation of Grimaldi, and immensely extended it. With splendid
success he applied the undulatory theory to the explanation of the
colours of thin plates, and to those of striated surfaces. He
discovered and explained classes of colour which had been previously
unnoticed or unknown. On the assumption that light was wave-motion,
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