was a statement of fundamental significance, but it required the
discoveries and the genius of Young to give it meaning. How he did so
will gradually become clear to you. You know that air is compressible:
that by pressure it can be rendered more dense, and that by
dilatation it can be rendered more rare. Properly agitated, a
tuning-fork now sounds in a manner audible to you all, and most of you
know that the air through which the sound is passing is parcelled out
into spaces in which the air is condensed, followed by other spaces in
which the air is rarefied. These condensations and rarefactions
constitute what we call _waves_ of sound. You can imagine the air of a
room traversed 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,