which have really been generated at the sides.  This crossing of waves may be seen on a small scale in any gutter after rain; it may also be seen on simply pouring water from a wide-lipped jug.  Where crest and furrow cross each other, the wave is annulled; where furrow and furrow cross, the river is ploughed to a greater depth; and where crest and crest aid each other, we have that astonishing leap of the water which breaks the cohesion of the crests, and tosses them shattered into the air.  The phenomena observed at the Whirlpool Rapids constitute, in fact, one of the grandest illustrations of the principle of interference.

Sec. 5. Analogies of Sound and Light.

Thomas Young’s fundamental discovery in optics was that the principle of Interference was applicable to light.  Long prior to his time an Italian philosopher, Grimaldi, had stated that under certain circumstances two thin beams of light, each of which, acting singly, produced a luminous spot upon a white wall, when caused to act together, partially quenched each other and darkened the spot.  This 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.