S.—LATER THEORIES.

Grey River Argus, 11 June 1904, Page 4 (Supplement)

S.— LATER THEORIES.

.If, however, both pipes are sounded together, instead of the intensity of the note being increased, as one would expect, it is found to be so greatly weakened that at a. short distance it is not even audible. So that sound added to ' sound produces silence under certain circumstances, impossible as it appears. The explanation is that the top of the wave of sound produced by one pipe is counteracted by falling exactly on the • bottom of the furrow of the wave produced by the other pipe, and the two neutralise each other, like the waves of water. This "interference" was Young's fundamental discovery in' light, though he was not the first to notice that light added to light does,, under certain circumstances, produce darkness. This has' been ; proved by a simple, experiment where a beam of light is allowed to fallonVtwo : mirrors placed close together, bub slightly inclined to eaob other, co as to get

i very wide angle between their faces. 1 from the mirrors the light is reflected m to a screen, and in the middle of the bright patch a series of dark lines is serceived, but they' vanish if one of the ( mirrors is covered up, and the whole 1 ratface appears to be illuminated. ] The fact that light acided to light pro- ( luces darkness, in just tho same Way as ; sound ud<led to Bound produces silence, aoid waves added to waves produce stillness, is i lie most perfect proof possible that light is transmitted by an "undulalory movement/ as it is called — that is to say, by a vibratory movement propagated from one particle to the next without any particle moving from its original position, at all events, to any cousideraule e^i^ant, and not by the transference or motion of the luminous mutter itself, like the passage of a projectile from the cannon to the object it strikes. There is, however, a difference, for when air is yet in motion by sound, ihe air, as a famous writer has expressed it, "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.