Light; a course of experimental optics, chiefly with the lantern . have the greatest tidal motion; at others the suns waveopposes the moons wave, and we have the least motion. 96. Single Interferences not Traceable in Lightand Sound.—But here we must make a very important dis-tinction, the want of which has caused many a student diffi-culty. In the foregoing cases we could trace the interferences IX.] SINGLE INTERFERENCES. 157 of swgle waves, because their motions were large to us, occu-pied considerable time, and thus enabled us to trace them —most clearly in the grand tidal waves, which are
Light; a course of experimental optics, chiefly with the lantern . have the greatest tidal motion; at others the suns waveopposes the moons wave, and we have the least motion. 96. Single Interferences not Traceable in Lightand Sound.—But here we must make a very important dis-tinction, the want of which has caused many a student diffi-culty. In the foregoing cases we could trace the interferences IX.] SINGLE INTERFERENCES. 157 of swgle waves, because their motions were large to us, occu-pied considerable time, and thus enabled us to trace them —most clearly in the grand tidal waves, which are longest ofall. The student is at first apt to fancy that, in a similarway, rays from any two points of light must be constantlydestroying one another by interference, much as in Fig. 92,supposed to represent the rays from two lighthouses. Andto some extent they undoubtedly do so. But they can onlythus act on each other at the detached/w^z^j where theundulations cross ; and in the case of light and sound thevibrations are so enormously rapid and numerous that. Fig. 92.—Two Lighthouses. comparatively few extinctions of this kind are not sensiblymissed. But if we can bring a whole wave series to act upon anotherexactly similar whole wave series, then any effect at one pointin any wave of the series is repeated throughout the series,and the effect becomes visible. In the case of sound wecan get similar wave series pretty easily by employing exactunisons; and so it will be found, if a tuning-fork be struckand held close to the ear, that on turning it round on thestem there is a position in which the sound is nearly orquite extinguished. This position differs, as it should do,with the key of the fork, but with either an A or a C fork 158 LIGHT. [chap. is when the two prongs are at an angle of nearly 45° with thedirection of the ear. If the fork is steadily rotated, the soundwill be alternately extinguished and reinforced-, accordingtothe phases in which the waves from each pro
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