1886.] on Thomas Young. 565 



these circular waves. The form of the wave moves forward, but 

 the motion of its individual particles, at any moment, is simply a 

 vibration up and down. Now each oscillating particle of every 

 moving wave, if left to itself, would produce a series of waves, 

 not so high, but in other respects exactly similar to those produced 

 by the stone. The coalescence of all these small waves produces 

 another wave of exactly the same kind as that which started them. 

 The princij^le that every particle of a wave acts independently of all 

 other particles, while the waves produced by all the particles after- 

 wards combine, is, as I have said, the great principle of Huyghens. 

 Taken in conjunction with the interference of light, first established 

 by Thomas Young, which proved that when waves coalesce or combine, 

 they may either support each other, or neutralise each other, the 

 neutralisation being either total or partial, according as the opposition 

 of the combining waves is complete or partial; taking, I say, the 

 principle of interference in conjunction with that of Huyghens, 

 Fresnel proved that although light does diverge behind an opaque 

 body, as Newton supposed that it would diverge, these divergent 

 waves completely efface each other, producing the shadow due to the 

 tranquillity of the medium which propagates the light. 



By reference to the waves of water, Young illustrates, in the most 

 lucid manner, the interference of the waves of light. He pictures 

 two series of waves generated at two points near each other in a 

 lake, and reaching a channel issuing from the lake. If the waves 

 arrive at the same moment, neither series will destroy the other. If 

 the elevations of both series coincide, they will, by their joint action, 

 produce in the channel a series with higher elevations. But if the 

 ridges of one series correspond to the depressions of the other, the 

 ridges will exactly fill the depressions, smooth water in the channel 

 being the result. " At least," says Young, " I can discover no alter- 

 native, either from theory or from experiment. Now," he continues 

 — gathering confidence as he reasons, " I maintain that similar effects 

 take place whenever two portions of light are thus mixed, and this I 

 call the genei*al law of the interference of light." 



The physical meaning of all the terms applied to light was soon 

 fixed. Intensity depended upon the amplitudes of the waves. Colour 

 depended on the lengths of the waves. Two series of waves coalesced 

 and helped each other when one was any number of complete un- 

 dulations or, in other words, any even number of half-undulations, 

 behind the other. Two series of waves extinguished each other when 

 the one series was any odd number of semi-undulations behind the 

 other. But inasmuch as white light is made up of innumerable 

 waves of different lengths, such waves cannot all interfere at the 

 same time. Some interfere totally, and destroy each other ; some 

 partially ; while some add themselves together and enhance the effect. 

 Thus, by interference, a portion only of the white light is withdrawn, 

 and the remaining portion is, as a general rule, coloured. Indeed 

 the most glowing and brilliant effects of coloration are thus pro- 



