12 



SIX LECTURES ON LIGHT. 



quenched each other and darkened the spot. 

 This was a statement of fundamental signifi- 

 cance, but it required the discoveries and the 

 genius of Young to give it meaning. How 

 he did so, I will now try to make 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 ren- 

 dered more rare. Properly agitated, a tun- 

 ing-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 par- 

 celled Out into spaces in which the air is con- 

 densed, 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 u series of such waves, and you 

 can imagine a second series sent through the 

 same air, and so related to the first that con- 

 densation coincides with condensation and 

 rarefaction with rarefaction. The conse- 

 quence of this coincidence would be a louder 

 sound than that produced by either system of 

 waves taken singly. But you can also ima- 

 gine a state of things where the condensa- 

 tions of the one system fall upon the rarefac- 

 tions of the other system. In this case th: 

 two systems would completely neutralize each 

 other. Each of them, taken singly, produces 

 sound; both of them, taken together, pro- 

 duce no sound. Thus, by adding sound to 

 sound we produce silence, as Grimaldi in his 

 experiment produced darkness by adding 

 light to light. 



The analogy between sound and light here 

 at once flashes upon the mind. Young gen 

 eralized this observation. He discovered a 

 multitude of similar cases, and determined 

 their precise conditions. On the assumption 

 that light was wave-motion, all his experi- 

 ments on interference were explained ; on the 

 assumption that light wai flying particles, 

 nothing was explained. In the time of Huy- 

 ghens and Euler a medium had been assumed 

 tor the transmission of the waves of light ; 

 but Newton raised the objection that, if light 

 consisted of the waves of such a medium, ' 

 shadows could not exist. The waves, he 

 contended, would bend round opaque bodies 

 and produce the motion of light behind them, 

 as sound turns a corner, or as waves of water 

 wash round a rock. It was proved that the 

 bending round referred to by Newton actually 

 occurs, but that the inflected waves abolish 

 each other by their mutual interference. 

 Young also discerned a fundamental differ- 

 ence between ti'.e waves of light and those of 

 sound. Could you see the air through which 

 sound-waves are passing, you would observe 

 every individual particle of air oscillating to 

 fcnd fro in the direction of propagation. 

 Could you see the ether, you would also find 

 every individual particle making a small ex- 

 cursion to and fro, but here the motion, like 

 that assigned to the water-particles above re- 

 ferred to, would be across the line of propa- 



gation. The vibrations of the air are longi- 

 tudinal, the vibrations of the ether are trans- 

 versal. 



It is my desire that you should realize with 

 clearness the character of wave-motion, both 

 in ether and in air. And, with this view. I 

 bring bcfcre you an experiment wherein the 

 air-particles are represented by small spots of 

 light. They are parts of a spiral, drawn upon 

 a circle of blackened glass, and, when the cir- 

 cle rotates, the spots move in successive pulses 

 over the screen. You have here clearly set 

 before you how the pulses travel incessantly 

 forward, while the particles that compose 

 them perform oscillations to and fro. This 

 is the picture of a sound-wave, in which the 

 vibrations are longitudinal, By another glass 

 wheel, we produce an image of a trans- 

 verse wave, and here we observe the waves 

 travelling in succession over the screen, while 

 each individual spot of light performs an ex- 

 cursion to and fro across the line of propaga- 

 tion. 



Notice what follows when the glass wheel 

 is turned very quickly. Objectively consid- 

 ered, the transverse waves propagate them- 

 selves as before, but subjectively the effect is 

 totally changed. Because of the retention of 

 impressions upon the retina, the spots of light 

 simply describe a series of parallel luminous 

 lines upon the screen, the length of these 

 lines marking the amplitude of the vibration. 

 The impression of wave-motion has totally 

 disappeared. 



The most familiar illustration of the inter- 

 ference of sound-waves is furnished by the 

 beats produced by two musical sounds slightly 

 out of unison. These two tuning-forks are 

 now in perfect unison, and when they are 

 agitated together the two sounds flow without 

 roughness, as if they were but one. But, by 

 attaching to one of the forks a two-cent piece, 

 we cause it to vibrate a little more slowly 

 than its neighbor. Suppose that one of them 

 performs 101 vibrations in the time required 

 by the other to perform 100, and suppose 

 that at starting the condensations and rare- 

 factions of both forks coincide. At the loist 

 vibration of the quickest fork they will again 

 coincide, the quicker fork at this point hav- 

 ing gained one whole vibration, or one whole 

 wave upon the other. But a little reflection 

 will make it clear that, at the 5oth vibration, 

 the two forks are in opposition; here the one 

 tends to produce a condensation where the 

 other tends to produce a rarefaction; by the 

 united action of the two forks, therefore, the 

 sound is quenched, and we have a pause of 

 silence. This occurs where one fork has 

 gained half a tuave-/cngl/i upon the other. 

 At the loist vibration we have again coinci 

 dence, and, therefore, augmented sound; at 

 the isoth vibration we have again a quench- 

 ing of the sound. Here the one fork is three 

 half-waves in advance of the other In gen- 

 eral terms, the waves conspire when the one 

 series is an even number of half-wave lengths, 



