MM// i (i.\ I l:Mn>l<.IRy .//'/JAi / s l.\ /•//) s/( s ill 2R3 



Inrm which .in- rt'qiiiri'd to .iccoiint for the ()l)srr\i'(l proptTlii's f)f 

 liy;l>i. 



Ma.wvrll'ri ihrorN iiniiUHliati-l>' arliii-xol ilu- si milling success of 

 prfscntinj; a valiif for the six-ifl of the iina^iiu-d elect roniannetir 

 waxes, determined e\chisi\ely from measurements upon ilie maKnelic 

 lields of electric currents, and a^;reein>; precisely with the observed 

 s(H'e<l of linlit- Two suppose<lly ilislincl jirovinces of jiliysics, each of 

 which li.id In-en organized on its own particular basis of experience 

 •md in its own particular manner, were suddenh' iniited i>y a stroke 

 of synthesis to which few if any parallels can be foimd in the history 

 of ihoui^ht. An<I this is l)\- no means the only achievement of the 

 electromagnetic theory of linht; there will shortly be occasion to 

 mention some of the others. 



Now that there was so much evidence that light travels as a wave- 

 motion, and that its speed and other properties are those of electro- 

 magnetic waxes, it became urjjentlx' desirable to inquire into the 

 n.iture of the sources of lijjht. Granted that light en ronle outxvards 

 from a luminous particle of matter is of the nature of a combination 

 of xvaxe- trains, what is taking place in the luminous particle? To this 

 ((uestion all our experience and all our hal)its of thought suggest one 

 sole obvious answer — that in the luminous particle there is a xibrating 

 something, a vibrator, or more likely an enormous number of \ibra- 

 tors — one to each atom, possibly — and the oscillations of these vibra- 

 tors are the sources of the xvaves of light, as the oscillations of a 

 xiolin-string or a tuning-fork are the sources of waves of sound. 

 This analogy draxvn from acoustics, this picture of the vibrating 

 xiolin-string and the vibrating tuning-fork, has been powerful — 

 indeed, it begins to seem, too powerful — in guiding the formation of 

 our ideas on light. It is profitable to reflect that the exolution of 

 thought in acoustics must hax-e traveled in the opposite sense from 

 the exolution of thought in optics. Whoexer it xx'as xvho was the 

 first to conceixe that sound is a xxave-motion in air, must certainly 

 haxe arrixed at the idea by noticing that sounding bodies vibrate. 

 One feels the trembling of the tuning-fork or the bell, one sees the 

 xiolin-string apparently spread out into a band by the amplitude 

 of its motion; it is not difficult to build apparatus which, like a slowed- 

 down cinema film, makes the vibrations separatelx' visible, or, like 

 the strolmscope, produces an equix-alent and not misleading illusion. 

 This was not possible in optics, and never will be. In acoustics, 

 one may sometimes accept the x'ibrations of the sounding body as 

 an independently-gix-en fact of experience, and reason forxvard to the 

 wave-motion spreading outwards into the enx-ironing air; in optics, 



