4i8 



NATURE 



[August 2>o, 1888 



electrical theory of light be true, to find that n 2 = Kfi ; 

 or that the index of refraction of any substance is the 

 geometric mean of its electrostatic and magnetic specific 

 capacities. 



That this is precisely true for all substances cannot at 

 present be asserted. There are some substances for 

 which it is very satisfactorily true : there are others 

 which are apparent exceptions. It remains to examine 

 whether they are not only apparent but real excep- 

 tions, and, if so, to what their exceptional behaviour 

 is due. 



It must be understood what the essential point is. It 

 has been proved by various methods, and with greater 

 approach to exactness as the accuracy of the methods is 

 improved, that electrical disturbances — such as the long 

 waves emitted by any alternating machine — travel through 

 air or free space with exactly the same velocity as light ; 

 in other words, that there is no recognizable difference 

 in speed between waves several hundred miles long and 

 waves so small that a hundred thousand of them can lie 

 in an inch. This is true in free ether, and it is a remark- 

 able fact. If it proves anything concerning the structure 

 of the ether, it proves that it is continuous, homogeneous, 

 and simple beyond any other substance ; or at least that 

 if it does possess any structural heterogeneity, the parts 

 of which it is composed are so nearly infinitesimal that a 

 hundred miles and the hundred-thousandth of an inch 

 are quantities of practically the same order of magnitude 

 so far as they are concerned : its parts are able to treat 

 all this variety of wave-length in the same manner. 



But directly one gets to deal with ordinary gross matter 

 we know that this is certainly not the case. Ordinary 

 matter is composed of molecules which, though small, 

 are far from being infinitesimal. Atoms are much smaller 

 than light-waves, indeed, but not incomparably smaller. 

 Hence it is natural to suppose that the ether as modified 

 by matter will be modified in a similarly heterogeneous 

 manner ; and will accordingly not be able to treat waves 

 of all sizes in the same way. 



The speed of all waves is retarded by entering gross 

 matter, but we should expect the smallest waves to be 

 retarded most. The phenomenon is well marked even 

 within the range of such light-waves as can affect the 

 retina : the smaller waves — those which produce the 

 sensation of blue — are more retarded, and travel a little 

 slower, through, say, glass or water, than the somewhat 

 larger ones which produce the sensation of red. This 

 phenomenon has long been known, and is called dis- 

 persion. Hence it is not easy to say at what rate waves 

 a few inches or a few yards or miles long ought to travel, 

 by merely knowing at what rale the ultra-microscopic 

 light-waves travel. 



But there is even more to be said than this. There is 

 not only dispersion, there is selective absorption possessed 

 by matter : not only does it transmit different-sized waves 

 at different rates, but it absorbs and quenches some 

 much faster than others. Few substances, perhaps none, 

 are equally transparent to all sizes of waves. Glass, for 

 instance, which transmits readily the assortment of waves 

 able to affect the retina, is practically quite opaque to 

 waves a few hundred times longer or shorter. And 

 whenever this selec'iva absorption occurs, the laws of 

 dispersion are^extraordinary — so extraordinary that the 

 dispersion is often spoken of as "anomalous"; which of 

 course means, not that it is lawless, but that its laws are 

 unknown. Dispersion in any case is an obscure and 

 little understood subject, but dispersion modified by 

 selective absorption is still worse. Until the theory of 

 dispersion is better understood, no one is able to say at 

 what speed waves of any given length ought to travel. 

 One can only examine experimentally at what rate they 

 do travel. This has been done for long electrical waves, and 

 it has been done for short light-waves : in the case of some 

 substances the speedy is the same, in the case of others it 



is different. But that the speed should be different is, as 

 I have now explained, very natural, and can by no means 

 be twisted into an admission that light-waves and 

 electrical waves are not essentially identical. That the 

 speed of both should agree at all is noteworthy ; the 

 agreement appears to be exact in air, and practically 

 exact in such simple substances as sulphur, and in the 

 class of hydrocarbons known as paraffins ; whereas in 

 artificial substances like glass, and in organic substances 

 like fats and oils, the agreement is less perfect. 



So much for the vital question of the speed at which 

 electrical and optical disturbances travel. In some cases 

 the speeds are accurately the same, in no case are they 

 entirely different ; and in those cases where the agreement 

 is only rough, an efficient and satisfactory explanation of 

 the difference is to hand in the very different lengths of 

 wave which have at present been submitted to experiment. 

 To compare t^e speeds properly, we must either learn to 

 shorten electrical waves, or to lengthen light-waves, or 

 both, and then compare the two things together when of 

 the same size. 



It cannot be seriously doubted that they will turn out 

 identical. 



Manufacture of Light. 



The conclusions at which we have arrived, that light is 

 an electrical disturbance, and that light-waves are excited 

 by electric oscillations, must ultimately, and may shortly, 

 have a practical import. 



Our present systems of making light artificially are 

 wasteful and ineffective. We want a certain range of 

 oscillation, between 7000 and 4000 billion vibrations per 

 second : no other is useful to us, because no other has 

 any effect on our retina ; but we do not know how to 

 produce vibrations of this rate. We can produce a 

 definite vibration of one or two hundred or thousand per 

 second, in other words, we can excite a pure tone of 

 definite pitch ; and we can command any desired range of 

 such tones continuously by means of bellows and a key- 

 board. We can also (though the fact is less well known) 

 excite momentarily definite ethere d vibrations of some 

 million per second, as I have at length explained ; but we 

 do not at present seem to know how to maintain this rate 

 quite continuously. To get much faster rates of vibration 

 than this we have to fall back upon atoms. We know 

 how to mike atoms vibrate : it is clone by what we call 

 "heating" the substance, and if we could deal with indi- 

 vidual atoms unhampered by others, it is possible that we 

 might get a pure and simple mode of vibration from them. 

 It is possible, but unlikely ; for atoms, even when isolated, 

 have a multitude of modes of vibration special to them- 

 selves, of which only a few are of practical use to us, and 

 we do not know how to excite some without also the 

 others. However, we do not at present even deal with 

 individual atoms ; we treat them crowded together in a 

 compact mass, so that their modes of vibration are really 

 infinite. 



We take a lump of matter, say a carbon filament or a 

 piece of quick-lime, and by raising its temperature we 

 impress upon its atoms higher and higher modes of 

 vibration, not transmuting the lower into the higher but 

 superposing the higher upon the lower, until at length we 

 get such rates of vibration as our retina is constructed for, 

 and we are satisfied. But how wasteful and indirect and 

 empirical is the process. We want a small range of npid 

 vibrations, and we know no better than to make the whole 

 series leading up to them. It is as though, in order to sourd 

 some little shrill octave of pipes in an organ, we wero 

 obliged to depress every key and every pedal, and to blow 

 a young hurricane. 



I have purposely selected as examples the more perfect 

 methods of obtaining artificial light, wherein the waste 

 radiation is only useless, and not noxious. But the old- 

 fishioijed plan was cruder even than this, it consisted 



