July 27, 1893] 



NA TURE 



307 



similar manner, by virtue of the forces brought into play by the 

 condensation and rarefraction of the ether. But there is one 

 whole class of phenomena which have actually no counterpart 

 in those cf sound; I refer to polarisation and double refrac- 

 tion." 



"The evidence for the truth of the theory of undulations as 

 regards the phenomena of common light depends in great 

 measure upon the fact of interference and the explanation which 

 the theory gives of the complicated phenomena of diffraction. 

 But in studying the interference of polarised light, additional 

 phenomena presented themselves which ultimately pointed out 

 that the vibrations with which we are concerned in the case of 

 the ether differ altogether in their character from those which 

 belong to sound. The phenomena of the interference of 

 polarised light prove incontestably that there exists in light an 

 element of some kind having relation to directions transverse to 

 that of propagation, and admitting of composition and resolu- 

 tion in a plane perpendicular to the direction of transmission 

 according to the very same laws as those of the composition and 

 resolution of forces, or velocities, or displacements in such a 

 plane. This requires us to attribute to the ether a constitution 

 altogether different from that of air. It points out the exist- 

 ence of a sort of elasticity whereby the ether tends to check 

 the gliding of one layer over another. Have we no example of 

 such a force in the case of ponderable matter ? We have. 

 We know that an elastic solid, which for simplicity I will sup- 

 pose to be uncrystalline, and alike in all directions, has two 

 kinds of elasticity, by one of which it, like air, tends to resist 

 compression and rarefraction ; while by the other it tends to 

 resist a continuous gliding of one portion over another, and to 

 restore itself to its primitive state if such a gliding has taken 

 place. There is no direct relation between the magnitude of 

 these two kinds of elasticity, and in the case of an elastic solid 

 such as jelly the resistance to compression is enormously great 

 compared to the resistance to a gliding displacement." 



"If we assume that in the ether there is really an elasticity 

 tending to restore it to its primitive condition when one layer 

 tends to glide over another, an elasticity which it appears to be 

 absolutely necessary to admit in order to account for the 

 observed laws of interference of polarised light, the question 

 arises, can we thereby explain double refraction?" 



"The earliest attempts to explain it in accordance with the 

 theory of transverse vibrations were made by attributing to the 

 ether a molecular constituiion more or less analogous to that 

 which we believe to exist in ponderable matter. Following out 

 speculations founded upon that view, the celebrated Fresnel was 

 led to the discovery of the actual laws of doable refraction ; the 

 theory, however, which he gave was by no means complete, inas- 

 much as the resultswere not rigorously deduced from the premises. 

 Cauchy and Neumann, independently and about simultaneously, 

 took up Fresnel's view of the constitution of the ether and 

 applied it to explain the laws of double refraction. In their 

 theory the conclusions an ived at were rigorously derived from 

 the premises; but the results did not altogether agree with 

 observation ; that is to say, although they could by the adoption 

 of certain suppositions he forced into a near accordance wilh»he 

 observed laws of double refraction, yet they pointed out the 

 necessity of the existence of other phenomena which were belied 

 by observation. Our own countryman Green was the first to 

 deduce Fresnel's laws from a rigorous dynamical theory, 

 although neatly simultaneously MacCullagh arrived at a theory 

 in some respects similar, though on the whole I think less satis- 

 factory. " 



" Still all these theories followed pretty closely the analogy of 

 ponderable matter ; and at least in the first three mentioned the 

 ether was even imagined to consist of discrete molecules, acting 

 on one another, like the bodies of the solar system regarded as 

 points, by forces in the direction of the joining line, and varying 

 as some function of the distance. I have already quoted the 

 very strong language in which Newton rejected the idea of the 

 heavenly bodies acting on one another across intervening spaces 

 which were absolutely void. But the conception has nothing 

 to do with the magnitude of the intervening spaces ; and the 

 conception of action at a distance across an inteivening space 

 which is absolutely void, is no: a bit easier when the sp.ace in 

 question is merely that separating two adjacent molecules, when 

 the ether is thought of as consisting of discrete molecules, than 

 it is when the space is that separating two bodies of the solar 

 system, though in this latter case it may amount to many mil- 

 lions of miles. If the ether be in so:ne unknown manner the link 



NO. 1239. VOL. 48] 



of connection whereby two heavenly bodies are enabled to exert 

 on one another the attraction of gravitation, then according to 

 the hypothetical constitution of the ether that we have been con- 

 sidering, we seem compelled to invent an ether of the second 

 order, so to speak, to form a link of connection between two 

 separate molecules of the luminiferous ether. But since the 

 nature of the ether is so very different as it must be from that of 

 ponderable matter, it may be that the true theory must proceed 

 upon lines in which our previous conceptions derived from the 

 study of ponderable matter are in great measure departed frorii." 

 "If we think of the ether as a sort of gigantic jelly, we can 

 hardly imagine but that it would more or less resist the passage 

 of the heavenly bodies — the planets for instance — through it. 

 Vet there appears to be no certain indication of any such resist- 

 ance. It has been observed indeed in the case of Encke's 

 comet, that at successive revolutions the comet returned to its 

 perihelion a little*i)»fore the calculated time. This would be 

 accounted for by the supposition that it experienced a certain 

 amount of resistance from the ether. Although at first sight we 

 might be disponed to say that such a resistance would retard 

 perihelion passage, yet the fact that it would accelerate it 

 becomes easily intelligible, if we consider that the resistance 

 experienced would tend to check its motion, and so prevent it 

 from getting away so far from the sun at aphelion, and would 

 consequently bring it more nearly into the condition of a planet 

 circulating round the sun in a smaller orbit." 



" Many years ago I asked the highest authority in this country 

 on physical astronomy, the late Prof. Adams, what he thought 

 of the evidence afforded by Encke's comet for the existence of a 

 retarding force, such as might arise from the ether. lie said to 

 me that he thought we did not know enough as to w hether there 

 might not possibly be a planet or planets within thetrbitof 

 Mercury which would account for it in a different « ay. But 

 quite independently of such a supposition it is worihy of note 

 that the remarkable phenomena presented by the tails of comets 

 render it by no means unlikely that even without the presence 

 of a resisting medium, and without the disturbing force arising 

 from the attiaction of an unknown planet situated so near to the 

 sun as not to have been seen hitherto, the motion of the head of 

 a comet might not be quite the same as that of a simple body 

 representing the nucleus, and being subject to the gravitation of 

 thesun and planets and nothing else. It appears ihat the tails 

 consist of some kind of matter driven from the comet with an 

 enormous velocity by a sort of repulsion emanating from the 

 sun. If the nucleus loses in this manner at each perihelion 

 passage an exceedingly small portion of its mass, which is re- 

 pelled from the sun, it is possible that the residue may expe- 

 rience an attraction towards the sun over and above that due to 

 gravitation, and that possibly this may be the cause of the ob- 

 served acceleration in the time of passing perihelion even 

 though there be no resistance on the part of the ether. So 

 that the question of resistance or no resistance must be left 

 an open one." 



" The supposition that the ether would resist in this manner 

 a body moving through it is derived from what we observe in 

 the case of solids moving through fluids, liquid or gaseous, as 

 the case may be. In ordinary cases of resistance, the main 

 represenlaiive of the work apparently lo'st in propelling the 

 solid is in the first instance the molecular kineticenergy of the trail 

 of eddies in the wake. The formation of these eddies is, how- 

 ever, an indirect effect of the internal friction, or if we prefer 

 the term visco-ity, of the fluid. Now il e viscosity of gases has 

 been explained on the kinetic theory of gases, and in the case of 

 a liquid we cannot well doubt that it is connected with the 

 constitution of the substance as not being absolutely continuous 

 but molecular. But if the ether be either non molecular, or 

 molecular in some totally diflferent sense from ponderable matter, 

 we cannot with safety infer that the motion of a solid through it 

 necessarily implies resistance." 



"The luminiferous ether touches on another mysterious agent, 

 the nature of which is unknown, although its laws are in many 

 respects known, and it is applied to the cvery-day wants of life, 

 and its applications are even regulated by Acts of Pailiament ; I 

 allude to electricity. I said that the nature of electricity is un- 

 known. More than forty years ago I was sitting at dinner beside 

 the illustrious Faraday, and I said to him that I thought a great 

 step would have been made if we could say of electricity some- 

 thing analogous to what we say of light, when we affirm that 

 light consists of undulations ; and he said to me that he thought 

 we were a long way off-that at present. But, as I said, relations 



