Feb. 19, 1880] 



NATURE 



569 



think, still less creditable to the writer. Whatever be one's 

 onn views on the subject, the question of the tenability of the 

 theory is still subjudice, and it is not becoming in " G. H. " to speak 

 so contemptuously of the author of the address for not taking 

 the same view as he does of the merits of the controversy. 

 74, Onslow Gardens, S.W. J. Fletcher Moulton 



On the Mode of the Transverse Propagation of Light 



In Nature, vol. xxi. p. 301, is a letter by Mr. W. M. Hicks 

 containing some critical remarks on a paper of mine, " On 

 a Mode of explaining the Transverse Vibrations of Light " 

 (Nature, vol. xxi. p. 256), which I shall be glad to notice 

 here. 



Firstly, it is, no doubt, understood that the theory proposed 

 by me cannot be regarded as in opposition to any existing theory, 

 from the simple fact that no theory or clear conception of the 

 constitution of the ether (in regard to the mode of propagation 

 of the transverse vibrations of light) appears really to exist. The 

 notion of the ether resembling a "solid" or an "infinitely 

 thin jelly," cannot, of course, be regarded otherwise than as a 

 resource in the face of a difficulty, which, however, we think 

 must appear to any impartial inquirer to increase rather than 

 diminish the difficulty ; and therefore the inference would seem 

 a not unreasonable one that any true theory of the constitution of 

 the ether would be something totally different from "statical'' 

 theories of this kind. As it has been one of my objects to prove, 

 after considerable attention given to the subject, that but one 

 view of the constitution of the ether is in principle conceivable 

 (or that one solution to the problem already exhausts the limits 

 of the conceivable), I may therefore be excused for having some 

 confidence in the fundamental groundwork (at least) of the view 

 adapted, and am therefore all the more ready to reply to any 

 criticisms on the subject, though no doubt (as in the case of any 

 theory possessing points of novelty) difficulties may be expected 

 at first to arise that may entail considerable thought to remove 

 them. It need not be premised that the attainment of truth is 

 the ultimate object of all. 



In the first place, in regard to the remarkable means of cor- 

 recting and adjusting their own motions that atoms moving 

 fr.cly among each other have been proved to possess, I may at 

 once withdraw the expression "instantly," in regard to the rate 

 at which this self-adjustment takes phce. The expression is at 

 best a vague one, and the idea arose from the know n fact of the 

 practically instantaneous adjustment that takes place in the ca-e 

 of an ordinary gas. The mean velocity of the ether atoms 

 would, of course, be necessarily equal to that of light, and all 

 that is essential is that the adjustment should be rapid enough to 

 maintain adequately the equilibrium of the ether. 



la regard to the second difficulty mentioned ; I do not see that 

 the fact of some of the atoms of ether moving at a greater or less 

 velocity than the mean velocity (which is equal to that of light) 

 Should put a difficulty in the way of accounting for the regularity 

 of the waves of light. For it has been proved in connection with 

 the kinetic theory that the number of atoms whose velocities 

 differ by any great proportion from the mean velocity is relatively 

 very small. These atoms would no doubt distribute the energy 

 irregularly over the beam of light, but the total effect would in 

 this way neutralise itself. The great majority of the atoms 

 would still be moving at the mean velocity and distributing the 

 energy in regular waves, and producing that sequence of energy 

 that we call light. I may note that in a paper on "The Mode 

 of the Propagation of Sound on the Basis of the Kinetic Theory 

 of Gases," published by me in the Phil. Mag. for June, 1877, 

 and where a mathematical determination of the velocity of the 

 wave was appended by the late Prof. Clerk Maxwell— the same 

 considerations regarding the varying velocities of the atoms would 

 be involved as above ; and yet we know that as a fact the 

 sequence of the waves of sound is in perfect regularity. 



In reference to the third difficulty mentioned by Mr. W. M. 

 Hicks, regarding the explanation of refraction and reflection. 

 This leads me more strongly to return to a detail in regard to 

 the constitution of the ether I had before adopted, but had not 

 fully grounded, probably from the absence of the requisite 

 encouragement to devote an adequate amount of thought to the 

 subject. I quote the following in substance from a paper already 

 written. I am led to regard the ether atoms as of frso grades 

 of dimensions Of course there is no a priori reason why they 

 should be all of one size, and the fact of their being of two sizes 

 does not alter the principle of the theory in the least. They 



may therefore be assumed, if facts require it, to be of two grades 

 of dimensions. The one set of atoms (specially concerned in 

 the effects of gravity) are to be considered as enormously smaller 

 than the atoms propagating light, and consequently their velocity 

 (which will adjust itself automatically in the inverse ratio of the 

 square root of their mass) very much greater. It might perfectly 

 well be assumed (for example), that the mass of the atoms 

 producing gravity is such that their velocity equals, say 10,000 

 times the velocity of light. I would just remark, in connec- 

 tion with this, that the expression "wonderful" sometimes 

 applied to the velocity of light is, I think, to be deprecated. I 

 would submit that there is nothing really " wonderful " in any 

 velocity, becau-e, however great a velocity is, it is always in- 

 definitely small compared with that which it might be conceived 

 to be, as one has in strict logic no power to limit arbitrarily 

 the conceptions in this respect. If, therefore, there be reason 

 for inferring a certain velocity to exist (no consequence what its 

 value), it seems to me there is' no ground for assuming it to be 

 "wonderful." If a body or atom moves in free space without 

 obstruction, there is nothing to curb its velocity, and its energy 

 may even become immeasurably small at this velocity; provided 

 the atom itself be small ; and, in the same way, we have nothing 

 to limit our conceptions as to the smallness of atoms. There 

 can be no difficulty whatever in these c .mcepti ins, as mechanical 

 principles are admittedly independent of scale, and therefore 

 there is nothing mysterious whatever in the subject. The real 

 mystery surely attaches to the spiritualistic assumptions about 

 "forces" which spoil the interest of physical inquiries', and 

 have involved that magnificent physical a?;ent, the ether, in such 

 a labyrinth of spurious mystery as to repel the inquirer. I cannot 

 avoid the inference that any one who reflects seriously and im- 

 partially on the subject, will be disposed to admit thdt there 

 really cannot be t-co methods in physical science, but only one 

 method (the dynamical), the so-called "statical" speculations 

 about "forces" leading nowhere. It lias been proved again and 

 again in connection with science that the so-called spiritualistic 

 "method" is utterly barren, and only involves one in an inextri- 

 cable maze of speculation from which there is no escape. I 

 have thought these few remarks necessary in view of the special 

 subject with which I am dealing. 



It will be observed that the whole of the dynamical effects 

 above referred to are automatic. The correction of the motion 

 of the atoms so as to move in the right way to produce gravity 

 and light is automatic ; the adjustment of the relative velocities 

 of the atoms between the two sets is automatic, or we make no 

 arbitrary postulate at all. The effect of an adequate velocity for 

 the smaller set of atoms would necessarily (trow well-known 

 dynamical principles) cause them to oppose no measurable resist- 

 ance k to the molecules of gross matter vibrating in them, and 

 consequently they could take no measurable part in the propaga- 

 tion of the energy of light. They would, on- the other hand, 

 produce an enormous pressure (adequate for gravity) on the 

 molecules of grcss matter— the pressure being as the sjuate of 

 the velocity. 1 



The main reasons for assuming that the atoms producing 

 gravity and th-.se producing light are separate, are first 

 the great pressure requisite for gravity, and the consequent 

 necessity for an adequately high velocity to produce this 

 pressure, and secondly (as Mr. W. M. Hicks points out), 

 there would appear to be a difficulty in explaining the 

 reflection of light from some bodies, and also the phenomena 

 of refraction, if we assumed the atoms propagating light to pass 

 through all bodies with perfect facility, as is necessary in the 

 case of the atoms which produce gravity. As this letter has 

 already grown to some length, I will at present confine myself 

 to this inference, reserving some ideas relative to polarisation (in 

 connection with the present theory) to a subsequent letter. 



To prevent misconception, the fact may be cited that the 

 above kinetic theory of the ether does not represent an emission 

 theory of light. The motion of translation (which the ether 

 atoms would possess if there were no light) merely serves as the 

 carrier of the energy impressed upon them by the vibrations of 

 the molecules of gross matter. On the other hand, the fact of 

 the theory resembling (in the translation of the atoms) one of the 

 ideas of Newton may possibly be regarded as rather a recom- 

 mendation than otherwise. If, however (as I have certainly set 



1 If observation shows light to suffer no (sennit le) diminution of velocity at 

 reflection, it would follow that the luminiferous atoms do not suffer a 

 (sensible) diminution of their translatory motion at rebound from gross 

 matter, and consequently these particular atoms could not be appreciably 

 concerned in the effects of gravity. 



