268 



NATURE 



[January 23, 1896 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ^ 



On Rontgen's Rays. 



Prof. Rontgen's remarkable discovery will materially affect 

 our views concerning the relation between the ether and matter ; 

 but further experimental evidence is required before any opinion 

 can be expressed as to the character of the rays, which behave in 

 so straightforward a manner that they seem to upset all one's 

 notions of the laws of nature. Prof. Rontgen, on the strength 

 of his carefully-conducted experiments, has arrived at a con- 

 clusion adverse to the idea that the rays only differ from light 

 rays by the smallness of wave-length. Perhaps the following 

 considerations may show that the evidence is not conclusive in 

 this respect. 



Rontgen's rays are not kathode rays — there can be no doubt 

 on that point— but they are generated at the point of impact 

 between the kathode ray and solid substances. 



The discoverer has not been able to obtain any interference 

 •effects, possibly, as he says, owing to the weakness of the 

 radiation. An absence of interference would not, however, be 

 sufficient to show that the radiation is not of the nature of 

 ordinary light, but only that it does not possess sufficient re- 

 gularity, or, in other words, that the disturbance is not sufficiently 

 homogeneous. That this is the case is not at all improbable, for 

 the radiation is produced by an impact, which in the first instance 

 may be an impulsive motion propagated outwards, and after 

 passing through the screen, would only possess such regularity as 

 is impressed on it by the absorption of the longer waves. 



The great argument against the supposition of waves of 

 very small length lies in the absence of refraction ; but is this 

 conclusive ? 



When we speak of the size of the atoms, we mean their 

 •distance in the solid and liquid state. The properties of the 

 ether may remain unaltered within the greater part of the sphere 

 of action of a molecule. The number of molecules lying within 

 a wave-length of ordinary light is not greater than the number 

 of motes which lie within a sound-wave, but, as far as I know, 

 the velocity of sound is not materially affected by the presence 

 of dust in the air. Hence there seems nothing impossible in 

 the supposition that light-waves, smaller than those we know of, 

 may traverse solids with the same velocity as a vacuum. We 

 know that absorption bands greatly affect the refractive index in 

 neighbouring regions ; and as probably the whole question of 

 refraction resolves itself into one of resonance effects, the rate of 

 propagation of waves of very small lengths does not seem to me 

 to be pre-judged by our present knowledge. If Rontgen's rays 

 contain waves of very small length, the vibrations in the mole- 

 cule which respond to them would seem to be of a different 

 order of magnitude from those so far known. Possibly we have 

 here the vibration of the electron within the molecule, instead of 

 that of the molecule carrying with it that of the electron. 



I should like, further, to express a certain sense of satisfaction 

 that Rontgen's rays are not deflected in a magnetic field. They 

 are thus clearly separated from kathode rays. The idea that 

 kathode rays are due to vibrations has become fashionable ; yet 

 the fact that the magnet deflects them just as it would an 

 •ejectrified molecule, has always seemed to me to be conclusive 

 against this view. No one has, so far, given any plausible reason 

 why a ray of invisible light should be able to run round in a 

 spiral, while a ray of visible light goes straight ; and, so far, 

 Rontgen's rays behave as we should expect well-conducted 

 vibrations to do. 



It is not my intention to argue in favour of any particular 

 theory, or against Rontgen's suggestion that we have at last 

 found the formerly missed longitudinal wave. I only desire to 

 put those points forward which at first sight seem to go against 

 the supposition of ordinary light vibrations, and to raise the 

 question whether they constitute an insuperable difficulty. 



Arthur Schuster. 



In connection with the wonderful discovery by Prof. Rontgen 

 of photographic rays, apparently hitherto unknown, and in 

 connection with the speculation which concludes Prof Rontgen's 

 most interesting paper, that these rays may perhaps be longi- 



NO. 1369, VOL. 53] 



tudinal vibrations of the luminiferous ether, the following ex- 

 tracts will probably be found of interest to the readers of 

 Nature. They are taken, by permission of Lord Kelvin, from 

 his Baltimore Lectures, delivered at the Johns Hopkins 

 University in 1884. 



The first extract is from the reprint (now in progress) of 

 Lecture IV. Referring to mathematical work immediately 

 preceding, Lord Kelvin says : — " . . . We can do that [obtain 

 certain forms of solutions of equations] for the purpose of illus- 

 trating different problems in sound, and in order to familiarise 

 you with the wave that may exist along with the wave of dis- 

 tortion in any true elastic solid which is not incompressible. 

 We ignore this condensational wave in the theory of light. We 

 are sure that its energy, at all events, if it is not null, is very 

 small in comparison with the energy of the luminiferous 

 vibrations we are dealing with. But to say that it is absolutely 

 null, would be an assumption that we have no right to make. 

 When we look through the little universe that we know, and 

 think of the transmission of electrical force, and of the trans- 

 mission of magnetic force and of the transmission of light, we 

 have no right to assume that there may not be something else 

 that our philosophy does not dream of We have no right to 

 assume that there may not be condensational waves in the 

 luminiferous ether. We only do know that any vibrations of 

 this kind, which are excited by the reflection and refraction of 

 light, are certainly of very small energy compared with the 

 energy of the light from which they proceed. The fact of the 

 case as regards reflection and refraction is this, that unless the 

 luminiferous ether is absolutely incompressible, the reflection 

 and refraction of light must generally give rise to waves of con- 

 densation. Waves of distortion may exist without waves of 

 condensation, but waves of distortion cannot be reflected at 

 the bounding surface between two mediums wit^iout exciting in 

 each medium a wave of condensation. When we come to the 

 subject of reflection and refraction, we shall see how to deal 

 with these condensational waves, and find how easy it is to get 

 quit of them by supposing the medium to be incompressible. 

 But it is always to be kept in mind as to be examined into, are 

 there or are there not very small amounts of condensational 

 waves generated in reflection and refraction, and may, after 

 all, the propagation of electric force be by these waves of 

 condensation ? 



" Suppose that we have at any place in air, or in luminiferous 

 ether (I cannot distinguish now between the two ideas) a body 

 that, through some action we need not describe, but which is 

 conceivable, is alternatively positively and negatively electrified ; 

 may it not be that this will give rise to condensational waves ? 

 Suppose, for example, that we have two spherical conductors 

 united by a fine wire, and that an alternating electromotive 

 force is produced in that fine wire, for instance by an ' alter- 

 nate current ' dynamo-electric machine ; and suppose that sort 

 of thing goes on away from all other disturbance — at a great 

 distance up in the air, for example. The result of the action of 

 the dynamo-electric machine will be that one conductor will be 

 alternately positively and negatively electrified, and the other 

 conductor negatively and positively electrified. It is perfectly 

 certain, if we turn the machine slowly, that in the air in the 

 neighbourhood of the conductors we shall have alternately posi- 

 tively and negatively directed electric force with reversals of, for 

 example, two or three hundred per second of time with a 

 gradual transition from negative through zero to positive, and so 

 on ; and the same thing all through space ; and we can tell 

 exactly what the potential and what the electric force is at each 

 instant at any point. Now, does any one believe that, if that 

 revolution were made fast enough, the electro-static law of force, 

 pure and simple, would apply to the air at different distances 

 from each globe ? Every one believes that if that process be 

 conducted fast enough, several million times, or millions of 

 million times per second, we should have large deviations from 

 the electro-static law in the distribution of electric force through 

 the air in the neighbourhood. It seems absolutely certain that 

 such an action as that going on would give rise to electrical 

 waves. Now it does seem to me probable that those electrical 

 waves are condensational waves in luminiferous ether ; and pro- 

 bably it would be that the propagation of these waves would be 

 enormously faster than the propagation of ordinary light waves. 

 " I am quite conscious, when speaking of this, of what has 

 been done in the so-called electro- magnetic theory of light. I 

 know the propagation of electric impulse along an insulated 

 wire surrounded by gutta-percha, which I worked out 



