September 17, 1896] 



NA TURE 



47: 



depended on the strength nf the magnetic field, or that 

 the magnetic field, by distorting the shape of the 

 boundary of the negative darU space, changed the direction of 

 the wave front, and so produced a deflection of the rays. The 

 chief reason for supposing that the kathode rays are a species of 

 wave motion is afforded by Lenard's discovery, that when the 

 kathode rays in a vacuum tulie fall on a thin aluminium window 

 in the tube, rays having similar properties are observed on the 

 side of the window outside the tube ; this is readily explained on 

 the hypothesis that the rays arc a sjjecies of wave motion to 

 which the window is partially transparent, while it is not very 

 likely that ])articles of the gas in the tube could force their way 

 through a piece of metal. This discovery of Lenard's does 

 not, however, seem to me incompatible with the view that 

 the kathode rays are due to negatively charged particles moving 

 with high velocities. The space outside Lenard's tube must 

 have been traversed by Rcintgen rays, these would put the 

 surrounding gas in a state in which a current would be readily 

 started in the gas if any electromotive force acted upon it. Now, 

 though the metal window in Lenard's experiments was connected 

 with the earth, and would, therefore, screen off from the outside 

 of the tube any effect arising from slow electrostatic changes in 

 the tube, it does not follow that it would be able to screen off 

 the electrostatic effect of charged particles moving to and from 

 the tube with very great rapidity. For in order to screen off 

 electrostatic effects, there must be a definite distribution of 

 electrification over the screen ; changes in this distribution, 

 however, lake a finite time, which depends upon the dimensions 

 of the screen and the electrical conductivity of the material of 

 which it is made. If the electrical changes in the tube take 

 lilace at above a certain rate, the distribution of electricity on 

 the screen will not have time to adjust itself, and the screen will 

 cease to shield off all electrostatic effects. Thus the very rapid 

 electrical changes which would take place if rapidly moving 

 charged bodies were striking against the window, might give 

 rise to electromotive forces in the region outside the window, 

 and produce convection currents in the gas which has been made 

 a conductor by the Riintgen rays. The Lenard rays would thus 

 be analogous in character to the kathode rays, both being 

 convective currents of electricity. Though there are some points 

 in the behaviour of these Lenard ra)'s which do not admit of a 

 very ready explanation from this point of view, yet the 

 <lifticulties in its way seem to me considerably less than that of 

 sup))osing that a wave in the ether can change its velocity when 

 mo\ing from point to point in a uniform magnetic field. 



I now pa.ss on to the consideration of the Rontgen rays. We 

 are not yet acquainted with any crucial experiment which shows 

 unmistakably that these rays are waves of transverse vibration in 

 the ether, or that they are waves of normal vibration, or indeed 

 that they are vibrations at all. As a working hypothesis, how- 

 ever, it may be worth while considering the question whether 

 there is any )>roperty known to be po.ssessed by these rays which 

 is not possessed by some form or other of light. The many 

 forms of light have in the last few months received a noteworthy 

 addition by the discovery of M. Becquerel of an invisible radia- 

 tion, possessing many of the properties of the Rontgen rays, 

 which is emitted by many fluorescent substances, and to an 

 especially marked e.xtent by the uranium salts. By means of 

 this radiation, which, since it can be polarLsed, is unquestionably 

 light, photographs through opaque substances similar to, though 

 not so beautiful as, those obtained by means of Rontgen rays, 

 can be taken, and, like the Rontgen rays, they cause an elec- 

 trified body on which they shine to lose its charge, whether this 

 be positive or negative. 



The two respects in which the Rontgen rays differ from light 

 is in the absence of refraction and perhaps of polari.sation. Let 

 us consider the absence of refraction first. We know cases in 

 which special rays of the spectnmi pa.ss from one substance to 

 another without refraction ; for example, Kundt showed that 

 gold, silver, copper allow some rays to pass through them with- 

 out bending, while other rays are bent in the wrong direction. 

 Pfliiger has lately found that the same is true for .some of the 

 aniline dyes when in a solid form. In addition to this, the 

 theory of di.spersion of light shows that there will be no bending 

 when the frequency of the vibration is very great. I have here 

 a curve taken from a paper by Helmholtz, which shows the rela- 

 tion between the refractive index and the frequency of vibration 

 for a substance whose molecules have a natural period of vibration, 

 and one only; the frequency of this vibration is represented by OK 

 in the di.tgram. The refractive index increases with the frequency 



NO. 1403. VOL. 54] 



of the light until the latter is equal to the frequency of the natural 

 vibration of the substance ; the refractive index then diminishes, 

 becomes less than unity, and finally approaches unity, and is 

 practically equal to it when the frequency of the light greatly 

 exceeds that of the natural vibration of the molecule 

 Helmholtz's results are obtained on the supposition that a 

 molecule of the refracting substance consists of a pair of 

 oppositely electrified atoms, and that the specific inductive 

 capacity of the medium consists of two parts, one due to the 

 ether, the other to the setting of the molecules along the lines of 

 electric force. 



Starting from this supposition we can easily see without 

 mathematical analysis that the relation between the refractive 

 index and the frequency must be of the kind indicated by the 

 curve. Let us suppose that an electromotive force of given 

 amplitude acts on this mixture of molecules and ether, and let 

 us start with the frequency of the external electromotive force 

 less than that of the free vibrations of the molecules : as the 

 period of the force approaches that of the molecules, the 

 effect of the force in pulling the molecules into line will 

 increase ; thus the specific inductive capacity, and therefore the 

 refractive index increases with the frequency of the external 

 force ; the effect of the force on the orientation of the molecules 

 will be greatest when the period of the force coincides with that 

 of the molecules. As long as the frequency of the force is less 

 than that of the molecules, the external field tends to make the 

 molecules set so as to increase the specific inductive capacity of 

 the mixture ; as soon, however, as the frequency of the force 

 exceeds that of the molecules, the molecules, if there are no 

 viscous forces, will all topple over and point so as to make the 

 part of the specific inductive capacity due to the molecules of 

 opposite sign to that due to the ether. Thus, for frequencies 

 greater than than of the molecules, the specific inductive 

 capacity will be less than unity. When the frequency of the 

 force only slightly exceeds that of the molecules, the effect 

 of the external field on the molecules is very great, so that 

 if there are a considerable number of molecules, the 

 negative part of the specific inductive capacity due to the 

 molecules may be greater than the positive part due to the 

 ether, so that the specific inductive capacity of the mixture of 

 molecules and ether would be negative ; no waves of this period 

 could then travel through the medium, they would be totally 

 reflected from the surface. 



As the frequency of the force gets greater and greater, its 

 effect in making the molecules set will get less and less, but the 

 waves will continue to be totally reflected until the negative 

 part of the specific inductive capacity due to the molecules is 

 just equal to the positive part due to the ether. Here the 

 refractive index of the mixture is zero. As the frequency of the 

 force increases, its effect on the molecules gets less and less, so 

 that the specific inductive capacity continually approaches that 

 due to the ether alone, and practically coincides with it as soon 

 as the frequency of the force is a considerable multiple of that 

 of the molecules. In this case both the specific inductive 

 capacity and the refractive index of the medium are the same as 

 that of the ether, and there is consequently no refraction. Thus 

 the absence of refraction, instead of being in contradiction to 

 the Rontgen rays, being a kind of light, is exactly what we 

 should expect if the wave length of the light were exceedingly 

 small. 



The other objection to these rays being a kind of light is, 

 that there is no very conclusive evidence of the existence of 

 polarisation. Numerous experiments have been made on the 

 difference between the absorption of these rays by a pair of 

 tourmaline plates when their axes are crossed or parallel. 

 Many observers have failed to ob.serve any difference at all 

 between the absorption in the two cases. Prince Galitzine and 

 M. de Karnojitsky, by a kind of cumulative method, have 

 obtained photographs which seem to show that there is a 

 slightly greater absorption when the axes are crossed than there 

 is when the axes are parallel. There can, however, be no 

 question that the effect, if it exists at all, is exceedingly small 

 compared with the corresponding effect for visible light. 

 Analogy, however, leads us to expect that to get polarisation 

 effects we must use, in the case of short waves, polarisers of a much 

 finer structure than would be necessary for long ones. Thus a wire 

 bird-cage will polarise long-electrical waves, but will have no effect 

 on visible light. Rubens and Du Bois made an instrument 

 which would polarise the infra-red rays by winding very fine 

 wires very close together on a franie\vi;trk ; this arrangement, 



