384 Scientijio Intelligence. 



electrostatic effect of charged particles moving to and from the 

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

 static effects, there must be a definite distribution of electrifica- 

 tion over the screen ; changes in this distribution, however, take 

 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 place 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 

 Rontgen 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 behavior of 

 these Lenard rays which do not admit of a very ready explana- 

 tion from this point of view, yet the difficulties in its way seem 

 to me considerably less than that of supposing that a wave in the 

 ether can change its velocity when moving from point to point in 

 a uniform magnetic field. 



I now pass 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 property known to be possessed 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 espe- 

 cially marked extent by the uranium salts. By means of this 

 radiation, which, since it can be polarized, 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 electrified 

 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 polarization. Let 

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

 which special rays of the spectrum pass 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. 

 Pfltiger 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 



