September 18, 1896.] 



SCIENCE. 



397 



off electrostatic effects, there must be a 

 definite distribution of electrification 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 strik- 

 ing against the window, would give rise to 

 electro-motive forces in the region outside 

 the window, and would produce convection 

 currents in the gas which has been made a 

 conductor by the Eontgen rays. The Le- 

 nard rays would thus be analogous in char- 

 acter to the cathode rays, both being con- 

 vective currents of electricity. Though 

 there are some points in the behavior of 

 these Lenard rays which do not admit of a 

 very ready explanation 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 

 Eontgen 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 in- 

 deed that they are vibrations at all. As a 

 working hypothesis,however,it may be worth 

 while considering the question whether there 

 is any property known to be possessed by 

 the^e 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 radiation, pos- 

 sessing many of the properties of the Eont- 



gen rays, which is emitted by many fluores- 

 cent substances, and to an especially marked 

 extent by the uranium salts. By means of 

 this radiation, which, since it can be polar- 

 ized, is unquestionably light, photographs 

 through opaque substances similar, though 

 not so beautiful to those obtained by means 

 of Kontgen rays, can be taken, and, like the 

 Eontgen 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 Eont- 

 gen rays differ from light is in the ab- 

 sence of refraction and perhaps of polariza- 

 tion. Let us consider the absence of refrac- 

 tion 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 

 allowed some rays to pass through them 

 without 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 ad- 

 dition to this, the theory of dispersion 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 vibra- 

 tion, and one only ; the frequency of this 

 vibration is represented by OK in the dia- 

 gram. The refractive index increases with 

 the frequency of the light until the latter is 

 equal to the frequency of the natural vibra- 

 tion of the substance ; the refractive index 

 then diminishes, becomes less than unity, 

 and finally approaches unity, and practi- 

 cally is 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 con- 

 sists of a pair of oppositely electrified atoms, 



