44 



SCIENCE. 



[N. S. Vol XII. No. 289. 



intensity and magnetic deflection.* In 

 passing through the air or other gases the 

 rays were observed to suffer diffusion simi- 

 lar to that experienced by light in a turbid 

 medium. It was found that the absorption 

 and diffusion of the rays were approxi- 

 mately proportional to the density. The 

 magnetic deflection, on the other hand, was 

 independent of the medium in which the 

 rays were observed, and remained the same 

 even after the rays had passed through thin 

 sheets of metal. 



By changing the conditions under which 

 the rays were generated, different kinds of 

 kathode rays were obtained, whose pene- 

 trating power and susceptibility to the ac- 

 tion of a magnetic field could be varied 

 through a wide range. Thus, upon reduc- 

 ing the pressure in the tube where the rays 

 were developed, the penetrating power of 

 the rays was found to increase, while at the 

 same time the magnetic deflection became 

 steadily less. In connection with this work 

 Lenard called attention for the first time to 

 the so-called ' magnetic spectrum ' of the 

 kathode raysf a phenomenon which was 

 rediscovered by BirkelandJ in 1896 and 

 has since attracted considerable attention. 

 It appears that a beam of kathode rays is 

 ordinarily not homogeneous, but that it 

 consists of rays which are magnetically de- 

 flected in different degrees. In conse- 

 quence, the fluorescent patch produced by 

 such a beam, after passing through a mag- 

 netic field, is no longer sharply defined. 

 In many cases it is drawn out into an in- 

 terrupted band, in which regions of bright 

 fluorescence alternate with regions of com- 

 parative darkness. The resemblance to a 

 banded or bright line spectrum is often 

 quite striking. The phenomenon is now 

 known to be due to the employment of a 

 fluctuating or interrupted current in devel- 



» Wied. Ann., 52, p. 23, 1894 ; 56, p. 255, 1895. 



t Wied. Ann., 52, p. 32, 1894. 



f Comptes rendus, 123, p. 492, 1896. 



oping the rays.* Since the character of the 

 kathode rays is so largely dependent upon 

 the conditions under which they are devel- 

 oped, it is natural to expect that when these 

 conditions are unsteady the rays obtained 

 will be non-homogeneous. If the rays are 

 developed by a steady current, the magnetic 

 spectrum is reduced to a single bright line. 



Without stopping to discuss further the 

 interesting and important phenomena in- 

 vestigated by Lenard, let us consider for a 

 moment the bearing of his work upon the 

 two opposing theories of the kathode rays. 

 Upon the assumption that the rays con- 

 sisted of some sort of wave motion, all 

 Lenard's results were readily explained. 

 That such waves should pass through air, 

 and even through thin layers of metal, was 

 to be expected ; the same is true with ordi- 

 nary light. To explain the diffusion of the 

 rays, it was sufiicient to assume that the 

 wave length was small compared with the 

 dimensions of a molecule. The same as- 

 sumption explained the observed relation 

 between absorption and density. The dif- 

 ficulty in accounting for the magnetic de- 

 flection of the rays still remained. But this 

 difiSculty was no greater than it had always 

 been, and seemed by no means insurmount- 

 able. 



On the other hand, to interpret Lenard's 

 results in accordance with the Crookes 

 theory, in the form that it then took, was a 

 matter of great difficulty. That exces- 

 sively short waves should be able to pass 

 through metal is reasonable enough ; but 

 that atoms or molecules should be able to 

 pass is hard to believe. Yet, according to 

 Lenard's experiments, not only must these 

 atoms pass through a grounded sheet of 

 aluminium, carrying with them their elec- 

 tric charge, but they must emerge from the 

 other side with their momentum sensibly 

 unaltered. The suggestion was indeed 

 made by the advocates of the Crookes the- 



*Strutt, Fhil. Mag., 48, p. 478, 1899. 



