AN-D SOME A^^ALOGOUS RAYS. 
483 
between orightness and darkness (as viewed in the screen) occurring in the plane of 
the anticathodal surface. As was pointed out in a communication to the Physical 
Society," and often since noticed by other observers, a similar oblique delimitation is 
visible in the yellow-green luminescence upon the walls of the bulb. At the time it 
was supposed that this internal luminescence extending over the region traversed by 
the Rontgen rays was caused by them on their way through the glass wall of the 
bulb. That they are not due to Rontgen rays may be, however, readily shown. If 
a magnet-pole be brought close to the bulb near the delimiting edge of the patch of 
luminescence upon the glass, that edge is seen to be distorted. It is due therefore 
to rays that possess magnetic deflectibility. It is also possible to produce an electro¬ 
static distortion of this delimiting edge. But if with a barium platino-cyanide 
screen one observes the corresponding delimiting edge of the Rontgen ray emission 
in the same oblique plane, one finds that it undergoes neither electrostatic nor 
magnetic deflexion. A small displacement in some cases to be noticed is due to 
want of exact complaneity of the anticathode surface, and to a displacement by the 
magnet of the focus of the incident (ortlio-)cathodic beam. 
It therefove aj^pecivs that fvofti the anticathode surface there are eynittecl simul¬ 
taneously with the Rontgen rays, and with a similar ahnormcd lateral distribution, 
para-cathodic rays ivhich differ from the Rontgen rays in respect of their power of 
penetration, as well in being electrostatically and magnetically deflectible. They also 
differ from the Rontgen rays in being emitted at a lower degree of exhaustion than is 
necessaiy for the production of the former, and from ordinary {ortho-fathodic rays 
in not exciting Rontgen rays where they impinge on a solid surface. 
From the similarity in the abnormal distribution of the para-cathodic rays and of 
the Rontgen rays, it may be inferred that the physical processes concerned in their 
emission at the anticathode are similar. 
7. Sifting of Cathode Rays. 
It has long been known that cathode rays at different stages of exhaustion of the 
tube, and excited under different electromotive forces, differ in character from ore 
another. The changes in mag’netic deflectibility during the process of exhaustion, 
noticed by Crookes,! are familiar. The cathode rays observed outside the Crookes 
tube, by Lenard,J differ from those within in several physical respects, and also 
apparently from one another under different conditions of production. Wiedemann 
and Ebert in particular have dwelt on the heterog'eneity of cathode rays, and 
front and back of the anticatbode, which, as viewed edgeway,s in a luminescent screen, appears a 
a dark line between two dim luminous regions. See also a case I’ecorded in § 7 below. 
* ‘ Proc. Physical Society of London,’ June 12, 1896, and ‘ Philosophical Magazine,’ August, 1890. 
t ‘ Philosophical Transactions,’ 1879, Part 1., p. 160. 
X Wiedemann’s ‘Annalen,’ vol. 51, p. 225, 1894. 
3 Q 2 
