482 
DR. S. P. THO]\JPSON ON CATHODE RAYS 
held outside the tube near p, there will appear upon it, if the exhaustion is sufficiently 
high, a second shadow cast hy the Rontgen rays ; the size and position of this shadow 
being such as to indicate that it is cast by rays which have their point of origin also 
at A, and which have traversed the glass wall of the tube. If no perturbing 
magnetic or electrostatic influences are present, these two shadows lie geometrically 
on the same lines projected from A as origin. Both are sharply defined ; the internal 
one more so than the external one. But the external shadow due to PtONTGEN rays 
does not shift when a magnet is placed between B and ^9, Neither does the external 
Bontgen shadow change its shape or size when B is made anodic or cathodic. If the 
shadows are simultaneously observed it will be seen that one and the same object’ 
illuminated from one and the same point of origin, is capable of casting two shadows 
of difierent shape in different directions at the same time. It is clear that there are 
present tv^^o kinds of rays, which differ in their deflectibility by magnetic and by 
electrostatic forces, and in their power of penetrating glass. 
These internal rays which are deflectible are not, however, ordinary cathode rays. 
Cathode rays proper, at a sufficiently high exhaustion, possess the characteristic 
property of exciting Eoxtgen rays wherever they impinge upon solid, or as Bom 
has shown,* upon liquid matter. These internal rays fail to exhibit, either at high 
or low exhaustion, any trace of this jmoperty. There are produced, as is shown below, 
under certain conditions, some other internal rays, which also differ from ordinary 
cathode rays. Hence it becomes necessary to distinguish the different species by 
adopting an appropriate nomenclature. The ordinary cathode rays may be called 
ortho-cathodic ; while the internal rays described above, which are emitted along 
with Bontgen rays at the surface of the anticathode, may be termed para-catliodic. 
The emission of these para-cathodic rays demands especial attention. They may 
be observed in any Bontgen ray tube of the focus type. As pointed out by the 
author,! in April, 1896, the emission of Bontgen rays from the surface of a plane 
anticathode follows a distribution entirely different from that of the emission of any 
known kind of light. It does not follow Lambert’s law of the cosine, the intensity 
remaining nearly uniform right up to a grazing angle, where it abruptly ends. This 
is demonstrated by photometric measurements made on the luminosity of a barium 
platino-cyanide screen placed near to the bulb. As viewed in such a screen the 
emission of Bontgen rays is confined to the region in front of the plane of the anti- 
cathode ; the whole hemispherical space in front being filled with them, while 
the whole hemispherical space behind is devoid of them;| a sharp delimitation 
* Mem. Accad. Lineri, s. V., vol. 2, July, 189G. 
t ‘ Comptes Rendus,’ loc. cit. See also ‘ Philosophical Magazine,’ August, 1896, p. 156, and ‘ Pi’oc. 
Royal Institution,’ May 8, 1896. 
t This is the state of things when the exhaustion is sufficient. But the autlior has many times 
obsei’ved, and has put on record (‘ Comptes Rendus,’ loc. cit.) the circumstance that during exhaustion, 
and at the stage at which Rontgen raj'S just first appear to be emitted, they are emitted from both 
