276 CATHODE EAYS AND RONTGEN RAYS. 



upon the clear fluorescent background the exact silhouette of the cross. 

 In this way perfect geometric shadows of the objects introduced can 

 be obtained in every case. 



This experiment necessitates the conclusion that the cathode emis- 

 sion is rectilinear. The cathode, the screen, and the silhouette are all 

 on a straight line. Things occur, in short, as if a single ray left each 

 point of the cathode, exciting luminosity at the very spot where it 

 encounters the walls. Without prejudging in any way the nature of 

 the phenomenon, it is proper to use the expression cathode rays. 



A close stud}" of the shadows formed by divers screens, of the sil- 

 houettes outlined by these rays, leads to a new and instructive point; 

 it shows that they are implanted at right angles to the surface of the 

 electrode; they are perpendicular to it at every point. It must be 

 added, however, following Goldstein, that it is not a strict rule; if 

 accepted, it results that the shape of the pencil varies in a simple man- 

 ner with that of the cathode. The latter is sometimes arranged as a 

 slightly convex disk; thereupon the rays form the trunk of a cone 

 which strikes the walls of the tube like a circular skullcap. If the 

 cathode disk is a mirror with spherical concave surface the perpen- 

 dicular lines at the surface form a conic pencil and converge toward 

 the center of the image of the sphere, where they form a focus. The 

 effects peculiar to cathode rays are magnified by this concentration, in 

 the same manner that the effects of luminous rays are increased in the 

 focus of a lens. In this manner Crookes was able to show the heating 

 action of his supposed radiant matter; that is to sa}', of cathode rays. 

 He succeeded in fusing, at one of these foci, not only glass, but a wire 

 of iridium-platinum, an operation which requires a temperature of 

 more than 2,000^. 



It is not only at the end of its path at the point where it strikes the 

 walls of the glass tube that the cathode pencil can be rendered visible. 

 Hittorf and Goldstein, in 1876, furnished the means of rendering it 

 visible at all points of its path by discovering the phosphorogenic 

 power of the new rays. The illumination which these dark rays excite 

 in the glass of the bulb they also produce on other bodies placed in 

 the interior. Rock crystal appears of a blue color, precious stones of 

 divers colors, rubies project a beautiful red glow, diamonds take on 

 an extraordinary brilliancy. The earthy sulphides which are naturally 

 phosphorescent — that is to say, able to store up the luminous rays and 

 yield them up afterwards— are lighted up most vividly. Wurtzite 

 (crystallized sulphide of zinc) becomes dazzling. By arranging a frag- 

 ment of one of these substances in the path of the pencil, the latter 

 becomes visible throughout. It becomes possible in this way to study 

 the properties of cathode rays. 



The results of this study should be briefl}^ mentioned. In the first 

 place the two laws already announced are verified — that the cathode ray 



