CATHODE RAYS AND RONTGEN RAYS. 283 



rays become the starting- point of Rontgen rays. It makes little differ- 

 ence whether a body is placed within the tube or whether it forms the 

 wall of the tube, nor is it of any importance whether it becomes fluor- 

 escent or not under the cathode action; from the moment that it 

 receives and arrests the first ray it generates the second. It has been 

 found advantageous to arrange a slight modification of the apparatus 

 in order to increase its power. An electrode is used having the form 

 of a spherical mirror which concentrates the cathode v^ys at a single 

 focus. Near it is arranged a platinum foil or some other infusible sub- 

 stance which intercepts the cathode emission and arresting - it trans- 

 forms it into Rontgen rays, which pass through the thinnest point of 

 the tube and may be collected without. This apparatus is called a 

 focussing tube. 



The Rontgen ray is plainly to be distinguished from the cathode 

 ray, which has given it birth by several characters, of which the two 

 most essential, from a theoretical point of view, are that it is not 

 attracted by the magnet, and that it is not electrified. The cathode 

 ray, on the contrary, carries an electric current and can be deflected 

 by a magnet. On these two characteristics has been founded the 

 theory of its materiality, as we have already said. The}^ are wanting 

 in the Rontgen ray, therefore we can not be sure that it results from 

 the emissiom of matter. On the contrary, circumstances are in favor 

 of its immaterial, etherial, vibratory nature. 



To these two distinctive, essential, traits must be added the two 

 following, which are no less important: The cathode ray has not the 

 power of penetration. It is immediately absorbed or diffused; whereas 

 the Rontgen ray is very penetrating and nondiffusible. 



We have just seen that the Rontgen ra} 7 s originate at the point 

 where the cathode rays encounter solid substances. The violence of 

 the blow of the cathode projectile against the material molecule dis- 

 turbs it and increases its calorific energy; at the same time it makes 

 the surrounding ether oscillate and produces the fluorescence of 

 Crookes's tube. The operation which produces the X-ray } r ields then, 

 at the same time and accessorily, luminous rays (visible fluorescence), 

 and at other times chemical rays, ultraviolet ra}^s (invisible fluores- 

 cence), and probably still other unknown radiations. 



Setting aside these accessory radiations — that moreover may be 

 absent — in order to consider the principal one, we have said that the 

 latter is disclosed by its chemical action on the salts of silver (photo- 

 graphic impression) and by its power of exciting the luminosity of 

 phosphorescent screens. If an opaque body is placed in a straight 

 line between the source of the ray in the screen its shadow appears 

 thereon with an astonishing distinctness. The formation of these geo- 

 metric shadows proves a perfectly rectilinear propagation and justifies 

 the name of "ray" here employed. 



