286 CATHODE BAYS AND KONTGEN BAYS. 



double reflection, which are here insufficient, it is not surprising that 

 the Rontgen rays are deprived of this property. Thus deprived of all 

 its burdens and functions it yet possesses transverse vibrations, which 

 place it in the family of spectra; but in these surroundings, after all 

 the diminutions, restrictions, and limitations which it has undergone, 

 it appears rather like a mangy sheep. We have said that some phys- 

 icists are contented with this state of affairs. 



The same difficulties arise if the longitudial vibrations of the ether 

 are introduced into the theory, and there is added, moreover, the 

 uncertaintv of the existence of these vibrations. There is nothing to 

 prove, in truth, that they do not exist; on the contrary, it is evident 

 that they are formed as soon as luminous ra3"S change their direction 

 are reflected or refracted. They could not be neglected except by 

 regarding the ether as strictly incompressible. Some ph3^sicists affirm 

 that it is, and, in short, if one relies upon experimental grounds it is 

 sufficient to say that the longitudinal component can be neglected, 

 owing to its insigniiicance. This is true if one ignores all the- phe- 

 nomena which can accompan}^ the manifestation of light. 



In fact, by disregarding the longitudinal vibration, satisfactory agree- 

 ment, as is known, is found to exist between theory and experiment. 

 It is possible that the Rontgeti rjiy may be due to this longitudinal 

 vibration, but this remains to be proved. .laumann has endeavoi'ed to 

 demonstrate this, but was refuted by M. H. Poincare. 



Besides these explanations there is a third, which consists in saying, 

 with M. A. Schuster, that the vibration of the ether which yields the 

 Rontgen ray is not strictly periodic; periodicity being a condition of 

 interference a troublesome objection is thus removed. On the other 

 hand, explanations founded on the theory of the emission of matter 

 are also problematical. M. Jean Perrin claims that the Rontgen ray 

 is due to the vibration of atomic corpuscles, and is produced by their 

 violent encounter with material molecules. This hypothesis has also 

 the advantage of taking into consideration the conditions of its pro- 

 duction. In conclusion, very little is positively known of the nature 

 of this physical agent, which, to quote M. Bouty, has remained exceed- 

 ingly mysterious in spite of the united efforts of the scientitic world. 



