312 A. A. Michelson— Theory of the "X-RaysP 



Art. XXXVII— A Theory of the " X-Rays ;" by Albert 

 A. Michelson. 



The principal facts, which any satisfactory theory of the 

 " X-rays " is called upon to explain, may be summarized as 

 follows : 



1. The production of the rays by electric impulse, at the 

 cathode,* in a highly exhausted enclosure. 



2. Propagation in straight lines and absence of interference, 

 reflection, refraction and polarization. 



3. The importance of density of the medium as the deter- 

 mining factor in the transmission of the rays. 



4. The production of fluorescence and actinic effects, and 

 the action on electrified conductors. 



Two theories have been proposed to account for these 

 remarkable phenomena: (1) the theory of longitudinal waves; 

 (2) the theory of projected particles. 



In reference to the first theory it may be said that unless it 

 is proved that an oscillatory discharge is essential to the pro- 

 duction of the X-rays, there can be no reason for supposing 

 that these rays are of aperiodic nature — that, they are wave- 

 motion as commonly understood. The absence of interference, 

 reflection and refraction is also a very formidable difficulty. 

 Attempts have been made to account for the absence of these 

 invariable accompaniments of every known form of wave- 

 motion, but, as I think, with very indifferent success. 



The most serious difficulty in the second theory is the 

 attempt to explain the passage of the electrified particles of 

 the residual gas (or of the electrode) through the walls of the 

 vacuum tube. The query at once arises, if glass is permeable 

 to these particles in virtue of their relatively great velocity, 

 why is it not permeable (in lesser degree) to the same particles 

 moving with smaller velocities ? That it is not, is evident from 

 the fact that vacuum tubes retain their high degree of exhaus- 

 tion unimpaired for years. 



In view of these difficulties, I would propose a third theory, 

 which may be called the " ether-vortex " theory. 



Let it be supposed that the X-rays are vortices of an inter- 

 molecular medium (provisionally, the etherf). These vortices 



* Even should further experiment prove that the X-rays proper originate at 

 the first obstruction encountered by the discharge, the fact remains that this dis- 

 charge originates at the cathode. 



f A possible objection occurs to the formation of ether -vortices in a medium 

 which is usually considered free from viscosity; but the fact that vibrating 

 molecules can and do communicate their motions to the surrounding ether shows 

 that the communication of vortex motion may also be possible. 



Though not a necessary part of. the theory, it may be considered that the 

 expulsion of the ether-vortices is due to an accumulation of ether in the cathode, 



