RONTGEN RAYS 313 



It is impossible, in the space at disposal, to give more than 

 a notion of the points involved ; the reader is referred to the 

 original papers. Marx has replied to his critics ; but one is left 

 with the unsatisfactory feeling that much reliance cannot be 

 placed on the value that experiment has so far given us for the 

 speed of the X rays. If such velocity were certainly known, it 

 would be of great value in discriminating between the two 

 theories of the Rontgen rays. 



Diffraction of Rontgen rays. — A good deal of ingenuity has 

 been exercised in testing whether, as in the case of light, there 

 are, on the boundaries of the shadows cast by small obstacles, 

 variations in the intensity of the X rays, corresponding to the 

 well-known diffraction fringes. Rontgen (1898) could not satisfy 

 himself on the point. Haga and Wind (1899-1901) worked with 

 a V-shaped slit, a few thousandths of a millimetre broad at its 

 widest point, and obtained in their photographs of the slit 

 broadenings of the narrow part of the image : if the effect were 

 one due to diffraction, the same amount of broadening with light 

 would be associated with a wave length of about 1*3 x io~ 8 cm. 

 On the pulse theory of the Rontgen rays, this would be the 

 order of the pulse thickness of the particular rays used. 



Walter and Pohl (1908) have recently repeated Haga and 

 Wind's experiments. The width of the slit-image, they find, 

 depends on the amount of energy sent through the slit, so that 

 different times of exposure give rise to different images. Haga 

 and Wind's measurements they consider to be in error, owing 

 to contrast effects produced by prolonged photographic develop- 

 ment and the method of measuring the slit-image on the plate. 

 Walter and Pohl conclude that the diffraction of the X rays 

 has not been established, and that the pulse thickness — if such 

 exists — is well under io -5 cm. 



We are now in a position to consider in turn the two theories 

 which offer interpretation of the nature and behaviour of the 

 Rontgen rays. 



The Ether Pulse Theory. — Sir George Stokes, in the Wilde 

 Lecture given before the Manchester Philosophical Society on 

 July 2, 1907, propounded the pulse theory of the Rontgen rays. 

 Of the nature of his conception we read that "... when these 

 charged molecules (from the cathode) strike the target, it is 

 exceedingly probable that by virtue of their charge they produce 

 some sort of disturbance in the ether. This disturbance ... or 



