SIE G, G. STOKES, BART., ON THE EONTGEN KAYS. 23 



ii concave speculum at a small angle of incidence, and which 

 show for certain a minute regular reflection of X rays, while 

 at the same time they prove that the quantity of X light 

 returned by regular reflection is extremely small compared 

 witb that which comes from the mirror by some dift'erent 

 process. 



Now there is another remarkable property of these rays, 

 or absence of property if you like so to call it. Rays of 

 light, as we know, admit of diffraction. If jon pass light 

 from a luminous point through a very small slit, or a small 

 hole, the riband, or the beam of light at the other side does 

 not follow merely the geometrical projection of the slit or 

 hole as seen from the source of light, but is more or less 

 widened, and certain alternations of illumination are visible, 

 a phenomenon referable to interferences which I have not 

 time to go into. How do these " X " rays behave under such 

 conditions? It is a very remarkable thing that they do not 

 show these enlargements or exliibit any sign of interference. 



The last number of the Comptes Rendus contains a paper 

 by M. Gouy in continuation of a former paper, but describing 

 experiments carried out in a still more elaborate manner, 

 which proves the truth of this to a very high degree of strict- 

 ness. He makes out that if these X rays are periodical, the 

 wave length cannot well be more than the one-hundredth part 

 of the Avave length of green light, indicating an enormously 

 high degree of frequency. 



Now if we assume that the X rays, like rays of light, 

 and unlike the cathodic rays, are a disturbance propagated in 

 the ether, ponderable matter being concerned only in their 

 origination, not in their propagation, the question arises, 

 what is the relation between the direction of vibration and 

 the direction of propagation ? Are the vibrations normal or 

 transversal? We know that the vibrations of the air which 

 constitute sound take place in a to and fro direction, or are 

 what is called normal, that is, perpendicular to the waves of 

 sound, AVe have the fullest evidence that the vibrations of 

 the ether which constitute light take place in directions per- 

 pendicular to that of propagation, or are Avhatis called trans- 

 versal. To which category do the vibrations belong which 

 constitute the X rays ? 



If we could obtain polarisation or even partial polarisation 

 of the X rays, that would settle the question, and prove that 

 they are due to transversal vibrations. But most of those 

 Avho have attempted to obtain indications of their polarisation 



