SIR G. G. STOKES, BART,, ON THE RONTGEN RAYS. 21 



ordinary light, and opaque even to the ultra violet with 

 which we were previously acquainted. They pass freely 

 through black paper, through cork, wood, or even through 

 the flesh of the hand, though less freely through the bones, 

 so that by simply laying his hand upon the case containing 

 the photographic plate, he actually got a photograph of the 

 bones of his hand. 



Well, what is the nature of these rays, and from whence 

 do they come ? As Rontgen said in his original paper, a 

 slight examination shows that they have their origin in the 

 part of the Crookes tube opposite to the cathode, and which 

 is rendered phosphorescent by the discharge from the 

 cathode. 



The rays, however, which come from this part of the tube, 

 and which appear to have their origin there, difier utterly in 

 some respects from the so-called cathodic rays. If you 

 isolate a portion of them, you find that a magnet has no 

 action upon them ; unlike the cathodic rays, they proceed 

 onwards without deflection, just as if the magnet were not 

 there. Like light they proceed in a straight course, but 

 these rays are able to pass through a variety of substances 

 that are opaque to ordinary light, while on the other hand 

 they are stopped by other substances which let light freely 

 through. That, however, does not prove that they are not 

 of the nature of light. You may have, suppose, a red glass 

 which is opaque to green rays, but lets red rays through 

 very freely, so that as regards merely the fact of the "X" 

 rays being stopped by substances transparent to light, while 

 they pass more or less freely through other substances which 

 are quite opaque to ordinary light, that establishes no greater 

 distinction than exists between green and red light. Are 

 they then of the same nature as light ? 



The "X" rays have some very remarkable properties by 

 which they appear at first sight to differ in totoixonx ordinary 

 light. They pass with either no refraction, or excessively 

 small refraction, through prism-shaped bodies, which we know 

 rays of light do not. They suffer hardly any, if any, regular 

 reflection, unless perhaps at a grazing incidence. 



Rontgen himself, in his original paper, dwelt on these 

 peculiarities of the new rays. He formed a prism of alu- 

 minium, with which he attempted to obtain deviation of the 

 new rays, but the experiment showed that if there were any 

 deviation at all, at any rate the refractive index could not 

 exceed 1-05. He speaks of the rays not being apparently 



