X-RAYS AND CRYSTALS 373 



with the rays from a bulb and it is almost incredible that the 

 reappearance of a definite amount of energy associated with 

 a cathode ray in these transformations should not be due to 

 the energy also being associated with the intermediate X-ray. 

 Yet this seems impossible when the energy of the X-ray is 

 spread over a wave front. However, the more paradoxical the 

 case seems, the more interesting it becomes ; indeed, there can be 

 no doubt that this new effect must go far towards solving that 

 puzzling problem, the nature of X-rays. 



Electromagnetic waves are now known to us of all wave 

 lengths over a range of many octaves. When Hertz first obtained 

 the electromagnetic vibrations predicted by Maxwell in his theory 

 of light, there was a vast gap between the wave lengths corres- 

 ponding to the frequencies of his oscillators and those of visible 

 light, the former being something like a million times the 

 latter. This gap has now been narrowed until it can be said 

 to have been abolished. On the one hand, the investigation 

 of the spectrum of light from hot bodies has been pushed far 

 into the infra red, heat waves of longer and longer wave length 

 being discovered by means of a radiomicrometer. These long 

 waves are isolated by different methods, such as continued 

 reflection on a surface of rock salt or sylvite or by making use of 

 their strong refraction by quartz ; their wave length may be 

 found by interferometer methods. In this way, Rubens and 

 Wood have been able to show the existence of heat waves as 

 long as T V mm. in the radiation from a Welsbach burner. 



On the other hand, by the use of improved forms of 

 oscillators, very much shorter electromagnetic waves have 

 been got than those which Hertz investigated. The shortest 

 waves as yet obtained have a wave length of 2 mm. Thus 

 there is hardly any gap left in the spectrum and one may now 

 say that all wave lengths greater than those of visible light 

 are at our command. 



The wave length of visible light lies between 4 x io -5 cm. 

 and 7 x io -5 cm. and the known range of the spectrum extends 

 on the other side to the smaller waves composing ultraviolet 

 light, the region investigated photographically. Here waves as 

 short as 1 x io~ 6 cm. have been found by Schumann and are 

 named after the discoverer. Until quite recently, the spectrum 

 as known to us has ended at this wave length. 



