26 Recent Advances in Electricity. 



electrified particles travelling with various velocities, and those 

 which travel slowest are most deflected. 



But what theory are we to form as to the nature of X rays, 

 and in particular how are we to account for the fact that they do 

 not undergo refraction ? Refraction is explained on the wave 

 theory of light as being due to difference of velocity in diiferent 

 media. If the index of refraction in a piece of glass is i"5, light 

 travels i\ times as fast in air or vacuum as in this glass. Light 

 of short wave length, such as violet light, is more refracted 

 than red, because the difference between velocity in vacuum 

 and velocity in glass is greatest for the shortest wave lengths. 

 In vacuum long and short waves travel with one and the same 

 velocity. In glass the siiortest waves travel slowest. Absence 

 of refraction indicates that if the X rays are a manifestation of 

 wave motion, the waves are propagated with one and the same 

 velocity in allsubstances. The view which has been almost unani- 

 mously adopted by the highest authorities appears at first glance 

 very paradoxical. They maintain that the X rays represent 

 waves of much shorter wave length than those which constitute 

 light. They explain the absence of refraction by the shortness 

 of the wave length, in spite of the fact that in the case of light 

 the shortest waves are the most refracted. This looks like 

 madness, but there is method in it. The mjst successful 

 attempts that have been made to explain by mechanical analogies 

 the dependence of refrangibility on wave length are discussed by 

 Lord Kelvin in his Baltim.ore lectures. He works out in detail 

 the suggestion that each particle of matter is to be regarded 

 as having a heavy nucleus in its centre, which is elastically 

 connected with its outside. The waves of ether, which consti- 

 tute light, are hampered by the presence of the particles of 

 matter between which they have to pass, and, as a result of 

 continually repeated impulses ' given by the ether to the 

 material particles, the particles are set in forced vibration. 

 These forced vibrations of the particles of matter react upon 

 the ether, and as the result of an elaborate mathematical 

 investigation it comes out that the velocity of propagation of 



