Manchester Memoirs, Vol. xli. (1897), No. 15. 5 



opaque to light, such, for example, as black paper, 

 board, and so forth. If that stood alone it would, 

 perhaps, not constitute a very important difference 

 between them and light. A red glass will stop green 

 rays and let red rays through ; and just in the same 

 way if the Rontgen rays were of the nature of the 

 ordinary rays of light, it is possible that a substance, 

 although opaque to light, might be transparent to them. 

 So, as I say, that remarkable property, if it stood alone, 

 would not necessarily constitute any great difference of 

 nature between them and ordinary light. But there are 

 other properties which are far more difficult to reconcile 

 with the idea that the Rontgen rays are of the nature 

 of light. There is the absence, or almost complete 

 absence, of refraction and reflection. Another remark- 

 able property of these rays is the extreme sharpness of 

 the shadows which they cast when the source of the 

 rays is made sufficiently narrow 7 . The shadows are far 

 sharper than those produced under similar circumstances 

 by light, because in the case of light the shadows are 

 enlarged as the effect of diffraction. This absence, or 

 almost complete absence, of diffraction is then another 

 circumstance distinguishing these rays from ordinary rays 

 of light. In face of these remarkable differences, those 

 who speculated with regard to the nature of the rays 

 were naturally disposed to look in a direction in which 

 there was some distinct difference from the process 

 which we conceive to go on in the propagation and 

 production of ordinary rays of light. Those who have 

 speculated on the dynamical theory of double refraction 

 have been led to imagine the possible existence in the 

 ether of longitudinal vibrations, as well as those trans- 

 versal vibrations which we know to constitute light. It 

 we were to suppose that the Rontgen rays are due to 

 longitudinal vibrations, that would constitute such a 



