Manchester Memoirs, Vol. xli. (1897), No. 15. 7 



can read the Russian language, which, unfortunately, I 

 cannot. I can, therefore, only guess what the method 

 was. It is something depending on the superposition 

 of sensitive photographic films. I suspect they had 

 several photographic films superposed, took the photo- 

 graphs on these, and then took them asunder for 

 development, and after development put them together 

 again as they had been originally. They consider that 

 they have succeeded in obtaining evidence of a certain 

 amount of polarisation. If we assume that evidence to 

 be undoubted, it decides the question at once. But as 

 the experiment, as made in this way, is rather a delicate 

 one, it is important for the evidence that we should 

 consider as well what we may call the Becquerel rays. 

 If time permits, I shall have something to say about 

 these towards the close of my lecture, but, for the 

 present, I shall say merely that they appear to be inter- 

 mediate in their properties between the Rontgen rays 

 and rays of ordinary light. The Becquerel rays un- 

 doubtedly admit of polarisation, and the evidence appears 

 on the whole pretty conclusive that the Rontgen rays, 

 like rays of ordinary light, are due to transversal, and 

 not to longitudinal, vibrations. It remains to be ex- 

 plained, if we can explain it, wherein lies the difference 

 between the nature of the Rontgen rays and rays of 

 ordinary light which accounts for the strange and 

 remarkable difference in the properties of the two. I 

 may mention that, although Cauchy and Neumann, and 

 some others who have written on the dynamical theory 

 of double refraction, have been led to the contemplation 

 of normal vibrations, Green has put forward what seems 

 to me a very strong argument against the existence of 

 normal vibrations in the case of light. The argument 

 Green used always weighed strongly with me against 

 the supposition that the Rontgen rays were due to 



