60 "N" RAYS 



numbers deduced from observation. Although 

 I cannot with certainty indicate the degree of 

 approximation of the results, I believe, never- 

 theless, that the relative errors do not exceed 

 4 per cent. 



The wave-lengths of " N " rays are much 

 smaller than those of light. This is contrary 

 to what I had imagined for a moment, and con- 

 trary to the determinations which M. Sagnac 

 thought he had deduced from the position of 

 the multiple images of a source, obtained with 

 a quartz lens, images attributed by him to 

 diffraction. I had previously observed that 

 while polished mica lets " N " rays pass, 

 roughened mica stops them, and also that 

 whereas polished glass reflects them regularly, 

 ground glass diffuses them. These facts were 

 already an indication that " N " rays could not 

 have large wave-lengths. If we desire to study 

 the transparency of a body, we must take care 

 that the surface is well polished. Thus I had 

 at first classed rock-salt amongst opaque sub- 

 stances, because the specimen I used, having 

 been sawn from a large block, had remained 

 unpolished ; in reality, rock-salt is transparent. 



