16 "N" RAYS 



transmitted through an aluminium sheet, are 

 reflected by a polished plate of glass in con- 

 formity with the laws of regular reflection, and 

 are diffused by a plate of ground glass. 



These radiations traverse all the sub- 

 stances whose transparency I tested, with the 

 exception of rock-salt 3 mms. thick (note 4), 

 platinum 4 mms. thick, and water. A slip of 

 cigarette paper, which is completely trans- 

 parent when dry, becomes absolutely opaque 

 when wetted with water. Figs. 2 and 3 are 

 reproductions of the impression made in four 

 seconds on a sensitive plate, without any 

 photographic apparatus, before and after 

 wetting the sheet of paper interposed 

 between the lens and the spark. The photo- 

 engraving, produced from a paper print, shows 

 that in the first case the spark is notably 

 brighter. 



These photographic prints are produced by 

 the spark influenced by the rays, and not by 

 the rays themselves, these latter producing no 

 appreciable photographic effect after an hour's 

 exposure. 



Amongst the bodies which are traversed, I 



