DISPERSION OF "N" RAYS 57 



axis is exactly underneath the slit of the second 

 wet cardboard. By turning the arm, the path 

 of the pencil is exactly marked out, and one can 

 verify that it is quite unique, and is accom- 

 panied by no lateral pencil, such as diffraction 

 could eventually produce in the case of large 

 wave-lengths. 



A grating is then placed in front of the slit 

 of the second wet cardboard (for instance, a 

 B runner grating of 200 lines per mm.). If, now, 

 the emerging pencil is explored by turning the 

 arm which bears the phosphorescent sulphide, 

 the existence of a system of diffraction fringes 

 is confirmed, just as with light, only these 

 fringes are much closer together, and are 

 sensibly equidistant. This already indicates that 

 " N " rays have much shorter wave-lengths 

 than luminous radiations. 



The angular distance of the fringes, or what 

 amounts to the same thing, the rotation of the 

 arm corresponding to the passage of the phos- 

 phorescent slit from one luminous fringe to 

 the next, is very small. It is therefore 

 determined by the method of reflection, with 

 the aid of a divided scale and telescope, a 



