54 " N n RAYS 



radiations foreign to " N " rays. In front of 

 those screens, and at a distance of 14 cms. from 

 the lamp filament, a large screen of wet card- 

 board is arranged, in which a slit has been cut 

 5 mms. wide and 3*5 cms. high, exactly opposite 

 the lamp filament. In this way I obtain a well- 

 defined pencil of " N " rays ; this pencil is 

 received on an aluminium prism whose refrac- 

 tive angle is 27 15', placed so that one of its 

 faces is normal to the incident pencil. 



It is, then, possible to prove that from the 

 other refractive face of the prism several pencils 

 of " N " rays, horizontally dispersed, emerge. 

 For this purpose a slit i mm. broad and i cm. 

 high, cut in a sheet of cardboard, is filled with 

 calcium sulphide rendered phosphorescent ; by 

 displacing this slit, the position of the dispersed 

 pencils is determined without difficulty, and the 

 deviations being known, their refractive indices 

 are deduced. This is the method of Descartes. 

 I thus established the existence of "N " radia- 

 tions, whose indices are respectively 1*04, 1*19, 

 1*29, 1*36, 1*40, 1*48, 1*68, 1*85. In order to 

 measure with more exactness the first two in- 

 dices, I made use of another aluminium prism 



