DISPERSION OF " N RAYS 



55 



having an angle of 60. I again found for one 

 of the indices the same value, i "04 ; and for the 

 other, 1-15 instead of 1*19. 



In order to control the results obtained by 

 the prisms, I determined the indices by produc- 

 ing, by means of an aluminium lens, images 

 of the lamp filament, and measuring their dis- 

 tances from the lens. The lens, which is plano- 

 convex, has a radius of curvature of 6*63 cms., 

 and an aperture of 6*8 cms. The slit of the wet 

 screen is widened so as to form a circular open- 

 ing 6 cms. in diameter ; the lens is placed at a 

 known distance (p cm.) from the incandescent 

 filament, and by means of the phosphorescent 

 sulphide, the position of the conjugate images 

 of the filament is determined. The following 

 table gives the values of the indices found, both 

 with the prism and the lens : 



Prisms. Lens. 



