of Refraction of various Substances for the Electric Ray, 299 



Fig. 1. — The electric refractometer : P, the plate with a diaphragm ; 0, the 

 semi-cylinder of glass ; S, the shield (only one shown in the diagram) ; 

 R, the receiver. 



than the true reading and gradually increasing the angle till there 

 was a response. The receiver was then placed at a greater angle, 

 and the angle gradually reduced till the receiver again responded. 

 In this way a series of readings for a particular angle of incidence 

 was obtained. These readings were found fairly concordant, the 

 maximum variation from the mean being not so great as 1°. One 

 set of readings being taken on one half of the spectrometer circle, the 

 cylinder was rotated in the opposite direction, and readings taken on 

 the other side. 



Angle of Incidence. 



Angle of Refraction. 



ft. 



Reading to 

 the right. 



Reading to 

 the left. 



Mean. 



15° 



31° 0' 

 31 

 31 30 

 31 30 



31° 30' 



30 30 



31 30 

 31 30 



31° 15' 



2 00 



20° 



45° 30' 

 45 30 



44 30 



45 30 



45° 30' 

 46 



44 



45 30 



45° 15' 



2-08 



22° 



48° 0' 

 50 



49 30 



50 



49° 30' 

 50 30 

 48 30 

 50 



49° 30' 



2-03 



Mean value of fi = 2'04 (3) . 



