1895.] of various Substances for the Electric Ray. 



161 



For the accurate measurement of deviation, the effect produced by 

 radiation on the receiver should undergo an abrupt variation. When 

 the radiation passes from a dense to a light medium, at a certain 

 critical angle of incidence, the radiation is totally reflected. From 

 the critical angle the index of refraction is easily determined. The 

 great advantage of this method lies in the fact that the transition 

 from refraction to total reflection is very sudden. 



I have determined the /t of various substances for the electric ray, 

 by the method of total reflection. 



It will be seen from the results of the experiments, that this method 

 is capable of giving very good results. 



The refracting substance is cut out or cast in the form of a semi- 

 cylinder, and mounted on the central table of a spectrometer ; the 

 electric ray is directed towards the centre of the spectrometer, and 

 its direction is always kept fixed. It strikes the curved surface and 

 passes into its mass without any deviation. It is then incident on 

 the plane surface of the semi-cylinder, and is refracted into the air 

 beyond. 



The incident angle on the plane surface is increased or decreased 

 by rotating the central table on which the cylinder rests. In practice, 

 it is more convenient to commence the experiment with an angle of 

 incidence greater than the critical angle, the incident ray being then 



Fig. 1. 



totally reflected. The angle of incidence is slowly decreased till the 

 critical value is reached. At this point the ray is all at once refracted 

 into the air, making an angle of 90° with, the normal to the surface. 

 If a receiver be fixed against the side of the semi-cylinder at R, it 

 will now respond to the refracted radiation. 



The platform on which the cylinder rests, carries the usual index. 

 By alternately rotating the platform in one direction or the other, 

 and observing the position of the index when the receiver just 

 responds, the reading for the critical angle is obtained with great 



