1895.] 



of various Substances for the Electric Ray. 



The angle of incidence is thus gradually increased, till the rajs ju&t 

 undergo total reflection. When this is the case the receiver ceases to 

 respond. Let A be the corresponding reading of the platform index. 

 A stationary index I' is now placed opposite the reading A of the 

 graduated circle. 



When the cylinder is rotated in the opposite direction a second 

 reading B for the critical angle is obtained. It is obvious that, 

 neglecting errors, A — B is equal to twice the critical angle. 



The platform index is now clamped and the circle as a whole is 

 rotated till B comes opposite to the fixed index I', The circle is now 

 clamped, the platform arm undamped, and the central table rotated 

 till another reading C for the critical angle is obtained. Then, as in 

 the previous case, B — C = 2i, where i is the critical angle. The 

 circle as a whole is now rotated till C comes opposite the fixed index. 



Fig. 4. 



E/, the Radiator ; C, the Coherer. 



Thus at each successive operation the circle is rotated past the 

 fixed index through 2i. The successive difference of readings of the 

 circle in reference to the fixed outside index, thus gives a series of 

 values of 2i. 



The result will be more accurate if we take the mean readings 

 -|(A + B), J?(B + C), .. .., and take their differences. Successive 

 readings are taken till the graduated circle is rotated as near as 

 possible through 360°. 



As has been said before, there are two semi-cylinders P and Q. 

 In the first set of experiments P is turned towards the radiator, 

 Q acting as a focussing lens. The circle at each successive operation 

 moves in a right-handed direction. 



In the second set of experiments Q is turned towards the radiator, 

 P acting as the converging lens. Successive readings are taken as 

 before, the circle now rotating in a hft-handed direction. 



