64 Prof. J. G. Bose on a Complete Apparatus for 



Determination of the Indices of Refraction. — For the deter- 

 mination of the index the prism-method is not very suitable. 

 I found the following to yield good results, the method de- 

 pending on the determination of the critical angle. Two semi- 

 cylinders of the given substance separated by an air-space are 



Fig. 5. 



placed on the platform. When the radiator is placed at the 

 principal focus of one of the semi-cylinders the rays emerge 

 parallel into the air-film, and are then focussed on the receiver 

 by the second semi-cylinder. A metallic plate with a narrow 

 rectangular opening is interposed between the semi-cylinders 

 to serve as a diaphragm, and cut off all but the central rays. 

 As the platform is rotated, the incident angle on the plane 

 surface separating the two media is gradually increased till 

 the rays undergo total reflexion. When this is the case the 

 receiver, which is- placed opposite the radiator, suddenly ceases 

 to respond. The trouble of following the deviated ray is thus 



Fig. 6. 



(The dotted lines show the two positions of the air-film for 

 total reflexion.) 



obviated ; the reading is also well defined as the transition 

 from refraction to total reflexion is abrupt. The index- 

 reading is now taken, and the cylinders rotated in an opposite 

 direction till total reflexion takes place a second time. The 

 difference of readings as given by the index in the two 

 positions is evidently equal to twice the critical angle. Hence 

 the value of the index can easily be deduced. 



A preliminary experiment gives the approximate value of 

 the index, from which the focal distance of the semi-cylinder 

 is roughly calculated. The spark-gap of the radiator is placed 



