162 Prof. J. C. Bose. On the Indices of Refraction [Dec. 12, 



accuracy. The receiver is now placed at R', and the cylinder rotated 

 in the opposite direction till total reflection again takes place. The 

 difference between the first and second readings is evidently equal to 

 twice the critical angle. 



To utilise only the central rays, a metallic screen with a small 

 central opening is placed against the plane face of the semi-cylinder. 

 In order that all the rays should undergo total reflection simultane- 

 ously, it is necessary that the rays incident on the plane of separation 

 of the two media should be parallel. This is effected by the cylinder 

 itself. From the approximate value of jx found from a preliminary 

 experiment, the focal distance of the semi-cylinder is roughly calcu- 

 lated. The spark-gap of the radiator is placed at this focus, and the 

 rays thus rendered very near parallel. Each subsequent experiment 

 gives a more accurate value of jll, and from the corrected value of the 

 focal distance thus obtained, a more accurate adjustment is made for 

 the next experiment. 



Apparatus Used. 



The apparatus used consists of an electric radiator emitting short 

 waves, a large graduated circle provided with a central circular plat- 

 form on which the refracting substance is placed, and a receiver, 

 which responds to the electric radiation. 



The Radiator. — I have given a detailed description of the electric 

 radiator in my paper on polarisation of electric rays (vide Journal, 

 Asiatic Society of Bengal, Part 2, ~No. 2, 1895). Electric oscillation 

 is produced by sparking between two metallic beads and an inter- 

 posed metallic sphere 1 cm. in diameter. By a single sudden break 

 of the primary in a Ruhmkorff's coil, a flash of radiation is emitted. 

 The spark gap is placed at one end of a brass tube 5 cm. in diameter. 

 By a sliding arrangement, the length of the tube may be varied. 

 The Ruhmkorff's coil is enclosed in a copper box. 



The Circle. — The circle has a diameter of 45 cm., and is graduated 

 into degrees, but one-fourth of a degree may be easily estimated. 

 The circle, as a whole, may rotate round a vertical axis which passes 

 though the centre of a massive stand. There is a raised circular 

 platform at the centre of the circle on which the refracting substance 

 is placed. This platform carries an index, and may be rotated inde- 

 pendently of the large circle. When the platform index is clamped, 

 the two circles rotate together. 



The h'efr acting Substance. — For substances which can be cast, the 

 molten mass is poured into a cylindrical mould with a thin partition 

 in the middle. In this way two equal semi-cylinders are obtained at 

 each casting. Substances like wood or stone are turned, and the 

 cylinder sawn into two equal halves. In my experiments different 

 sized cylinders were used. I have successfully used small ones with 



