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



leading wires from the coherer are enclosed in a double coating of tin- 

 foil. 



Method of Experiment. 



I first tried to determine the iudex of refraction of sulphur. The 

 material used was ordinary commercial sulphur. A semi-cylinder 

 was made, and the two positions for total reflection determined by 

 the method which has already been described. The difference of 

 readings found for the two positions varied from 69° to 71° and the 

 value of the critical angle would from these experiments seem to lie 

 between 34<'5 and 35°. This approximate value for the critical angle 

 having been obtained, the experiment was modified to secure a greater 

 amount of accuracy. 



Two equal semi- cylinders P and Q were taken and placed on the 

 rotating table face to face, with an air film between. A metallic 

 plate with a narrow rectangular opening was also interposed between 



Fig-. 2. 



the semi-cylinders to serve as a diaphragm and cut off all but the 

 central rays. When the spark gap is placed at 0, the principal 

 focus of P, the rays emerge parallel into the air film, and are then 

 focussed by the second cylinder at an equal distance / on the other 

 side. 



The spark gap is placed at 0, and the receiver at 0', 00' being extre- 

 mities of a diameter passing through the centre of the circle. The 

 air film is for convenience placed parallel to the index. 



The platform carrying the cylinders is now rotated, say to the left. 



Fig. 3. 



(The dotted lines represent the two positions of the air film for total reflection.) 



