296 Dr. J. C. Bose. On the Determination of the Indices 



With small radiators the intensity of radiation is not very great. 

 This is a positive advantage in many experiments. It sometimes 

 becomes necessary to have greater intensity without the attendant 

 trouble inseparable from too long waves. I have made a new radiator, 

 where the oscillatory discharge takes place between two small circular 

 plates 12 mm. in diameter and an interposed ball of platinum 9*7 mm. 

 in diameter. The sparking takes place at right angles to the circular 

 plates. The intensity of radiation is by this expedient very greatly 

 increased. 



In my previous experiments to determine the index of refraction, 

 I used tubes to surround the radiator. This I was obliged to do to 

 protect the receiver as much as possible from external disturbances. 

 But this procedure maybe open to the objection that the sides of the 

 tube may send reflected waves. It is preferable to have a divergent 

 beam from a single source form a well-defined image after refraction. 

 Owing to the successful removal of the disturbing causes it is now 

 possible to allow the radiator to be placed in open space, a plate 

 with a rectangular aperture allowing the radiation to fall on the 

 refracting cylinder along a given direction. The size of the plate is 

 26 x 15 cm., and the aperture is 7 X 6 cm. (see fig. 1). The radiator 

 and the receiver are placed on opposite sides of the plate. Absence 

 of disturbance due to lateral waves was tested by closing the 

 aperture and observing whether the waves still affected the receiver 

 by going round the plate. The plate was found to act as an effective 

 screen. 



I have hitherto preferred the null method in my experiments, as it 

 possesses many advantages. The sensitiveness of the receiver can 

 be pushed to the utmost extent, and observations taken when no 

 effect is produced on the receiver. The total reflection method also 

 dispenses with the difficulty of making accurate measurement of the 

 deviation produced. After obtaining the value of the index by the 

 method described above, I was desirous to see whether it was not 

 possible to obtain fairly good results by measuring the angle of 

 refraction corresponding to a given angle of incidence. I shall pre- 

 sently describe the difficulties met with in these experiments, and 

 the manner in which they were to a great extent removed. 



The preliminary experiment was carried out with a single semi- 

 •cylinder. The angle of incidence was gradually increased by rotating 

 the cylinder, and the refracted beam was followed with the receiver. 

 In this way it was found that the rays ceased to be refracted when 

 the angle of incidence was about 28° 30'. The critical angle is 

 therefore 28° 30' and 



/i = 2-08 (1). 



T next used two semi-cylinders. The plane vertical face of the 



