298 Dr. J. 0. Bose. On the Determination of the Indices 



however, be placed on this expedient, when a very sensitive receiver 

 is used. 



After repeated trials with different forms of receiving tubes, I 

 found a form, to be described below, to obviate many of the diffi- 

 culties. Instead of a continuous receiving tube, I made two doubly 

 inclined shields, and placed them one behind the other, on the radial 

 arm which carries the receiver. The first shield has a tolerably 

 large aperture, the aperture of tke second being somewhat smaller. 

 The size of the aperture is determined by the wave-length of radia- 

 tion used for the experiment. It will be seen from this arrangement, 

 that the rays which are in the direction of the radial arm, can 

 effectively reach the receiver, the slanting rays being successively 

 reflected by the two shields. With this expedient, a great improve- 

 ment was effected in obtaining a definite reading. 



When the deviated rays are convergent, the receiver is simply 

 placed behind the shields, at the focus of the rays. But when the 

 rays are parallel, the use of an objective (placed behind the first 

 shield) gives very satisfactory results. As objectives I used ordinary 

 glass lenses ; knowing the index from my experiments, I was able to 

 calculate the focal distance for the electric ray. This is of course 

 very different from the focal distance for the luminous rays. I at 

 first used a lens of 6 cm. electric focal distance, but this did not 

 improve matters sufficiently. I then used one with a longer focus, 

 i.e., 13 cm., and this gave satisfactory results. 



The receiver used to be enclosed in a metallic case, 2 cm. in 

 "breadth, with an open front for the reception of radiation. The case 

 was nsed to protect the receiver from stray radiations. But by the 

 new arrangement and improved construction, these disturbances 

 were effectively removed. I therefore discarded the use of the 

 metallic enclosing ceil, as it seemed to me that the rays which did 

 not actually fall on the sensitive surface might be reflected from the 

 back of the metallic cell and thrown on to the sensitive layer. The 

 layer of spirals, only 15 mm. in breadth, is laid on a groove in 

 ebonite (which is transparent). This linear receiver without any 

 metallic case was placed at the focus of the lens. 



I now proceeded to measure the angle of refraction corresponding 

 to a given angle of incidence. In the first series observed, the 

 refraction was from glass to air ; the cylindrical surface of the semi- 

 cylinder was turned to the radiator, which was placed at its principal 

 focus. The receiver was mounted on the radial arm with the double 

 shields, and the objective in the manner already described. The 

 reading for refracted rays was taken in the following manner. 

 Having adjusted the semi-cylinder for a given angle of incidence, 

 the receiver was moved round till it responded to the refracted ray. 

 Readings were taken first by placing the receiver at an angle less 



