Index of Refraction of Ebonite. 197 



pair of delicate Bell's telephones, we had succeeded in showing, 

 1st, that there was refraction when intermittent radiation from 

 the oxyhydrogen light passed through an ebonite prism ; and, 

 2ndly, that the index of refraction of that ebonite was approxi- 

 mately 1°7. 



Exceedingly great care had to be taken, in consequence of 

 the feebleness of the sounds given out by the telephones; and, 

 from the nature of the experiment, we obtained the index of 

 refraction for that narrow band of rays which experienced least 

 absorption. 



Shortly after these results were published, Prof, Fitzgerald, 

 of Dublin, suggested, in conversation, the possibility of check- 

 ing them by measuring the polarizing angle of light reflected 

 from ebonite, on the assumption that the refracted ray is at 

 right angles to the reflected one when giving maximum pola- 

 rization. Subsequently Dr. Jellett was so kind as to make 

 these experiments, the results of which Prof. Fitzgerald per- 

 mits us to quote. The mean index of refraction for ebonite 

 thus obtained, on Fresnel's theory, was 1*611. 



Later on we repeated our selenium experiments, replacing 

 the intermittent oxyhydrogen light with an intermittent elec- 

 tric light, and increasing the electromotive force in the sele- 

 nium telephone-circuit to about 60 volts. A confirmation of 

 our former result was obtained; but, although we were able to 

 take greater precautions to ensure accuracy, we obtained no 

 more than a confirmation ; and from the difficulty of hearing 

 the weak sounds in the telephones, we felt that the index of 

 refraction thus measured might be as much as 1*8 or as small 

 as 1*6. 



In the course of these experiments, however, it was noticed 

 that visible red rays were certainly refracted; and consequently 

 we proceeded to make measurements according to ordinary 

 optical methods, using the apparatus shown in the figure. 

 L (fig. 1) is a fairly powerful electric light produced by a 

 Gramme machine ; is a glass lens giving a parallel beam of 

 light, part of which passes through the slit, S, ^ inch wide, 

 and falls on the edge of the ebonite prism P. F is a frame 

 holding tissue-paper, which can be moved about P as centre, 

 and which carries an index, I, pointing to the graduations on 

 the circle, as seen in fig. 2. There was a fine vertical line in 

 the middle of the tissue-paper; andHGr, forming about one third 

 of the paper, was well blackened. First this screen was 

 moved into such a position that the edge of the prism threw a 

 black shadow which was bounded by the fine central line, and 

 between that line and the blackened portion H G was a thin 

 band of white light. In fact a narrow beam from the slit fell 



