198 



On the Index of Refraction of Ebonite. 

 Fig. 1. Fig. 2. 



LX- 



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on the edge of the prism; and half was stopped by the prism, 

 the other half going on. The index-reading in this position was 

 taken; and now the screen was moved ronnd until a red spec- 

 trum was seen. At the least-refrangible end this spectrum termi- 

 nated nearly abruptly, as the ordinary visible spectrum usually 

 does ; and this end was made to coincide with the central line 

 in the screen, and the index-reading taken when, after moving 

 the prism itself, it was supposed that we had minimum devia- 

 tion. The index-reading was also taken in the same way for 

 the most-refrangible end of the visible spectrum ; but as this 

 did not die away at all abruptly, and as the whole spectrum 

 was very faint, the second set of measurements merely gives 

 a rough idea of the amount of spectrum that was visible. The 

 mean of a number of observations made by different observers, 

 and the results of which were closely in accord, gave 1*66 as 

 the index of refraction for the well-defined least-refrangible 

 end, and 1*9 as the average result for the badly-defined most- 

 refrangible end. 



As the slit in the metal diaphragm used with the selenium 

 experiments had to be much wider than that employed with the 

 simple light-experiments, we cannot of course tell what exact 

 part of the spectrum produced the sound: probably it was at 

 about the least-refrangible end of the visible spectrum; but it 

 may have been the dark rays just beyond. 



Summing up the results of the various experiments, we have 

 for the index of refraction of certain specimens of ebonite: — 



Ebonite prism, selenium, and telephones . . about 1*7 



Measurement of polarizing angle by reflection „ 1*611 



Least refracted end of visible spectrum produced by 1 1 «,, 

 ebonite lens having an angle of 28 0, 5 , about J 



