of Refraction of various Substances for the Electric Ray, 295 



There is thus a serious difference between the two views of the 

 variation of K (and therefore of ,w) with the frequency of vibration. 

 In a previous paper,* I alluded to the probability of the variation 

 of fi with the frequency of vibration. The value of fi may at first 

 undergo a diminution with the increase of frequency, reach a 

 minimum, and then have the value augmented when the frequency- 

 rises above the critical rate. The result obtained by Lecher is, how- 

 ever, too divergent from the others to be explained by such a suppo- 

 sition. 



The direct determination of fi for glass for electric oscillations of 

 high frequency, seemed to me of interest, as throwing some light on 

 the controversy ; so, on the conclusion of my determination of the 

 index for sulphur, I commenced an investigation for the determina- 

 tion of /t for glass. This w^as, however, greatly delayed by repeated 

 failures to cast glass here, and by my long absence from India. I 

 have now obtained from England two semi-cylinders of glass, with 

 a radius = 12 ■ 5 cm. and height = 8 cm. 



The method of experiment followed is the same as that described 

 in my previous paper. The radiator is placed at the principal focus 

 (obtained from a preliminary experiment) of one of the semi-cylinders. 

 The cylinder mounted on the platform of a spectrometer is rotated 

 till the rays are totally reflected. From the critical angle the value 

 of jx is deduced. 



I shall here describe some modifications introduced in the appa^ 

 ratus, which have been found to be great improvements. One of the 

 principal difficulties met with was in connexion with the disturbance 

 caused by stray radiation. It is to be remembered that the receiver 

 is extremely sensitive. Comparatively long waves are found to 

 possess very great penetrative power; shielding the receiver then 

 becomes very difficult. Even after the receiver, the galvanometer*, 

 and the leading wires had been screened, disturbances were met with 

 which it was difficult to localise. Part of the disturbance may have 

 been due to that set up by the generating coil. A double box made 

 of soft iron and thick copper removed this difficulty. But the 

 greatest immunity from disturbance was secured by using short 

 waves. In this case it was not at all necessary to take very special pre- 

 cautions to shield either the galvanometer or the leading wires, the 

 sensitive layer in the receiver alone being affected by the radiation* 

 I exposed the bare leading wires to the strong action of the radiator 

 by putting them in close proximity to the source of radiation, anfi 

 yet no response was observed in the galvanometer. This freedom 

 from disturbance is not due to the opposite action on the two wires,,, 

 for a single wire may be exposed to the radiation without any action 

 on the receiver. 



» Vide ' Eoy. Soc. Proc.,' ySI. 60, p. 168. \ 



Y 2 " 



