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



from Cauchy's formula, which is admittedly faulty when applied to 

 rays below the visible spectrum. It would therefore be of interest 

 to be able to measure directly the index for long electric waves, and 

 compare it with the value of K for rapidly alternating electric fields, 

 the periodicity of which is preferably of the same order as that of 

 the electric waves for which the index is determined. 



Among the substances in which great divergence is exhibited 

 between the values of K and jll 2 , glass may be taken as typical. In 

 the very carefully conducted series of experiments by Hopkinson 

 the valne of K (later results) was found to be 6'61 for light flint and 

 9*81 for extra dense flint glass. He found no variation of K with 

 the time of charge, which varied from 1/4 to 1/20,000 part of a 

 second.* Messrs. Romich and ISTowakf found the value to be 7*5 for- 

 alternation of field of abont once in a second, while for steady fields 

 they obtained the abnormally high value of 159. Schiller J found K 

 for plate glass to be 6"34, with a frequency of alternation of 25 in a 

 second. With a higher frequency of about 1*2 X 10 4 , the valne 

 obtained was lower, i.e., 5*78. Gordon, with a frequency of 1*2 X 10 4 , 

 obtained 3'24 as K for common glass. 



From the experiments of Schiller it would appear that the value 

 of K for glass diminished with the increase of frequency of alterna- 

 tion of the field. 



.Rubens and Arons§ compared the velocities of propagation of 

 electro-magnetic action through air and glass, and obtained the ratio 

 of the velocities or f.i ~ 2"33. The deduced value of K wonld there- 

 fore be 5*43. M. Blondlot|| fonnd K to be 2*84 when the frequency 

 of vibration was of the order 2'5 x 10 7 . Professor J. J. Thomson 

 found the specific inductive capacity of glass to be smaller nnder 

 rapidly changing fields than in steady ones. He deduced the valne of 

 K by measuring the lengths of wave emitted by a parallel plate con- 

 denser with air and glass as dielectrics. The value for glass was 

 found to be 2*7.^f 



On the other hand, Lecher** found that the dielectric constant rose 

 with the frequency of vibration. Thus for plate glass — 



Frequency. K. 



2 4-64 



2 x 10 3 5-09 



3-3 xlO 6 6-50 



* Hopkinson, 'Phil. Trans.,' 1881, Part II. 

 t ' Wien. Ber.,' vol. 70, 1874. 

 X ' Pogg. Ann.,' p. 152, 1874. 

 § ' Wied. Ann.,' vol. 42, p. 581 ; vol. 44, p. 206. 

 i || ' Corai t. Rend,' May 11, 1891, p. 1058. 



f ' Roy. Soc. Prop.,' vol. 46, p. 292. 

 ** ' Phil. Mag.,' vol. 31, p. 205. 



