160 Prof. J. C. Bose. On the Indices of Refraction [Dec. 12, 



December 12, 1895. 



Sir JOSEPH LISTER, Bart., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



Pursuant to notice, Professor Albert Gaudrv, Professor F. Kohl- 

 rausch, Professor S. P. Langley, Professor Sophus Lie, and Professor 

 E. Metschnikoif were balloted for and elected Foreign Members of 

 the Society. 



The following Papers were read : — 



I. ** On the Determination of the Indices of Refraction of 

 various Substances for the Electric Hay. I. Index of 

 Refraction of Sulphur." By Professor J. C. Bose, B.A. 

 (Cantab.), B.Sc. (Lond.). Communicated by Lord 

 Rayleigh, Sec. R.S. Received October 20, 1895. 



The indices of refraction of transparent substances have been 

 determined by the usual optical methods. There is still a large 

 number of substances like the various rocks, wood, brick, coal-tar, 

 and others which are not transparent to light, so that their indices 

 could not be obtained. These substances are, however, transparent 

 to electric radiation ; and the present investigation was undertaken 

 to find a direct method of determining their indices w r ith a sufficient 

 amount of accuracy. 



Even in the case of optically transparent substances, the indices 

 are only known for the narrow range of light waves. For greater 

 wave lengths, the index is inferred from Cauehy's formula. Professor 

 Langley has, however, shown that this formula fails to give trust- 

 worthy results when applied to the dark radiations in the infra-red 

 portion of the spectrum. It does not, therefore, seem at all likely 

 that the above formula will give accurate results when applied to the 

 electric radiation. 



For the determination of the index for the electric ray, the prism 

 method is unsuitable. In the well-known Hertz's experiment with 

 the pitch prism, the deviation of the refracted rajs extended from 11° 

 to 34°. The approximate value of /*, = 1'69, obtained from this 

 experiment, is probably higher than the true value by about 15 to 20 

 per cent. 



