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On the oblique Refraction of Iceland Crystal. By William Hyde 

 Wollaston, M.D. F.R.S. Read June 24, 1802. [Phil. Trans. 

 1802, p. 381.] 



In the preceding communication Dr. Wollaston inserted two dif- 

 ferent measures of refractive powers distinctly observable in the Ice- 

 land crystal, as well as an estimate of its dispersive power ; but he 

 has reserved for this treatise some remarks, which the same mode of 

 investigation has enabled him to make on its oblique refraction. To 

 this he was led by the consideration that the law to which Huygens 

 had reduced this refraction, however founded in truth, could not be 

 easily verified by any of the former methods of measurement. 



According to the Huygenian hypothesis, light proceeding from any 

 luminous centre is propagated by vibrations of a medium highly 

 elastic, that pervades all space. In ordinary cases the incipient un- 

 dulations are of a spherical form ; but in the Iceland crystal they 

 appeared to him to be portions of an oblate spheroid, of which the 

 axis is parallel to the short diagonal of an equilateral piece of crystal, 

 and its centre the point of incidence of the ray. Hence he deduced 

 a ratio between the sine of incidence, and the sine of refraction (that 

 is, the ordinate of the spheroidical undulation) in any section of the 

 spheroid. 



In a geometrical deduction our author shows that his observations 

 on this substance accord throughout with the hypothesis of Huygens, 

 the measures he has taken corresponding more nearly than could well 

 happen in case of a false theory. This is illustrated by various ex- 

 amples, in which the refractive power is estimated according to va- 

 rious directions of the plane of incidence ; and the data are pointed 

 out for the construction of the spheroid, by which these refractions 

 are regulated. Lastly, a comparative view of the angles observed, 

 and those obtained by computation, is reduced into a table, from 

 which, by their near agreement, we collect an additional proof of. 

 the accuracy of the results. 



An Account of some Cases of the Production of Colours, not hitherto 

 described. By Thomas Young, M.D. F.R.S. F.L.S. Professor of 

 Natural Philosophy in the Royal Institution. Read July 1, 1802. 

 [Phil. Trans. 1802,j. 387.] 



In a former paper Dr. Young, treating of certain phenomena of 

 coloured light, mentioned a law, according to which it appears, that 

 whenever two portions of the same light arrive at the eye by different 

 routes, either exactly or very nearly in the same direction, the light 

 becomes most intense when the difference of the routes is any mul- 

 tiple of a certain length, and least intense in the intermediate state 

 of the interfering portions, and that this length is different for light 

 of different colours. In the same paper he showed the sufficiency of 

 this law for explaining all the phenomena in the second and third 

 books of Newton's Optics ; and in the present communication he il- 



