REPORT ON PHYSICAL OPTICS. 365 



crown-glass, and then varying the incidence on the latter until 

 the intensities are observed to be equal. The intensitj^ of the 

 light reflected from crown-glass at various incidences had been 

 already obtained from a detailed series of experiments ; and the 

 results were embodied in an empirical law, in which the intensity is 

 represented by the ordinate of a rectangular hyperbola, the cor- 

 responding abscissa being the sine of incidence. This formula 

 then gives the intensity of the light reflected from crown-glass, 

 and therefore also from the substance examined, at the corre- 

 sponding incidences. Mr. Potter concludes in this manner, that 

 the intensity of the light reflected from diamond at a perpendicular 

 incidence is 9-3, and that from glass of antimony 8'2; the in- 

 tensity of the incident light being represented by 100. The 

 intensities calculated from the refractive indices, by the formulae 

 of Young, Poisson, and Fresnel, are 18-36, and 13*33, respec- 

 tively. This variance in the results of theory and experiment is 

 undoubtedly beyond the limits of the errors of observation ; and, 

 were it otherwise, the partial results obtained by Mr. Potter, in 

 these and other experiments of the same nature, agree too closely 

 to permit us to refer the discri|)ancy to such a source. The 

 principle of the method however, appears, to say the least, un- 

 certain ; and it cannot but be wished that some of the various 

 photometrical methods recently proposed should be applied to 

 the examination of this interesting question. 



The formulae of Fresnel supply the account of the re- 

 markable phenomenon observed by M. Arago; — namely, that 

 when Newton's rings are formed between a lens of glass and a 

 metallic reflector, one of the two images into which they are 

 divided by a double-refracting crystal whose principal section 

 is parallel or perpendicular to the plane of reflexion, changes its 

 character as the incidence passes the polarizing angle of the 

 glass ; the colours being the same as in the other image when 

 the incidence is less than the polarizing angle, but comple- 

 mentary/ to them when it is greater. In fact, when the incident 

 light * is polarized perpendicularly to the plane of reflexion, the 

 amplitude of the reflected vibration (which vanishes at the angle 

 whose tangent is equal to the refractive index,) changes sign in 

 passing through zero ; being negative when the incidence is 

 less than that angle, and positive when it is greater. Conse- 

 quently, if the wave reflected from the glass, at the central spot, 

 is in complete discordance with that reflected from the metal in 

 the former case, it will be in complete accordance with it in the 

 latter ; and the centre, which before was black, will then be 



• Tlie effect is the same whether the light be polarized before or after re- 

 flexion. 



