[ 105 ] 



XIV. On the Refractive Indices and the Dispersion of Opaque 

 Bodies. By W. Wernicke*. 



VARIOUS indirect methods have been employed for the de- 

 termination of the refractive indices of bodies which appear 

 opaque when they are of such thickness as is requisite for direct 

 determination by means of prismatic deflection. The one mostly 

 used is the determination of the angle of the greatest polariza- 

 tion (or of the nearly identical principal angle of incidence f) of 

 the substance, the tangent of which is assumed to be equal to the 

 refractive index. But, apart from the contradictions to which we 

 are led by a comparison of the refractive indices of metals deter- 

 mined by this method with those obtained by another method of 

 determination, the observation of the angle of polarization is very 

 uncertain : even with transparent substances of great refractive 

 power, results are at times obtained which differ even in the first 

 decimal J from those correctly determined by means of prismatic 

 deflection. The same uncertainty occurs in a still higher degree 

 with Wollaston's method for the determination of refraction by 

 means of the total reflection : most opaque bodies of great refrac- 

 tive power, when placed in contact with an hypothenuse-surface 

 of the prism, afford no definite limiting angle of total reflection. 

 Still less applicable is Arago's method, who, by means of Poisson's 

 formula?, determined the refractive index of mercury to be 5*829, 

 from the ratio of the quantity of perpendicularly incident light 

 reflected to that which is transmitted. If this method is applied 

 to silver, which reflects 95 per cent, of light, the refractive index 

 is found to be 71 '8, whilst the method by the angle of polariza- 

 tion only gives 4*8. A fourth method, more recently applied by 

 Quincke §, according to which the velocity of light in the metal 

 is determined by means of the displacement of fringes which is 

 produced by two interfering pencils of light, one direct and the 

 other passing through a thin lamina of the metal, has given the 

 refractive indices of a few metals less than 1, which is in agree- 

 ment with Cauchy's formula, but in contradiction to the results 

 of the method by the angle of polarization. All these methods 

 can at most give, in the most favourable case, an approximate 

 idea of the magnitude of the mean refraction ; not one is suitable 

 even for an approximate determination of the dispersion. 



In the present research I describe a method which admits the 

 determination of the refractive indices and of the dispersion of a 



* Translated from Poggendorff' s Annalen, 18/0, No. 1. 

 t Haughton, Phil. Trans, vol. cliii. pp. 81-125. 

 X Cf. de Senarraont, Ann. de Chim. S. 2. vol. lxviii. p. 337. 

 § Monatsber. d. Berl. Akad. 1863, p. 125. Phil. Mag. S. 4. vol. xxvii. 

 p. 161. 



