of the Metals. 169 



From the experiments of Jamin, Eisenlohr has deduced the 

 values of the optical constants which he denotes by e and d for 

 a series of metals. The index of refraction n, and the extinction- 

 coefficient g for light falling perpendicularly on the metallic 

 plates, are related to these constants so that 



n = 3.cose, ^=d.sine (3) 



For other angles of incidence I*, the index of refraction v, and 

 the extinction-coefficient 7, have the values 



= n 2 +sin 2 I,l (4) 



= <7 2 +sin 2 lJ 



^=9 



The index of refraction, therefore, in the case of the metals is 

 dependent upon the angle of incidence. 



The values of n and^ which Beerf, by means of an approxima- 

 tion, deduces from the observations of Jamin, differ from those 

 which are furnished by Eisenlohr's calculation. By both methods, 

 however, the refraction index n of silver is given less than unity ; 

 in other words, light ought to travel through silver with greater 

 velocity than through a vacuum. 



Light penetrates, as the above researches show, to an appre- 

 ciable depth into metal. It must also, according to the view 

 entertained by the author, be reflected back from the interior ; 

 for the great difference of phase of the components of reflected 

 light seems to be only explicable on the supposition that the 

 reflected ray has to pass twice through the boundary between the 

 metal and the medium lying adjacent to it. The author's mea- 

 surements show that the light penetrates into the metal to a 

 depth which is as great as, or very little less than the length of 

 an undulation j and DoveJ remarked, a long time ago, how the 

 so-called metallic lustre was produced by a conjoint action of 

 exteriorly reflected and interiorly reflected or dispersed light. 



Since the theory, apart from the assumptions which it makes, 

 takes no account of this penetration of light into metal, the 

 author has endeavoured to determine directly the velocity of 

 light through metals. He has obtained, in the course of this 

 investigation, the remarkable result that light travels faster 

 through gold and silver than through a vacuum. 



For these experiments an interference-apparatus was used 

 which had previously been employed in other optical investiga- 

 tions §. A heliostat reflected sunlight in a horizontal direction 

 upon a vertical slit which was situated in the focus of an achro- 



* Beer, Pogg. Ann. vol. xcii. p. 412. f Ibid. p. 417. 



% Monatsber. d. Berl. Akad. 1851, p. 262. 



§ Ibid. 1862, pp. 718, 719; and Pogg. Ann. vol. cxviii. p. 449. 



