and prime Azimuth of Fraunhofer's Lines. 325 



frequently alter the optical qualities of a mirror very consider- 

 ably ; and it is probably much more difficult to obtain faultless 

 reflecting surfaces than transparent substances free from veins. 

 Besides these irregularities, disturbance is occasioned by un- 

 avoidable scratches and roughnesses of the surface, which act 

 like a diffraction -grating. As in the case of coefficients of elas- 

 ticity or electric conducting-power, properly the value of H and 

 B must be determined for each definite portion. 



The following Table (p. 326) contains a series of determinations 

 of the prime angle of incidence and prime azimuth for Fraun- 

 hofer's lines C, D, E, F, and G, executed by the method of 

 parallel mirrors; and the numbers in general, so far as they 

 relate to substances previously investigated, agree with the 

 above-mentioned measurements by Jamin and Haughton. 



The gold was burned in upon plate glass by heating a solu- 

 tion with essential oil; copper and nickel were electro-deposited ; 

 the cobalt consisted of polished pieces such as occur in com- 

 merce. Antimony, bismuth, zinc, and tin were cast and po- 

 lished plates ; aluminium, brass, and platinum were rolled ; the 

 silver was obtained by Martin's process (Pogg. Ann. vol. cxxix. 

 1866, p. 55) from a solution several months old. 



For reflection in glass, the silver was fixed on the long, pa- 

 rallel sides of two Fresnel parallelepipeds, which had an acute 

 angle of 56° 52' and the exponents of refraction 



c. 



D. 



E. 



F. 



G. 



•5220 



1-5246 



1-5279 



1-5308 



1-5362 



In the calculation of the prime azimuths, account is taken of 

 the rotation of the plane of polarization on the entrance into and* 

 the issue from the glass. To the selenium a reflecting surface 

 had been imparted by pressing cold plates of glass on the melted 

 substance. 



The most perfect surfaces were possessed by nickel, silver, 

 gold, and selenium. 



Where not stated otherwise, the reflection took place in air; 

 and shortly before the observation the metals had been polished 

 with clear buckskin and a trace of jeweller's rouge. 



With the exception of gold, in the case of all the metals here 

 mentioned the prime angle of incidence (polarization-angle) di- 

 minishes with the diminution of the wave-length — the reverse 

 of what takes place with transparent bodies. The diminution, 

 however, is very different with the different metals. 



The prime azimuths partly increase, and partly diminish as 

 the wave-lengths become less. Platinum shows a maximum 

 value for the line D, cobalt and bismuth for the line E, and tin 

 for the line F. 



