and prime Azimuth of Fraunhofer' s Lines. 327 



According* to my earlier investigations (Pogg. Ann. vol. cxxviii. 

 p. 562, vol. cxxix. p. 211, 1866), a prime angle of incidence and 

 a prime azimuth can still be spoken of when the light is reflected 

 in air from a thin, transparent lamina of metal lying on a glass 

 plate. The difference of phase and ratio of amplitudes of the 

 components polarized parallel and perpendicular to the plane of 

 reflection can thence be calculated for any angle of incidence, 

 just as with non-transparent metallic mirrors. 



I have therefore determined the prime angle of incidence and 

 prime azimuth on transparent laminae of gold, platinum, and 

 silver, as well as on non-transparent metals. (See Table II.) 

 They lay on plates of mirror-glass, the back of which was blackened 

 with oil-colour in order to destroy their reflection. 



The gold was obtained by Wernicke's process (Pogg. Ann. 

 vol. cxxxiii. p. 183, 1868). The silver was obtained thus : frotn 

 the same freshly prepared Martin's silvering-fluid, on strips of 

 the same plate glass, dressed simultaneously in the same manner, 

 I let silver be precipitated during ^, 2, 3, 6, and 15 minutes 

 respectively. The* thicker layers were converted into silver 

 iodide ; and from the colour of the iodide layer the thickness of 

 the silver was calculated (cf. Pogg. Ann. vol. cxxix. p. 208, 1866). 

 The platinum was a mirror, such as have been sold in France 

 for some years past. The thickness given of the gold and pla- 

 tinum layers is only from a very rough estimate. 



The observations on the thinnest layer of silver, as it was not 

 everywhere of equal thickness, deserve but little confidence. 

 The rest of the mirrors of silver^ however, were most perfect and 

 homogeneous, and reflected the light strongly, although they 

 had merely been rinsed with water and dried but not polished 

 with leather. 



The polarization-angles of the unsilvered glass plates were 

 calculated from the refraction-exponents of the surface, deter- 

 mined by a peculiar process. The glass for the gold layers had 

 exactly, and that for the platinum very nearly, the same expo- 

 nents of refraction as for silver. 



These measurements, agree substantially with the results of 

 the previous investigations, which yet were made with only one- 

 coloured red light. They show that the prime angle of inci- 

 dence and the prime azimuth increase as the thickness of the 

 metal increases, but in different degrees for different colours. 

 The values of the prime azimuths for the different Fraunhofer 

 lines approximate to one another with increasing thickness of 

 silver, recede from one another with greater thickness of gold. 



With silver the prime azimuth exhibits a maximum value for 

 a certain Fraunhofer line, which maximum moves towards the 

 red end of the spectrum as the thickness is augmented. 



