and prime Azimuth 0/ Fraunhofer's Lines. 329 



Gold, in transparent as in non -transparent layers, has a mi- 

 nimum angle of incidence for Fraunhofer's line F. 



The values of H and B were not altered by magnetizing or 

 electrizing the metallic mirrors. 



By polishing and pressure of the layer of metal the value of 

 the prime angle of incidence is augmented, that of the prime 

 azimuth diminished. I have observed this also with other me- 

 tals than those cited in Tables I. and II., for example cobalt and 

 nickel. The treatment may vary much for the different colours. 



Only with reflection from silver into glass was there shown a 

 slight increase of the prime azimuth through pressure, which, 

 however, was not exerted in this case on the reflecting front sur- 

 face of the metal, but on its hinder surface bounded by the air. 



The effect of pressure, or of the distance of the particles of 

 silver, upon the constants of reflection was most strikingly exhi- 

 bited with silver-collodion films, for which I am indebted to 

 M. F. A. Nobert, of Barth. They were obtained by the process 

 used in photography — a glass plate coated with collodion (con- 

 taining iodide of potassium ?) being immersed in a silver solu- 

 tion, browned by exposure to daylight, and made black by a 

 strong solution of pyrogallic acid. After drying, the plate was 

 strongly heated, in order to make the collodion film adhere more 

 fimly to the glass. 



Such a collodion film contains uniformly distributed finely 

 divided particles of silver, is opaque even when very thin, but 

 reflects too little light for the double-reflection method. With 

 Babinet's compensator and homogeneous red or blue glass, trial 

 was made with reflection in air : — 



Red. Blue. 



Ol O 1 



Prime angle of incidence . 50 29 50 

 Prime azimuth .... 21 20 20 44 



After gentle rubbing with soft buckskin, when the plate took a 

 polish : — 



-c a ,• Red. Blue. 



J? or reflection in air, 



' O I O i 



Prime angle of incidence . 64 50 62 2 

 Prime azimuth .... 11 26 13 28 



For reflection in glass, 



Prime angle of incidence . 57 25 54 41 

 Prime azimuth . . : . 25 7 41 13 

 An ordinarily transparent collodion film, rendered opaque by 

 finely divided particles of silver, shows therefore a slight altera- 

 tion of the prime angle of incidence, but a great alteration of the 

 prime azimuth, which approximates to that of pure silver. 

 If by a slight pressure the silver particles are brought nearer 



