394? Prof. Stokes on the Metallic Reflexion 



the metallic yellowish-green reflexion which this substance ex- 

 hibits. It occurred to me that the almost metallic opacity of the 

 medium with respect to green light was connected with the 

 reflexion of a greenish light with a metallic aspect. I found, in 

 fact, that the medium agreed with a metal in causing a retarda- 

 tion in the phase of vibration of light polarized perpendicularly 

 to the plane of incidence relatively to light polarized in that plane. 

 The observation was made by reflecting light polarized at an azi- 

 muth of about 45° from the surface of the medium to be examined, 

 the angle of incidence being considerable, about equal to the 

 angle of maximum polarization, and viewing the reflected light 

 through a NicoFs prism capped by a plate of calcareous spar cut 

 perpendicularly to the axis. Now by using different absorbing 

 media in succession, it was found that with red light, for which 

 safflower-red is comparatively transparent, the reflected light 

 was sensibly plane-polarized, while for green and blue light the 

 ellipticity was very considerable. 



In the case of a transparent medium, light would be polarized 

 by reflexion, or at least very nearly so, at a proper angle of inci- 

 dence. Hence if the light reflected by such a medium as saf- 

 flower-red were decomposed into two pencils, one, which for 

 distinction's sake may be called the ordinary, polarized in the 

 plane of incidence, and the other, or extraordinary, polarized 

 perpendicularly to the plane of incidence, the extraordinary 

 pencil would vanish at the polarizing angle, except in so far as 

 the laws of the reflexion deviate from those belonging to a trans- 

 parent substance. Hence the light remaining in the extraordinary 

 pencil might be expected to be more distinctly related to the light 

 absorbed with such energy. Accordingly, it was found that 

 under these circumstances the extraordinary pencil (in the case 

 of safflower-red) was of a very rich green colour, whereas with- 

 out analysis the light was of a yellowish-green colour. Similar 

 observations were extended to specular iron. 



These phenomena recalled to my mind a communication which 

 Sir David Brewster made at the meeting of the British Association 

 at Southampton in 1846*; and on referring to his paper, I 

 found that the appearance of differently coloured ordinary and 

 extraordinary pencils in the light reflected by safflower-red was 

 the same phenomenon as he has there described with reference 

 to chrysammate of potash. 



The observations above-mentioned were made towards the end 

 of 1851. Accordingly, when Dr. Herapath's first paper on the 



* Notice of a new property of light exhibited in the action of chrysam- 

 mate of potash upon common and polarized light. (Transactions of the 

 Sections, p. 7.) 



