1879.] 



some Experiments on Metallic Reflexion. 



249 



when the light was incident npon the surface of the first at angle of 

 about 70°, it was reflected once by either plate. 



The incident light being polarized in a plane inclined at an angle of 

 45° to the plane of incidence, the position of the stage and of the 

 analysing nicol were altered till the reflected light was reduced to a 

 minimum . 



Plane of polarization Principal 

 of incident light. incidence. Azimuth. 



+ 45 75 45 —30 20 • 



75 52 , 31 30 



75 23 31 05 



76 02 31 55 



-45 75 54 +29 30 



76 29 50 



75 57 29 05 



76 03 28 45 



Mean 75 52 30 15 



A rectangular glass trough was placed on the horizontal stage of 

 the goniometer so as to surround the gold plates; the trough filled 

 with water, and the principal incidence and the azimuth observed. 



The ray of light which had been twice reflected by the plates being 

 parallel to the incident ray, and the trough having been placed with its 

 front perpendicular to the direction of the incident light, the polariza- 

 tion of the ray could not be altered in any way by the glass, as indeed 

 was verified by experiment. 



The light having been twice reflected, the square root of the 

 tangent of the angle which the plane of polarization of the reflected 

 ray makes with the plane of incidence, is equal to the tangent of 

 the principal azimuth. 



The principal incidence and principal azimuth determined by this 

 method from eight observations, four with the plane of polarization 

 of the incident light on either side of the plane of incidence are — 



Gold in air 75 52 37 22 



„ water 72 28 .... 37 48 



The principal incidences agree fairly well with those obtained by the 

 other method ; but the azimuths are somewhat higher. 



The following table contains the values of these constants for gold 

 in air, as previously determined by — 



VOL. XXVIII. T 



