1884.] Some Experiments on Metallic Reflection. 



39 



The calculated and observed values for the light polarised in the 

 plane of incidence agree very fairly, the calculated values being 

 slightly the lowest. 



For light polarised perpendicularly to the plane there is consider- 

 able difference between the two sets of numbers, the calculated 

 values being considerably the highest. 



As has already been stated the silver film was, to some extent at 

 least, transparent, and it was found that when a Nicol was held 

 between the eye and the silvered glass, and sunlight was incident 

 obliquely upon the film, the brightness and colour of the transmitted 

 light varied with the position of the Nicol ; the image of the sun 

 being brightest when the short diagonal of the Nicol was in the plane 

 of incidence, and darkest, and of a deep blue colour, when the 

 long diagonal was in that plane. Hence it would appear that at 

 oblique incidences light which is polarised perpendicularly to the 

 plane of incidence penetrates to a greater depth in the film than that 

 polarised in the plane, a result that is in accordance with the con- 

 clusion drawn from the experiments with silver films already referred 

 to, and one that may account for the difference in the observed and 

 calculated intensities of light polarised perpendicularly to the plane 

 of incidence reflected by the silver film.* 



In order to ascertain whether the difference between the observed 

 and calculated results was really due to this cause or not, a thicker 

 film was prepared by depositing a second coating of silver on a 

 freshly-prepared film. 



The same glass plate was used ; the silver weighed 0'0072 grm., and 



Table IV. Double Silver Film, with Light polarised in the Plane of 



Incidence. 



* Quincke, " Pogg. Ann.," vol. cixix, p. 177, is of opinion that light polarised in 

 and perpendicularly to the plane of incidence penetrates to an equal depth, but that 

 the former is more rapidly diminished in intensity. 



