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XX. — On the Polarisation of Light by Rough and White Surfaces. By Sir David 



Brewster, K.H., D.C.L., F.R.S. 



(Read 2d March 1863.) 



The laws of the polarisation of light when reflected from the surfaces of solids 

 and fluids, and when refracted and transmitted by translucent and transparent 

 bodies, have been successfully investigated ; but no experiments^ I believe, have 

 been made on the polarisation of light by rough or unpolished surfaces, such as 

 ground glass, painted surfaces, pounded glass, snow, white powders, and solids 

 and fluids reflecting white light from their interior. When studying the polarisa- 

 tion of the atmosphere, and anxious to discover the cause of its partial polarisa- 

 tion, and of the three neutral points or spots, in which there is no polarisation, 

 I investigated the action of rough surfaces upon light, under the conviction that 

 the sky or atmosphere was a rough surface like any aggregation of white or 

 coloured particles. Had the atmosphere been specular like water or any body 

 with a polished surface, the image of the sun would have been seen in it by 

 reflexion, but being composed of aerial and aqueous molecules, it must reflect 

 the sun's rays like pounded glass, or any white or coloured powders. 



The results of this inquiry, which I now submit to the Society, are such as I 

 anticipated, and afford an explanation not only of the partial polarisation pro- 

 duced by the atmosphere, but of each of the three neutral points, which, it will 

 be shown, can be produced artificially by the combination of rays polarised by the 

 reflexion and refraction of any rough or molecular surface. 



The experiments by which these results were obtained were made chiefly 

 with rectangular plates of glass, 9 inches by 7, of various degrees of roughness, 

 some with only one side, and others with both sides rough. These plates, some 

 of w^hich are now on the table, were made at the Smethwick Glass Works, near 

 Birmingham, and were kindly presented to me by Messrs Chance and Brothers, 

 the proprietors of that great establishment. 



The angle of complete polarisation for light reflected from the polished surface 

 of this glass is about 56^°, and the polarisation is a maximum at this angle, 

 diminishing on one side to 0° of incidence, and on the other to 90°. When the 

 light thus polarised is examined by the band polariscope, the bands are nowhere 

 interrupted, and therefore there can be no neutral point, the polarisation of the 

 bands being everywhere positive or vertical. 



VOL. XXin. PART II. 3 L 



