by Rough and White Surfaces. 345 



snow, white powders, and solids and fluids reflecting white light 

 from their interior. When studying the polarization of the 

 atmosphere, and anxious to discover the cause of its partial 

 polarization, and of the three neutral points or spots in which 

 there is no polarization, I investigated the action of rough sur- 

 faces upon light, under the conviction that the sky or atmo- 

 sphere 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 polarization produced by the atmosphere, but of each 

 of the three neutral points, which, it will be shown, can be pro- 

 duced artificially by the combination of rays polarized 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 which 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 polarization for light reflected from the 

 polished surface of this glass is about 56^°, and the polarization 

 is a maximum at this angle, diminishing on one side to 0° of 

 incidence, and on the other to 90°. When the light thus polar- 

 ized is examined by the band polariscope, the bands are nowhere 

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

 polarization of the bands being everywhere positive or vertical. 



In order to observe the effect of a single rough surface, such 

 as that of glass ground with the finest emery, I blackened with 

 melted wax the polished surface of a plate which was ground 

 only on one side, in order to prevent the light scattered by the 

 ground surface from being reflected at the second surface. When 

 this single rough surface reflected the light of a gas-flame, it 

 polarized it almost completely at nearly the polarizing angle of 

 56|° ; and there is no interruption or neutral point in the bands 

 of the polariscope. The rotation, therefore, or measure of polar- 

 ization, is nearly 45°. 



If this single rough surface is placed at an open window when 

 the sun is not shining, and reflects the light of the sky or clouds, 

 the light is only partially polarized, and the degree of polariza- 

 tion R is only 19°. In the open air it is much less. Within 



