206 



SIR DAVID BREWSTER ON THE POLARISATION OF LIGHT 



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 scat- 

 tered by the ground surface from being reflected at the second surface. When 

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

 completely at nearly the polarising angle of 56^°, and there is no interruption or 

 neutral point in the bands of the polariscope. The Rotation, therefore, or measure 

 of polarisation, 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 

 polarised, and the degree of polarisation R is only 19'. In the open air it is much 

 less. Within the room the polarisation increases as the glass recedes from the 

 window. In the open air it is a maximum when the plane of reflexion passes 

 through the sun and the observer. The rotation is then 15°, and it diminishes 

 with the distance of the plane of reflexion from the plane passing through the sun 

 and the observer,* 



When the second or polished surface is not blackened, it reflects the light 

 scattered by the ground surface, and the maximum polarisation of the rough 

 surface is greatly diminished. The rotation is 16° with a gas burner very near, 

 and 20° when the flame is still nearer. The polarisation is diminished by holding 

 a lighted candle on one side of the plane of reflexion. 



With a similar plate having a rougher surface, the degree of the polarisation 

 of the sun's rays directly reflected was Wh° close to a south window. Before noon 



the angle of maximum polarisation was 44°, and 

 at noon, with a brighter sun, it was 40°. 



When the whole light of the sky fell upon the 

 rough surface MN, the rotation was 21°. When 

 AB was covered up the rotation was diminished, 

 and when CD M'as covered up the rotation was 

 increased. 



When both the surfaces of the plate of glass are rough, the polarisation is a 

 maximum, at an angle greater than the normal polarising angle, or 56|°, and the 

 degree of polarisation is about 20°. 



In making similar experiments with ivory, bone, porcelain, white or coloured 

 surfaces painted in oil, paper, parchment, silk, linen, and cotton cloths, milk, flour, 

 and other white powders, &c., I found that the polarisation was partial in all of 

 them, the normal or complete polarisation being reduced by its combination with 

 the oppositely polarised rays produced by refraction. 



In all these experiments the partial polarisation was positive or vertical from 



* The rouoliness of the glass surface used in the preceding experiments is such, that a gas 

 flame, distant 5| feet, ceases to be visible at an angle of incidence of 79" 50'. 



FiK. 1. 



