by Hough and White Surfaces. 



347 



Fig. 2. 



^ 



A a 



The rough or white surface 

 MN being placed vertically, 

 was illuminated with the flame 

 of a gas-burner, or a moderator 

 lamp, placed at F. The ob- 

 server at E, or in any direction 

 between F and N, observes the 

 condition of the bands of the 

 polariscope when they are placed 

 parallel to M N. 



If we now take a single rough 

 surface of plate-glass blackened 

 on its polished side and place 

 it at M N, we shall observe a 

 neutral point about H, the polariscope-bands being negative or 

 horizontal with a white centre on the M side of H, and positive 

 or vertical with a black centre on the N side of H. This neu- 

 tral point is obviously produced by the equal and opposite action 

 of light polarized by reflexion and refraction. As these two 

 lights, proceeding from the light incident at H, proceed from F, 

 they cannot reach the eye at E by the ordinary law of reflexion. 

 They are portions, therefore, of oppositely polarized rays scat- 

 tered in every direction by the rough surface of the glass. 



If we take the same plate of glass with its surface not black- 

 ened and place it at M N, we shall find the neutral point at G. 



If we invert this plate, so that its polished side is uppermost, 

 the neutral point is advanced to A, all the bands on the M side 

 of A being, as before, negative, and those on the N side of A 

 positive — a result proving that the intensity of the negative 

 bands had been increased, and the positive ones diminished. 



Conceiving that this effect was produced by the light scattered 

 by the rough surface being polarized negatively by the refraction 

 of the polished surface, I took the plate with the blackened side 

 and laid upon its surface a plate of transparent glass, in order 

 to imitate the action of the unpolished surface in the preceding 

 experiment. The neutral point which, by the action of the 

 single rough surface, was at H, was now advanced to A by the 

 refractive polarization of the two surfaces of the transparent plate. 

 "When this plate was placed at m n parallel to M N, the very 

 same effect was produced. By inclining mn, the neutral point 

 was advanced from A to a, still further by increasing the incli- 

 nation, and still further by using several plates, the plane of 

 refraction or incidence being parallel to a plane passing through 

 FGA. 



If we turn the plate or plates at mn round 90°, keeping the 

 same inclination to the incident rays from M N, the neutral 



