348 Sir David Brewster on the Polarization of Light 



points will return to their respective places a, A, G, H, the 

 refractive polarization of the plate or plates reducing the nega- 

 tive bands between M and the neutral points, and increasing 

 the positive bands between the neutral points and N. 



Hence we have a method of determining whether the polari- 

 zation of the bands is positive or negative, when they are so 

 faint or indistinct that we cannot see whether the central band 

 is black or white. When they become less intense, or perhaps 

 disappear, by viewing them through one or more plates of glass 

 at m n, having their planes of reflexion parallel to the plane pass- 

 ing through FA, they are positive. When they become more 

 intense they are negative. 



The place of any neutral point maybe advanced from H to G, 

 or from G to A and a, by placing a sheet of white paper behind 

 the rough-surfaced plate M N, or by one or more plates of glass 

 with rough-surfaces, each plate advancing it further. A sheet 

 of paper advances it further than four surfaces of ground glass. 

 The rays scattered by the paper are negatively polarized by 

 refraction at their emergence from the plate MN; and the in- 

 creased intensity of the negative bands shifts the neutral point 

 towards N. 



We have already seen that a single rough surface of glass 

 blackened behind has its neutral point about H, and when not 

 blackened, about G. A plate with both sides rough carries the 

 neutral point towards A, and two or more such plates still 

 further. The smoothest ground plates carry the neutral point 

 further than the roughest towards M. 



In the prosecution of this subject I submitted to examination 

 the following substances :— 



Calcareous spar, ground. 



Marble, white. 



Painted board. 



Ivory. 



Ivory, artificial. 



Bone. 



Porcelain. 



White fir wood, planed. 



Yellow fir wood, planed. 



Silk, white. 



Satin, white. 



Linen cloth, white. 

 Cotton cloth, white. 

 Cashmere, white. 

 Paper of all colours. 

 Parchment. 

 White kid leather. 

 Pith of the sola. 

 Rice paper. 

 Cotton wool. 

 Ermine. 



Swan's down. 

 Snow. 



Pearl-oyster shell. 

 Pounded sugar. 

 Pounded glass. 

 Rochelle salts. 

 Soda. 

 Magnesia. 

 Milk. 

 White shell. 



In all these substances I found a neutral point in the mixture 

 of the positively and negatively polarized rays which they reflect. 

 In many of them the neutral point was on the A side of G, or 

 when the angle of incidence was less than 90° ; while in others 

 it was on the H side, or when the angle of incidence was nega- 

 tive and greater than 0°, but on the other side of the perpendi- 

 cular F G. r 



