208 



SIR DAVID BREWSTER ON THE POLARISATION OF LIGHT 



to the incident rays from MN, the neutral points will return to their respective 

 places a. A, G, H, the refractive polarisation of the plato or plates reducino- the 

 negative 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 polarisation 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 mn, 

 having their planes of reflexion parallel to the plane passing through Yk, the}-- 

 are positive. When they become more intense they are neriatire. 



The place of any neutral point may be advanced from II to G, or from G to A 

 and a, by placing a sheet of white paper behind the rough surfaced plate MN, or 

 by one or more plates of glass with rough surfaces, each plate advancing it 

 farther. A sheet of paper advances it farther than/owr surfaces of ground glass. 

 The rays scattered by the paper are negatively polarised hy refraction at their 

 emergence from the Plate MN, and, by increasing the 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 and more such 

 plates still farther. The smoothest ground plates carry the neutral point farther 

 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. 



Rochclle salts. 



Soda. 



Magnesia. 



Milk. 



White shell. 



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

 and negatively polarised 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 negative 

 and greater than 0°, but on the other side of the perpendicular FG. 



As in ground glass, one or m^ore plates of glass inclined to tlie reflected ray, as 

 previously described, brought the neutral point, in almost all these substances, to 

 the A or N side of G. 



