553 



The intensity of the light reflected from an infinite number of plates, 

 as we see from (18), is a- 1 ; and since a is changed into a~ l by 

 changing the sign of a or of A, 



*'-A), ..... (21) 



which is equal to 1 in the case of perfect transparency. Accordingly 

 a substance which is at the same time finely divided, so as to present 

 numerous reflecting surfaces, and which is of such a nature as to be 

 transparent in mass, is brilliantly white by reflected light, for 

 example snow, and colourless substances thrown down as preci- 

 pitates in chemical processes. 



The intensity of the light reflected from a pile consisting of an 

 infinite number of similar plates falls off rapidly with the transpa- 

 rency of the material of which the plates are composed, especially at 

 small incidence. Thus at a perpendicular incidence we see from the 

 above Table that the reflected light is reduced to little more than one 

 half when 2 per cent, is absorbed in a single transit, and to less than 

 a quarter when 10 per cent, is absorbed. 



With imperfectly transparent plates, little is gained by multiplying 

 the plates beyond a very limited number, if the object be to obtain 

 light, as bright as may be, polarized by reflexion. Thus the Table 

 shows that 4 plates of the less defective kind reflect 79 per cent., 

 and 4 plates of the more defective as much as 94 per cent., of the 

 light that could be reflected by a greater number, whereas 4 plates 

 of the perfectly transparent kind reflect only 60 per cent. 



The Table shows that while the amount of light transmitted at the 

 polarizing angle by a pile of a considerable number of plates is mate- 

 rially reduced by a defect of transparency, its state of polarization is 

 somewhat improved. This result might be seen without calculation. 

 For while no part of the transmitted light which is polarized perpen- 

 dicularly to the plane of incidence underwent reflexion, a large part 

 of the transmitted light polarized the other way was reflected an even 

 number of times ; and since the length of path of the light within 

 the absorbing medium is necessarily increased by reflexion, it follows 

 that a defect of transparency must operate more powerfully in redu- 

 cing the intensity of light polarized in, than of light polarized perpen- 

 dicularly to the plane of polarization. But the Table also shows that 

 a far better result can be obtained, as to the perfection of the polari- 



