288 
SIR J. CONROY ON THE AMOUNT OF LIGHT REFLECTED 
Sir Davip Brewster stated, many years ago (‘Phil. Trans.,’ 1815, p. 126), that 
glass “ acquires an incrustation or experiences a decomposition by ex230sure to the air 
which alters its polarising angle without altering its general refractive power,” and 
added that by the action of heat alone he had produced a variation of 9° in the 
polarising angle of flint glass. 
Seebeck (‘Poggendoref, Annalen,’ vol. 20, 1830, p. 27) made a number of deter¬ 
minations of the polarising angles of different kinds of glass, and found that there was 
considerable difference between the observed values and those calculated from the 
refractive index, except in the case of surfaces which he himself had ground and 
polished (with emery and colcothar). 
With one specimen of flint glass the difference was originally — 38’; he then 
polished it himself, and found that the difference was only -p 3'; after bemg polished 
by an optician, the difference became -p 28'. 
Seebeck was of opinion that these differences were due to the treatment which the 
glass had received whilst being polished and cleaned, and that lapse of time made no 
change. The only experiments he appears to have made on this latter point w^ere with 
crown glass, the surface of which, as the experiments here recorded show, does not 
alter, or at least only alters very slowly. 
Lord Rayleigh found (‘Roy. Soc. Proc.,’ vol. 41, p. 275) that repollshing prisms of 
crown glass caused a considerable increase in the amount of light they reflected; but 
his experiments do not show that when the prisms were first examined by him they 
reflected less light than when they were originally polished. 
Conclusion. 
It seems probable that the amount of light reflected by freshly polished glass 
varies with the way in which it has been polished, and that, if a perfect surface could 
be obtained without altering the refractive index of the surface-layer, then the amount 
would be accurately given by Fresnel’s formula, but that usually the amount differs 
from that given by the formula, being sometimes greater and sometimes less. 
The formation of a film of lower refractive index on the glass would account for the 
defect in the reflected light; but, to account for the excess, it seems necessary to 
assume that the polishing has increased the optical density of the surface-layer, and 
the changes produced in the amount of light transmitted and in the angle of polarisa¬ 
tion support this view. 
After being polished, the surface of flint glass seems to alter somewhat readily, the 
amount of the reflected light decreasing, and the amount of the transmitted increasmg, 
whilst with crown glass the change, if any, proceeds very slowly. 
There is no evidence to show to what particular cause these changes are due. 
The values of the transmission coefficients for light of mean refranaibility for the two 
particular kinds of glass are given, and show that for 1 centimetre the loss by obstruction 
amounts to 2‘62 per cent, with the crown glass, and 1T5 per cent, with the flint glass. 
