230 
Dr, Brewster on the Polarisation of LighL 
the Royal Society, I shall communicafe to you the results of a 
set of experiments on the Polarisation of Light by r flection. In 
this paper I shall be able to shew, by the most satisfactory 
details, that the principle discovered by Malus is not, as has 
been supposed, a general law, but that it depends upon a cir- 
cumstance which he appears to have completely overlooked, viz. 
the relation between the quantities of reflected and transmitted 
light when the pencil is incident at the polarising angle. When 
light is polarised by r^'flection from water, the reflected light is 
only To^ incident pencil, and in the case of glass 
it is only rioo’ when’ realgar^ diamond^ and chromate of lead 
are employed, the light reflected at the polarising angle is 
above and therefore none of these bodies have the power 
of polarising the whole of the reflected pencil. 
I have the honour to be. 
Sir, your most obedient humble servant, 
Edinburgh, Nov. 24, 1813. 
DAVID BREWSTER. 
To Taylor. Combe, Esq. Sec. R. S. 
