228 br. Brewster on the Polarisation of Light. 
polarised hy every successive plate will force its way through all 
the remaining plates j and will reach the eye at E, without having 
lost a single ray hy reflection, the light reflected at each surface 
being taken from the unpolarised portion of the transmitted light. 
Hence it follows, that the light which penetrates through a 
parcel of plates is not a maximum when it falls with a perpen- 
dicular incidence ; that the principle employed by Bouguer for 
computing the intensity of the light transmitted by several 
plates is completely erroneous; and that the method adopted 
by the same distinguished philosopher for measuring the ab- 
sorption of light is affected with a similar error.* 
The celebrated discovery made by Malus, of the polarisa- 
tion of light by oblique reflection, is perhaps the most impor- 
tant that optics has received since the discovery of the principle 
of the achromatic telescope ; but though it developed a new 
set of phenomena, analogous to those produced by doubly 
refracting crystals, yet as the polarisation of one of the images, 
at least, formed by these crystals, was effected by refraction, 
and not by reflection, it did not furnish us with any information 
respecting the manner in which they polarised the transmitted 
light. The discovery, however, of the polarisation of light by 
oblique refraction, forms the connecting link between these 
two classes of facts, and holds out the prospect of obtaining a 
direct explanation of the leading phenomena of double re- 
fraction. 
* In estimating the quantity of light absorbed, Bouguer {Traite d’Optique sur 
la gradation dc la Lumiere, p. 156 — 160,) compared the quantity transmitted by 
four pieces of glass with the quantity transmitted by one piece having the same thick- 
ness with all the four. The pencil of light was iiicident at an angle of 1 5*, and hence 
the 177th part of the light transmitted by each plate was polarised, and therefore 
not altogether subject to the general law of reflection. 
