polarisation of light by reflexion from transparent bodies. 137 
it may be shown that the angle A.¥>C,flg. 4, is a right angle ; 
but BC being a continuation ofBG, ABG will also be a right 
angle, and consequently the angle of incidence EBG will be 
half a right angle, or 45 0 . 
Prop. xnr. 
IVhen a ray of light is incident on the second surface of a trans- 
parent body at an angle whose sine is greater than the reciprocal 
of the index of 1'efraction, or at an angle greater than the angle 
of total reflexion, the reflected light will consist of two pencils, 
one of which is polarised in the plane of inflexion, and the other 
in a plane perpendicular to the plane of reflexion. 
The experiments by which I ascertained this singular pro- 
perty were conducted in a manner similar to those of Malus 
upon polished metals. A ray of light moving horizontally 
in the direction of the meridian, after having been polarised 
in the plane of the horizon, was made to fall upon the second 
surface of a transparent body facing the south-east or south- 
west, and inclined at such an angle to the horizon, as to re- 
ceive the ray near the limit of total reflexion. The polarised 
ray was depolarised by the action of the second surface, so 
that the images of the object from which it proceeded, when 
viewed through a prism of calcareous spar, continued visible 
in every part of its revolution, an effect which could only be 
produced by the power of the second surface to form two 
oppositely polarised images. 
When the plane of the second reflexion is either parallel, 
or perpendicular to the plane in which the ray was originally 
polarised, the ray will suffer no change by the second re- 
flexion, one of the images formed by a prism of calcareous 
spar vanishing in every quadrant of its circular motion. 
mdcccxv. T 
