polarisation of light by reflexion from transparent bodies. 153 
I at first imagined that bodies with a high refractive power 
approached to the metals in their mode of action upon light, 
but this conjecture was refuted by experiments which showed 
that the pencil did not consist of two oppositely polarised 
portions. When I discovered the law of successive reflexions, 
as stated in Prop. XVI. and XVII., the difficulty of explaining 
the phenomenon seemed to increase. What Malus would 
have called the unpolarised portion of a beam of light, re- 
flected at 62° from glass, was now shown to be so far pola- 
rised, that its polarisation was completed by a second reflexion 
at the same angle, so that it became still more improbable 
that unpolarised light could exist at the polarising angle itself. 
All these difficulties, however, were immediately removed by 
the discovery of the law of the tangents, and of the polarisa- 
tion of the differently coloured rays at angles of incidence 
depending on their respective indices of refraction. The 
explanation which now suggested itself was confirmed by 
experiment, and I was thus led after much fruitless investiga- 
tion to the results expressed in the following Propositions. 
Prop. xxiv. 
If a pencil of white light is incident at the maximum polarising 
angle upon any transparent body whatever , a portion of the 
reflected pencil , consisting of the mean refrangible rays, will be 
completely polarised, while another portion of the beam, consist- 
ing °f th e blue and redrays, will not be completely polarised, and 
will therefore not vanish when the image from which the light 
proceeds is examined with a prism of calcareous spar. 
It is obvious from Prop. II. that all the rays which compose 
a beam of white light cannot be polarised at the same angle 
MDCCCXV. X 
