152 Dr. Brewster on the laws which regulate the 
Sect. V. On the nature and origin of the apparently unpolarised 
light which exists at the maximum polarising angle. 
I have already shown in a former paper,* that in substances 
of a high refractive power, such as realgar and diamond, there 
is a great quantity of apparently unpolarised light reflected 
at the polarising angle, so that the image which would have 
vanished by the application of calcareous spar, had the re- 
flexion been made from water or crown glass , possessed in 
these cases a considerable brilliancy. The comparative inten- 
sity of the light of this image is indeed so great, that only a 
very small portion of the incident light seems to be polarised. 
I was at first much surprised at witnessing this phenomenon, 
as I had been led to believe from the first Memoir of Malus, 
that one of the pencils formed by calcareous spar vanished 
when the light was reflected from all other bodies as well as 
from water and glass. There is some reason to think, however, 
that Malus afterwards observed the same fact, for in a subse- 
quent Memoir, he makes use of the term maximum polarising 
angle , in which the knowledge of it seems to be implied. 
The extreme difficulty of accounting for such an unex- 
pected phenomenon, probably deterred him from even men- 
tioning the subject in any of his Memoirs ; and I am not 
ashamed to avow that the investigation of this point alone has 
cost me more labour than any other branch of the polarisa- 
tion of light. The existence of a quantity of apparently un- 
polarised light, in a pencil reflected at the polarising angle, 
appeared completely paradoxical, and it was obvious that no 
satisfactory generalisation of the phenomena could be given 
while this difficulty remained unsolved. 
• See Phil. Trans, for 1814, Part I. p. 230. 
