polarisation of light by reflexion from transparent bodies 127 
anomalous result, I found that one of the surfaces had suf- 
fered some chemical change, and reflected less light than any 
other part of the glass. This artificial substance acquires 
an incrustation, or experiences a decomposition by exposure 
to the air, which alters its polarising angle without altering its 
general refractive power. The perplexing anomalies which 
Bouguer observed in the reflective power of plate glass, were 
owing to the same cause, and so liable is this substance to 
these changes, that by the aid of heat alone, I have produced 
a variation of 9 0 on the polarising angle of flint glass, and 
given it the power of acting upon light like the coloured 
oxides of steel. 
Having thus ascertained the cause of the anomalies pre- 
sented by glass, I compared the various angles which I had 
measured, and found that they were all represented by the 
following simple law. 
The index of refraction is the tangent of the angle of polarisation. 
In the course of last summer, when I had the pleasure of 
seeing M. Arago, I mentioned to him the relation which I 
had discovered between the refractive powers, and the tan- 
gents of the polarising angles. He informed me, that he had 
found the polarising angle of air to be 45 0 or 47 0 , which being 
at the very extremity of the scale would afford a good test 
of the accuracy of the law. Now, if we take the refractive 
power of air at 1.00031 the polarising angle will be 45 0 o' 32'', 
a result which agrees most strikingly with the observed 
angle. 
In the following table I have given the polarising angles 
of eighteen transparent bodies, as determined by experiment, 
and as deduced from the law of the tangents. I have added in 
