PLANE POLARIZATION. 223 
flected at the surface of water at an angle of 37°, or 
from the surface of glass at an inclination of 35° 25/, is 
as completely polarized as the two rays, ordinary and 
extraordinary, proceeding from a crystal of Iceland spar. 
The reflexion of light long ago occupied observers in 
the age of Plato and of Euclid: since that epoch it had 
been the object of thousands of experiments, of hundreds 
of theoretical speculations ; the law according to which it 
proceeds serves as the basis of a great number of instru- 
ments, ancient and modern. Among the multitude of 
enlightened minds, of men of genius, of skilful artists, 
who, during more than 2300 years, have been occupied 
with this phenomenon, no one ever aimed at any other 
object than the means of making the rays divide, or of 
causing them to diverge or converge ; no one ever imag- 
ined that reflected light ought not to possess all the same 
properties as the incident light, or that a change of path 
would be the cause of a change of nature. Generations 
of observers thus succeeded each other during several 
thousands of years, every day touching closely on the 
most beautiful discoveries without actually making them. 
Malus, as I have already explained, gave a means of 
polarizing light different from that which Huyghens had 
formerly announced. But the polarizations produced by 
the two methods were identically the same. The re- 
flected rays and those which proceed out of an Iceland 
erystal possess exactly the same properties. Since that 
time a member of this Academy (Arago) has discovered 
a kind of polarization* entirely distinct, and which mani- 
* It may be necessary for some readers to explain that, in this some- 
what paradoxical mode of speaking, the author is referring to his own 
discovery of the polarized tints; and his meaning is simply that if, in 
polarized light, there be placed a thin film, e. g., of selenite or mica, 
and it be viewed through a doubly refracting crystal as an analyzer, 
