220 FRESNEL. 
There must be then, in each of these rays, north and 
south sides in some way different from their east and 
west sides. And further, the north and south sides of 
the ordinary ray ought to have precisely the same prop- 
erties as the east and west sides of the extraordinary ray ; 
so that if this last ray make a quarter of a circuit about 
the line of its length it will be impossible to distinguish 
one from the other. The rays of light are so subtle that 
thousands of millions of these rays can pass simultane- 
ously through the eye of a needle without interfering : 
yet we find ourselves obliged to take into account the 
idea of their s¢des, and to recognize, on their opposite 
sides, dissimilar properties. 
When speaking of a magnet, natural or artificial phys- 
icists affirm it to have poles. They mean only that cer- 
tain points on its surface are found endowed with certain 
properties which are not found, or at least only show 
themselves feebly, at any other points. We have, then, 
equal reason to say the same thing of the ordinary and 
extraordinary rays of light which proceed from the divis- 
ion of the beam which passes through Iceland spar; and 
in contradistinction to the natural rays in which all points 
appear alike, we may rightly call them polarized rays. 
In order, however, that we may not extend beyond its 
proper limits the analogy of a polarized ray and a mag- 
net, it is important to remark well that in the ray, the 
poles diametrically opposite appear to possess exactly the | 
same properties ; whilst the dissimilar poles are situated 
on sides of the ray whose positions are at right angles to 
each other. 
The lines resembling diameters, which join the similar 
poles, in every ray deserve particular attention. "When- 
ever, in two distinct rays, these lines are parallel, we say 
