218 FRESNEL. 
on the same subject, I find myself still carried back, by 
the nature of Fresnel’s labours, to the subject of double 
refraction: but, this time, instead of occupying myself 
with the manner in which the rays divide in passing 
through certain crystals, I will examine the permanent 
modifications which they receive: I will present, in one 
word, the principal features of the new branch of opties 
which bears the name of polarization of light. 
Every ray of light falling even perpendicularly on any 
surface, natural or artificial, of the transparent crystals of 
carbonate of lime, called also cale spar, or Iceland spar, 
is divided into two. One portion passes through the 
crystal without deviation, which we call the ordinary ray ; 
the other undergoes a sensible refraction, and for that 
reason has very justly the name of the extraordinary ray. 
Both the ordinary and extraordinary ray lie in one plane 
perpendicular to the face of the crystal. The considera- 
tion of this plane is important, for it is this which deter- 
mines the direction which the extraordinary ray will 
take ; and in consequence a special name has been given 
to it, “the principal section.” 
These points being premised, I will suppose, to fix the 
ideas, that a particular crystal of cale spar has its princi- 
pal section directed north and south. Below this, and at 
any distance, we will place another similar crystal turned 
similarly ; that is, so that its principal section shall also 
lie in the meridian. What will result from this disposi- - 
tion, if light traverse the whole system? A single ray 
impinges on the first crystal but it emerges in two rays: 
each of these again seems as if it should undergo a double 
refraction in the second erystal ; and thence we might 
expect four emergent rays. Yet this does not happen. 
The rays emanating from the first crystal are not divided 
