226 FRESNEL. 
that a ray may have the same properties round all points 
of its circumference, and yet not be common light. To 
show by a single example that these two species of light 
comport themselves differently, and ought not to be con- 
founded, I will observe that, in undergoing double re- 
fraction, a natural ray after traversing a plate of crystal 
gives two white images, while under the same conditions 
the’ ray of Fresnel is decomposed into two beams, each 
brilliantly coloured. . 
This new modification, which, having no reference to 
the different sides of a ray, has been designated circular 
polarization, can be impressed upon rays ordinarily 
polarized, by making them undergo two successive total 
reflexions from the internal surfaces of a piece of glass 
suitably formed.* The pleasure of having his name as- 
sociated with a new kind of polarization hitherto unsus- 1 
pected, would probably have sufficed for the vanity of 
an ordinary experimenter, and his researches would not 
have extended beyond that point. But Fresnel was 
actuated by more elevated sentiments; in his eyes 
nothing seemed to have been done while any thing re- 
mained to do. He sought, therefore, if there were not 
the double refracting crystal, the two images in the former case will 4 
be always white, in the later coloured. | 
* In the instance mentioned, Fresnel showed, by a remarkable in- 
stance of theoretical prediction, that a ray polarized at 45° to the 
plane of incidence, and twice reflected internally from glass, will 
emerge in the condition of two rays polarized in planes at right 
angles, and one retarded by one fourth of a wave-length behind the 
other; these being superimposed will, by mathematical consequence, 
give rise to vibrations, no longer plane, but performed in circles; or in 
ellipses, if the retardation be any other fraction of a wave-length. 
Such a piece of glass is called Fresnel’s Rhomb. The course of the 
ray will be apparent by inspection of the annexed diagram, which 
needs no further explanation. 
