CHROMATIC POLARIZATION. 225 
as we gradually make the mirror go through all possible 
changes of position, Here there are not only four poles 
placed in two rectangular directions, which we must ad- 
mit in the constitution of the ray, but we see that there 
are thousands ; that every point in the circumference 
round the ray has a special character; that every face 
which it presents produces in the reflexion a particular 
tint. This strange dislocation of the natural ray (I may 
be allowed this word, since it exactly expresses the fact) 
thus affords the means of decomposing white light by 
means of reflexion. The colours, it must be avowed, 
have not all the homogeneity of those which Newton ob- 
tained by the prism; but also the object from which they 
originate does not undergo any distortion, as in prismatic 
refraction : and in a multitude of researches this is a point 
of material importance. 
To discover whether a ray has received the polariza- 
tion of Huyghens and Malus, or that of which I have 
just spoken, and which we call chromatic polarization, it 
suffices as we have seen, to make it undergo double re- 
fraction: but from the fact that a ray in traversing a 
crystal of Iceland spar always gives two images of white 
light and of equal intensity, it will not follow that it is 
formed originally of common light: this is again the 
discovery of Fresnel.* It is he who first pointed out 
* The author would have expressed his meaning more clearly to 
general apprehension if he had said, that natural or unpolarized white 
light, on traversing Iceland spar, gives two white images in all posi- 
tions: an ordinarily polarized ray does not; but there is a kind of 
light which gives always two images, and yet is not unpolarized: this 
is the circularly polarized light, discovered by Fresnel. One test 
which distinguishes it from common light is, that on interposing a 
crystallized plate of selenite, mica, &c., before receiving the light on 
10 * 
