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 * 



