CIRCLLAR POLARIZATION OF LIGHT. 79 



so-called moveable polarization the double refraction was not considered 

 as a necessary consequence of the appearance of its colour in the recti- 

 linearly polarized light, it is desirable to confirm by new experiments 

 the proofs that these colours originate in the difference of path of the rays 

 passing through the glass. The following therefore, for the explanation 

 of the colours upon the principle of interference, seems to me not un- 

 important. 



When a ray polarized rectilinearly in the azimuth of 45°, after two 

 total i-eflexions in the interior of a Fresnel's parallelepiped, exhibits a 

 difference of phase of a quarter-undulation, between the quantities of 

 light polarized perpendicularly to each other, of uniform intensity, this 

 difference will in this case, after four reflexions, become a half-undu- 

 lation ; the ray consequently will be again polarized rectilinearly, but 

 perpendicularly to the plane of primitive polarization. After six re- 

 flexions it is again circular, but left-handed, if after the two reflexions 

 it was right-handed, since the azimuth of the rectilinearly polarized in- 

 cident light is now — 45° instead of -f- 45°. Finally, after eight re- 

 flexions the plane of the restored polarization coincides with that of the 

 primitive one. The explanation of the observed phaenomena of circular 

 polarization in the above-mentioned experiments, depended upon making 

 the difference of path of the two rays exactly equal to the quarter- 

 undulation, by means of a determinate change of heat in the interior of 

 the Irody made use of, its thickness remaining unaltered. If this expla- 

 nation is correct, precisely the same phaenomena would be obtained by 

 \ gradual heating as by successive reflexions in the interior of the Fresnel's 

 rhomboid, but with this difference, that instead of the direction of the 

 polarization varying by successive steps we should expect a continual 

 transition through all degrees of elliptic polarization. The experiments 

 confirm this perfectly. They must of course be made in homogeneous 

 light. 



3. Phccnormna during the Heating and Cooling of the Glasses. 



The apparatus (Plate II. fig. 1.) more particularly described in the 

 succeeding paper was adjusted before a monochromatic lamp giving 

 yellow light, so that the plate of Iceland spar in the ring I, cut perpen- 

 dicularly to the axis, exhibited distinctly the black rings with the dark 

 cross, when the glass cube reduced by a new heating and cooling to 

 perfect loss of action upon polarized light, was thus intei-posed be- 

 tween k and o, before the Nicol's polarizing pi-ism. In order to heat it 

 conveniently over a lamp, the three-sided prism or rod be, carrying all 

 the polarizing arrangements, was placed in such a manner in its case as 

 to bring those arrangements from their vertical situation over the rod to 

 a position in which they projected on one side of it; their position as re- 

 presented in the figure must therefore be imagined as altered 120°. In 



