CIRCULAR 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 reflexions in the interior of a Fresnel’s parallelopiped, 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- 
eident light is now — 45° instead of + 45°. Finally, after eight re- 
flexions the plane of the restored polarization coincides with that of the 
primitive one. The explanation of the observed phenomena 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 body made use of, its thickness remaining unaltered. If this expla- 
nation is correct, precisely the same phenomena 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. Phenomena 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 /, 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 interposed be- 
_tween k and 0, before the Nicol’s polarizing prism. In order to heat it 
conveniently over a lamp, the three-sided prism or rod be, carrying all 
e polarizing arrangements, was placed in such a manner in its case as 
bring those arrangements from their vertical situation over the rod to 
osition in which they projected on one side of it; their position as re- 
resented in the figure must therefore be imagined as altered 120°. In 
