so-called moveable polarization the double refraction was not considered as a necessary consequence of the appearance of its colour in the rectilinearly 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 unimportant.
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-undulation; the ray consequently will be again polarized rectilinearly, but perpendicularly to the plane of primitive polarization. After six reflexions it is again circular, but left-handed, if after the two reflexions it was right-handed, since the azimuth of the rectilinearly polarized incident light is now – 45° instead of + 45°. Finally, after eight reflexions the plane of the restored polarization coincides with that of the primitive one. The explanation of the observed phænomena 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 brody made use of, its thickness remaining unaltered. If this explanation is correct, precisely the same phænomena 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. Phænomena 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 perpendicularly 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 between and , before the Nicol's polarizing prism. 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 represented in the figure must therefore be imagined as altered 120°. In
