Page:Scientific Memoirs, Vol. 1 (1837).djvu/105

Rh and in the vacant space $$p$$, on the contrary, the partially polarized incident light is changed in the direction of the second reflexion. The light of the vacant space m differs from that in a Fresnel's parallelopiped by having the planes of the two reflexions perpendicular to each other, instead of coinciding as they do when that is used.

The phænomena of cooled glasses in circular light have not yet been described particularly, and those of compressed glasses not at all; we shall therefore add a few words respecting them.

In circular analysis solid cooled cylinders have the same properties as Iceland spar plates. In linear analysis they exhibit the system of rings without the cross displaced in the quadrants. The same may be said of the rings of colours of hollow cylinders, which are concentric with the inner black ring, and abruptly separated. The cross in three-sided plates consists of four black points (with two plates placed upon each other it consists of four triangles), which, united by bright gray shades, form a Y. In six- and eight-sided plates the black central spot becomes a six- and eight-sided star, while the colours of the angles are arranging themselves into a very regular inclosure particularly when by turning the analysing prism the centre becomes white: figs. 8. and 9. represent the figures of cooled cubes. The isochromatic lines of rectangularly crossed parallelopipedal plates remain, with regard to their form, identical with those in the linear light, which appear when the plane of polarization bisects the right angle between the plates. All the figures remain unchanged when the glasses are turned in their plane at the time of circular polarization and analysis. The irregularities of the figures produced by unequal cooling appear in the circular light, particularly with thin plates, and often even with those which appear regular in the linear light ; nevertheless, I have also observed precisely the reverse, and that indeed with a six-sided plate.

A cylinder compressed by brass wire wound round it had the same properties as a cooled one. Square and circular plates diametrically compressed by a screw, exhibit between the rings originating at the compressing points of the screw a coloured junction without a cross. If the axis of compression lies in the plane of polarization of the rectilinearly polarized incident light, the figure is also here displaced in the quadrants, when the light becomes circularly analysed.