FOR EXHIBITING THE POLARIZATION OF LIGHT. 93 
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 phznomena 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, forma 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 irregu- 
larities of the figures produced by unequal cooling appear in the cir- 
cular light, particularly with thin plates, and often even with those 
which appear regular in the linear light; nevertheless, I have also ob- 
served 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 diametri- 
cally 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 recti- 
linearly polarized incident light, the figure is also here displaced in the 
' quadrants, when the light becomes circularly analysed. 
* This application of Weber’s method of compressing glass to the phenomena 
of polarization was shown to me by Prof. Mitscherlich. (Compare Poggen- 
dorff’s Annalen, vol. xx. p. 1.) 
