quarter-interval; but the phænomenon is in that case not reciprocal, as a revolution here takes place similar to that which occurs when we look from the opposite side at an electric current in which the circuit is complete, and which is made to proceed in a circular form ; the first and third quadrant then become the second and fourth, and vice versâ. By placing the tourmaline axes and the axes of compression parallel severally to each other, we obtain the phænomena of rectilinearly polarized light.
If between the crossed mirrors we insert a round or square plate compressed to a certain degree, so that the axis of compression coincides with one of the planes of reflexion of the mirror, we see upon it a black cross with white vacant spaces at the corners. If by means of the plate of Iceland spar these four white vacant spaces be examined, we find that those which belong to the same diagonal are similar to each other, but in opposition to the two white vacant spaces of the other diagonal; and it will be found that the light proceeding from them is circularly polarized, in the one diagonal to the right and in the other to the left. Hence it directly follows, that when the pllate is turned in its plane 90°, all the white vacant spaces have exactly exchanged their effect in the diagonals. The plates I made use of in these experiments were 11½ lines in diameter, and 3¾ lines in thickness.
2. Circular Polarization by Cooled Glasses.
I carefully cooled a glass cube of 17 lines each side, so that when the diagonals of the surface of the cube turned towards the eye form with the plane of polarization an angle of 45°, it exhibited between the crossed mirrors in the centre a dark cross, and in the four corners only the white surrounding it. The light of the four white vacant spaces was exactly similar to the light of the four white vacant spaces of the compressed plate, when their axis of compression lay perpendicularly to, or within, the plane of polarization. By turning the cube excentrically round the ray perpendicularly escaping through one of the white vacant spaces, as round an axis of revolution, similar variations are produced, whilst at 90° revolution the diagonals interchange their effect. Instead of turning the cube round, it may, in order to obtain the same variations, be so moved that two of the parallel sides of the surface turned towards the eye are carried forwards perpendicularly to their direction, whilst the other two advance in their own path. We pass from the white vacant space of the one diagonal into that of the other. The combinations of the cooled glasses, for the purpose of analysing circularly a circularly polarized light, explain themselves. In order to obtain the system of rings without the cross with the black spot in the centre, they must be combined as in Plate II. fig. 5.
So far as I am aware, we possess as yet no direct experiments upon the double refraction of the cooled glass; and as in the theory of the
