78 DOVKS EXPERIMENTS ON THK 



quarter-interval; but the phaenomenon 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 com- 

 plete, and which is made to proceed in a circular form ; the first and third 

 quadrant then become the second and fourth, and vice versa. By placing 

 the tourmaline axes and the axes of compression parallel severally to each 

 other, we obtain the phaenomena of rectilinearly polarized light.] 



If between the crossed mirrors we insert a round or square plate com- 

 pressed 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 jilate 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 11^ lines in dia- 

 meter, and 3^ 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 cxcentri- 

 cally 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 cirodarly 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 tlie theory of the 



