73 DOVE'S EXPERIMENTS ON THE 
quarter-interval; but the phenomenon 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 versd. By placing 
the tourmaline axes and the axes of compression parallel severally to each 
other, we obtain the phenomena 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 plate 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 114 lines in dia- 
meter, and 33 lines in thickness. 
2. Cireular 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 excentri- 
eally 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 refraetion of the cooled glass ; and as in the theory of the 
