as exhibited in its propagation along plates of glass. 95 
spaces. When polarised light is reflected from this plate at 
the polarising angle, the preceding phenomena are very 
finely displayed. The minute fringes mentioned in Prop. 
XXVI. are also seen by looking through the edges of the 
plate, and are not affected by the circular crystallization. 
Proposition XXXVI. 
When a cylinder of glass is brought to a red heat, and cooled in 
the open air, it acquires a permanent crystallization, in which 
the principal sections of all the elementary crystals are directed 
to the axis of the cylinder. 
The phenomena exhibited by transmitting polarised light 
along a cylinder of this kind, about 9 ,~ inches long, and of 
an inch in diameter, are. shown in Figs. 37 and 38. (PI. IV.) 
where A B CD, Fig. 37. (PI. IV.) is the principal image, and 
abed , Fig. 38. (PI. IV.) the complementary image. The 
dark cross AC, BD, instead of having its arms inclined 45 0 
to the horizon, as in Fig. 19. (PI. III.), has them parallel and 
perpendicular to the horizon, as the light transmitted through 
the cylinder happened to be polarised in the plane of the 
horizon. The luminous spaces between the arms of the cross 
contain about 10 beautiful rings of coloured light. The com- 
plementary image abed is marked with four dark spots, cor- 
responding to the four luminous portions round the central 
part of the cross, and the outer part has four dark sectors 
A, B, C, D, corresponding with the light ones in the other 
image, and formed of small concentric arches of a dark hue, 
fringed with tints of different colours. In ord r to see this 
phenomenon in all its beauty, it is necessary th.t the polarised 
ray beexactly parallel to the axis of the cylinder, as the slight- 
est deviation completely destroys the regularity of the figure. 
