as exhibited in its propagation along plates of glass. 87 
No. 1 . One of the most curious specimens of crystallized 
glass which I have obtained, is a parallelopiped. about 0.38 of 
an inch broad and deep, and 1.11 inch long. It depolarises a 
faint yellow of the first order in the central fringe* when 
polarised light is transmitted through the faces of the paral- 
lelopiped. But when the light is transmitted along the axis 
of the parallelopiped, and when the lines AC, AB are parallel 
or perpendicular to the plane of primitive polarisation, the two 
images formed by calcareous spar exhibit the forms repre- 
sented in Figs. 25, 2 6. (PL III.) The first of these consists of 
a black cross surrounded with beautiful fringes of contrary 
flexure, and has bright green spots of the third order with a 
little yellow of the same order; their centre at the four angles, 
A, B, C,D. Figure 25. (PL III.) exhibits a form exactly com- 
plementary to Fig. 2 6 . (PL III.) and remarkable like it for the 
symmetry of its form. The coloured spots at the angles are 
now a brilliant pink, with a spot of blue in the middle of 
them. When the lines AB, AC are inclined 45 0 to the plane 
of primitive polarisation, the two images exhibit the forms 
represented in Figs. 27, and 28, (PL IV.) 
No. 2. Is another piece of glass of a square form, and 0.3 
of an inch thick, it produced the central cross, and exhibited at 
the angles all the tints up to the blue of the second order 
arranged in circles, having the blue or maximum tint in the 
centre. See Figs. 29 and 30. (PL IV.) 
No. 3. A third plate 0.4 of an inch thick produced the 
same effect, the angular tints rising in this case to the yellow 
of the second order. 
No. 4. A fourth plate, 1.2 inch thick, produced fringes of 
contrary flexure like those of No. 1, but rising to the pink of 
the fourth order. 
