102 Dr. Brewster on new properties of heat , 
other crystal a situated out of the diagonal AH, will be acted 
upon by forces proportional to its distances am, an, from the 
edges AB, AD, and in the direction of these lines. It will 
therefore turn its axes in the direction a A the diagonal of the 
parallelogram A nam. In like manner it may be shown, that 
all the other crystals will turn their axes towards A in lines 
diverging from A as a centre. Each angular portion, there- 
fore, exactly resembles an inverted quadrant of the cylindrical 
piece of glass represented in Fig. 37, (PI. IV.) and described 
in Prop. XXXIV., and consequently an arm of the black cross 
will appear in the diagonal AH in every quarter of a revolu- 
tion. The diagonal portions AH will be dark when all the 
other fringes are visible, and the diagonal fringes will appear 
in their full beauty, when the rest have vanished. Since the 
diagonal fringes at A and C have their axes AH, CH parallel, 
they will exhibit tints of the same character, and opposite to 
those of B and D which have their axes BpI, DH at right 
angles to the former. The reason is therefore manifest, why 
each diagonal fringe changes its character by inverting the 
plate, for when this inversion takes place the axis of the dia- 
gonal portion is put into a position at right angles to its first 
position. 
These observations enable us to explain the appearances 
shown in Fig. 10, and 11, (PI. II.) and described in Prop. XVII. 
In Fig. 10, where the plate is narrow, the black spaces at C and 
D, bisecting the angles, interfere and nearly obliterate the 
interior fringes, but in Fig. 11, where the plate is consider- 
ably broader, the influence of the angular crystallization does 
not extend so far, and therefore the interior fringes are seen 
at 2, Fig. 11. The state of the crystallization at the angles 
