as exhibited in its propagation along plates of glass. 55 
The result of these combinations is the production of a dark 
cross, as represented in Fig. 3. (PL II.) This cross is extremely 
regular and beautiful when the two plates have the same breadth, 
polarise the same tints, and have their exterior fringes of the 
same magnitude at both edges. When some of these circum- 
stances are varied, the cross changes its form in a manner 
which can easily be ascertained from a previous examination 
of the separate fringes ; but, when the one plate polarises higher 
tints than the other, the cross is no longer produced. The 
fringes of the plate which polarises the highest tint, are bent 
from their rectilineal direction, as represented in Fig. 4. (PI. II.) 
As the figures exhibited at the intersection of two plates can 
always be determined, a priori, from a knowledge of the fringes 
which each plate produces separately, so the nature of the 
separate fringes, and the rate at which the tints change, may 
be easily predicted from the figures which are exhibited at 
the place of intersection. When the tints polarised by the 
two plates are numerous and brilliant, the intersection al 
figures are singularly beautiful. 
