as exhibited in its propagation along plates of glass. 81 
Proposition XXIV. 
When a plate of glass, crystallized in the manner described in the 
preceding Proposition , is inclined to the polarised ray in a plane 
perpendicular to the direction of the fringes, the central tints 
ascend in the scale of colours , as if the plate had increased in 
thickness ; but , when it is inclined in a plane parallel to the 
direction of the fringes, the central tint descends in the scale , as 
if the plate had become thinner. When the plane of inclina- 
tion forms an angle 0/ 45 0 with these planes, no change is pro- 
duced in the tints. 
I took a plate of crystallized glass which polarised in the line 
a b , Fig. 2. (PL II.) a broad but very faint tinge of yellow; 
when it was inclined in a plane perpendicular to the direction 
of the fringes, the tint which it polarised became a dark orange 
yellow — but, when it was inclined in a plane at right angles 
to the former, the tint became a pale bluish white. A similar 
result was obtained, when the colours belonged to higher 
orders in the scale. 
The effect of inclination may be seen more advantageously 
when two plates that polarise the very same tint, are placed 
transversely, so as to exhibit the cross represented in Fig. 3. 
(PI. II.) By inclining one of the plates, the other is necessa- 
rily inclined in an opposite plane, so that the tints of the 
one ascend, while those of the other descend in the scale of 
colours. 
The consequence of this is a separation in the middle of the 
cross, producing two curved black fringes, having the same 
appearance that is afforded by crossing two plates that pola- 
rise different tints. 
MDCCCXVI. 
M 
