as exhibited in its propagation along plates of glass. 91 
Proposition XXX. 
In all the phenomena which have hitherto been described, the results 
are precisely the same, whether the anterior or the posterior face 
of the glass plate is exposed to the polarised ray; but, in the 
portions N, N' S, S' the tints change their character, according 
as one or other of the faces first receives the polarised light. 
If the plate a b, Fig. 34. (PI. IV.) has its lower surface ex™ 
posed to the polarised light, the portions n, n' exhibit, when 
combined with sulphate of lime, a tint due to the difference 
of their action ; and 5, 5' a tint due to the sum of their 
action ; but when the upper surface is exposed, as in Fig. 35. 
(PL IV. ) the portions .9, s' exhibit, in combination with sul- 
phate of lime, a tint due to the difference of their action, 
and the portions n, n' , a tint due to their sum. This curious 
phenomenon arises from the axes of the elementary crystals 
suffering an angular change of position, amounting to 90°, by 
turning the other side of the plate to the polarised ray, as 
shall be more particularly explained in a subsequent Pro- 
position. 
N 2 
