as exhibited in its propagation along plates of glass. 89 
developed, and will possess the same properties as if the whole 
plate had been regularly crystallized. . 
Proposition XXIX. 
At the extremities A, B of every plate of crystallized glass, there 
are four portions N, S, N' S', at the boundary between the 
terminal and the lateral fringes , which possess a structure diffe- 
rent from the rest of the plate. These portions have their axes 
inclined to axes of the other parts of the glass. The portions 
N, N have their axes in the same direction, and S, S’ in a 
direction opposite to those of N, N'. 
When a plate of crystallized glass is exposed to a polarised 
ray, so that its length in the direction of the lateral and cen- 
tral fringes is parallel or perpendicular to the plane of primi- 
tive polarisation, it will exhibit the appearance shown in 
Fig. 31. (Pi. IV.) where all the lateral, central, and terminal 
fringes have vanished. Four luminous spots, however, N, S, 
N', S', will be seen at the extremities A, B, exhibiting tints 
which, in general, vary from the white of the first order to the 
pink of the second order, and sometimes exceed, and some- 
times fall below, the maximum tint of the central fringes. In 
order to examine the nature of these tints, I took a plate of 
glass, which when held in the position already mentioned, 
polarised at the points N, S, N', S', a blue of the second order. 
I then combined with it a plate of sulphate of lime which pola- 
rised the same tint, and which had its axis inclined 45°to the 
plane of primitive polarisation. The resulting tints at the 
angles N, N', were black, or that which was due to the 
difference of their actions, while the resulting tint at S, S', 
MDCCCXVI. N 
