Dr. Brewster on the laws of polarisation , &c. 227 
PI. XV. figs. 4 and 5, may be called the meridian of direct polari- 
sation, because the tints ascend directly from O.otoAand B,and 
the great circle COD 0 the meridian of inverse polarisation , not 
only because the tints descend from O to P and P', and ascend 
from P to C and from P to D, but because there is a real 
inversion in the character of the tints on each side of PP'.* 
In mica, topaz, and other crystals, where the distance PP' 
is 45 0 and upwards, the system of rings represented in PI. XV. 
fig. 4, cannot easily be seen at once ; but in nitre and other 
crystals where PP' is very small, the whole system is finely 
developed, and every individual curve may be examined with 
attention. 
In order to observe these rings to advantage, let a plate of 
nitre ACBD, PL XV. fig. 6 , about T ~ or of an inch thick, 
be cut perpendicular to the axis of the hexaedral prism. If 
this plate is exposed to polarised light, so that either A B 
or CD is in the plane of primitive polarisation ; and if the 
transmitted light is analysed with an achromatic prism of 
calcareous spar, the extraordinary image will exhibit the 
system of rings shown in PI. XVI. fig. 7, while the ordinary 
one will exhibit a system exactly complementary. 
The poles P,P' of no polarisation, are distant 5 0 20', what- 
ever be the thickness of the plate ; and through them passes 
one of the branches CD of the rectangular cross A, B, C, D. 
The breadth of each ring is least between C and P and D 
* The preceding names given to the three great circles, have been drawn from 
the most prominent physical characters which belong to them. Other names, such 
as the meridian of the principal axis , the meridian of the secondary axis, tic. would 
have been preferable, had they not involved an hypothesis ; for it will afterwards be 
seen that the phenomena may be equally well explained by axes varying both in 
number and position. 
