228 Dr. Brewster on the laws of polarisation , &c. 
and P, and gradually increases towards AB, owing partly to 
the different thicknesses at which the light is transmitted 
through the plate. 
When the plate of nitre is turned round its axis, the black 
cross ABCD immediately breaks up, according to laws which 
will afterwards be explained ; and when AB forms an angle 
of 4.5 0 with the plane of primitive polarisation, the system of 
rings has the appearance shown in PL XVI. fig. 8. The rings 
themselves have suffered no change by this change of position 
in the plate ; but the black cross is separated into two hyper- 
bolical branches M'P'N', MPN. When the thickness of the 
plate is above T ^ths of an inch, the portions of the rings 
included between the two hyperbolic branches, are con- 
founded into one mass of white light, while those between 
C and P, or D and P, are distinctly visible. The concave 
sides of the hyperbolic branches are strongly fringed with a 
red and yellow colour, while the convex or inner sides are 
equally affected with blue rays. By diminishing the thick- 
ness of the plate, the rays gradually appear between the 
hyperbolic branches, but they consist only of pink and green 
tints, like those in the 5th order of Newton’s scale ; and 
even when the rings almost cease to appear by a great dimi- 
nution of thicknesses, the tints, though at the very com- 
mencement of the first order, are not the same within as 
without the poles of no polarisation. In the position of PL XVI. 
fig. 7, however, and when the tint at O, in the position of PL 
XVI. fig. 8, is only blue of the first order, this irregularity of 
the tints is almost imperceptible, and the system of rings is dis- 
tinguished from that produced by crystals with one axis only, 
by a slight degree of ellipticity. This approximation to the 
