2o8 Dr. Brewster on the Affections of Light 
of reflection from GH being perpendicular to the plane of 
reflection from the topaz AB. A part of this beam will be 
reflected at C in the direction Cr, and part of it transmitted at 
C in the direction CF, no light being reflected from the flrst 
surface AB. The rays transmitted at C having been polarised 
before their incidence at R' are depolarised in passing from 
R' to C along the oblique depolarising axis, and the rays re- 
flected at C are polarised by reflection trom the surface ab, 
and again depolarised in their passage from C to r along the 
other oblique depolarising axis. 
If the observer now’ looks into the topaz in the direction ;r', 
he will perceive the first set of elliptical coloured rings, as re- 
presented in Plate VI., fig. i. These rings are now peculiarly 
distinct and brilliant, and it was therefore from them that I 
drew up the table of colours referred to from Plate VII., fig. i. 
Let the ray rP be now received upon a plate of agate hav- 
ing its lamiiice perpendicular to the section hBab, and a thb'd 
set of rings will be seen like those in Plate VI., fig. 3. This 
third set differs from the first set only in the central parts. 
All the rings have the same colours in both, but the central 
spots are much smaller in the third set than in the first, and 
the mass of darkness with which they are surrounded en- 
croaches considerably upon the blue part of the first ring. 
In the third set of rings the distance of the outsides 
of the two central spots is - "3° 3' 
Conjugate diameter of each spot - - ^ j 
Ditto of the black space between the 
spots - - - -10 
The third set, indeed, may be considered as the exact coun- 
terpart of the second set, all the colours of the former being 
