6 Dr. Brewster on the Optical Properties 
the other had an inclination of 27° 28’. The tints of the 
Brazilian Topaz deviate more than those of the other crystals 
from the colours of Newton’s scale, and are produced by a 
polarizing force of inferior intensity. 
The phenomena of the coloured rings in Scotch Topaz which 
I have described in the Phil. Trans. for 1814, are neither seen 
with the same distinctness, nor to the same extent, in the Topaz 
of Brazil. This circumstance arises from the difference in the 
inclination of their resultant axes. In the Scotch Topaz the 
angle formed by their axes is such, that when light incident 
along the one axis is reflected in the direction of the other, 
the angle of reflection is almost the same as that of maximum 
polarization for Topaz, and hence the rings appear with peculiar 
brilliancy, and exhibit themselves under the new modifications 
which I have distinguished by the names of the third and 
fourth Set* in the Paper already quoted. In the Brazilian 
Topaz, however, the inclination of the resultant axes deviates 
considerably from twice the polarizing angle, and consequently 
the preceding phenomena are very indistinctly displayed. 
The Brazilian Topazes are in general phosphorescent when 
placed upon a heated iron, although I have found several, 
especially among the finer crystals, that do not possess this 
property in the slightest degree. The tesselated crystals display 
their phosphorescence in a very singular manner. Sometimes it 
is of a rich orange red colour, and in many cases the external 
border is phosphorescent, while the interior nucleus discharges 
no light at all. This phosphorescent light is, in general, most 
brilliant in the outer lamine, though I have seen in some 
crystals the greatest intensity of light at the boundary of the 
* The fourth set of rings which has a peculiar character, is a combination of the 
first and second set, or of the direct and complementary systems. 
