2i8 Dr. Brewster on the laws of polarisation, &c. 
i 
combine the system of rings formed by the attractive crystals, 
with the system formed by the repulsive ones; for rock 
crystal being the only attractive substance of one axis with 
which he was acquainted, was quite unfit for this purpose, 
from the imperfection of the system of rings which it pro- 
duces. The discovery of the optical structure of zircon * has 
enabled me to perform the experiment with success. 
This opposition of structure between quartz and the other 
four crystals with one axis, contained in M. Biot's table, is 
ascribed by this eminent philosopher to an opposition in the 
forces which act upon the extraordinary ray ; and he consi- 
ders it as an established point, that in rock crystal, the extra- 
ordinary ray is attracted to the axis, while in the other 
crystals it is repelled from it. Hence he has concluded, that 
the phenomena may be represented in the first case by a 
prolate ellipsoid, and in the second case by an oblate ellipsoid 
as Huygens had already shown ; and has thus been led to 
divide crystals into two great classes, attractive and repulsive. 
This division, however, is entirely hypothetical, in so far as 
the polarising forces is concerned, for it will afterwards be 
shown that the phenomena of rock crystal may be explained 
by a negative force emanating from two equal rectangular 
axes, or that the phenomena of the other class may be ex- 
plained by a positive force in a similar manner ; or if we 
prefer the analogies which the sciences of magnetism and 
• The extreme difficulty which attends experiments of this kind, will be understood 
from the fact, that I cut more than fifteen plates out of a large piece of zircon without 
discovering its axis. By a singular accident, however, Mr. Morton, jeweller, in 
Edinburgh, procured for me no fewer than sixty plates of zircon with parallel faces, 
and it was only in two of these that the system of rings was developed. 
