Dr. Brewster on the laws of polarisation , &c. 263 
sary for the proper classification of the phenomena, I would 
have no hesitation in reversing the name, and giving the 
appellation of imperfect crystallization to the phenomena 
produced by sulphate of lime, and the greater number of 
crystals of the mineral kingdom. If we employ pieces of 
pure glass of regular forms, and with straight and smooth 
edges, and if we transmit through them the heat in a uni- 
form and careful manner, we shall obtain a structure which 
will develope all the phenomena of polarisation with the 
utmost beauty and perfection. 
There is one point, however, and a very instructive one, in 
which the artificial differ from the real crystals. If we cut a 
plate of calcareous spar into any number of pieces, each 
piece, however minute, will produce the system of coloured 
rings in as great perfection as the whole plate of which it 
formed a part ; and its polarising force will suffer no dimi- 
nution by the reduction of its size, provided its thickness 
remain the same. If we subdivide, however, a circular plate 
of glass, the polarising force of each portion is not only 
much diminished, and sometimes even destroyed, if the por- 
tion is very small, but the polarising influence is distributed 
in a new manner, according to the outward shape of the 
fragment.* Hence it follows, that in glass the polarising 
structure depends entirely on the external form oj the plate, 
and on the mode of aggregation of its particles. When its 
form is circular, it has only one axis of polarisation ; and this 
axis is positive, if the density diminishes towards the centre, 
and negative, if it increases towards the centre ; but when 
See Phil. Trans. 1816, pp. 71, 8 z. 
