196 Mr Brooke's Remarks on Dr Brewster's Reply 
express an imaginary line , bearing a certain relation to the po- 
sition in which this property may be observed. 
Dr Brewster complains, that, in my former allusion to his 
theory, 64 the reader is not told what the system is that is as- 
sayed ; he is not made aware that it has any foundation at all 
as a general principle ; he is not told of its successive triumphs 
over everypositive objection that has been urged against it ; but 
he is hurried at once, and without ceremony, to witness its de- 
molition." 
I shall now endeavour to atone for my former negligence, by 
presenting an outline of Dr Brewster's Optical System to the 
reader, and then stating those objections to it, over which it has 
not yet triumphed. 
But it will first be convenient to insert an outline of Professor 
Mohs's Arrangement of Crystalline Forms, with which Dr Brew- 
ster states that he has found the optical classification very gene- 
to agree. 
© 
Mohs has separated what I have termed the primary forms 
of crystals, into four systems of primitive forms. 
1st, The Rhomboidal, Comprehending the rhomboid, and 
the hexagonal prism. 
%d, The Pyramidal, Comprising the octahedron with a 
square base , and the right square 
prism. 
3d, The Prismatic, Comprising the following prisms : 
the right rectangular, 
the right rhombic , 
the right oblique-angled, 
the oblique rhombic , 
the doubly oblique, 
And also comprising 
the octahedron with a rectangular base , 
— — rhombic base. 
1th, I he Tessular, Comprising the 
cube, 
r egu la r tetrahedron , 
regular octahedron, 
rhombic dodecahedron . 
