to his Observations on the Optical System of Mineralogy. 1 97 
To which Dr Brewster has proposed to add the composite sys- 
tem j'—an addition to which there can be little exception, as a 
matter of amusement, if it be established merely to receive those 
specimens, which, from the superior beauty of their optical phe- 
nomena, may claim a distinction in rank from their Doric, Ionic , 
and Corinthian neighbours. But it does not at present appear 
necessary for crystallographical purposes. 
Dr Brewster appears to have examined a very considerable 
number of mineral and other bodies, by means of polarized 
light, and he has been led by this examination to the following 
optical classification of crystals : \st, Those in which only one 
axis of double refraction has been yet observed ; which include 
nearly all the crystals comprised in the first and second of Mohs’s 
Systems. 
2d, Those which possess two axes , consisting of nearly, if not 
entirely, the whole of Mohs’s third system. But some which 
erystallographically belong to the first and second systems, are 
also included by Dr Brewster in this. 
3d, Those in which no double refraction has been yet disco- 
vered, either by direct experiment, or by means of polarised 
light, and which belong to the fourth system of Mohs. 
Out of these facts, which, we must allow, are very numerous, 
and lienee very imposing, Dr Brewster has produced two op- 
tical systems ; one relating to mineralogy generally ; and the 
other relating to compounds of sulphuric and tartaric acids, 
with earthy, alkaline, and metallic bases, all of which are said to 
have two or more axes of double ref raction. It will appear after 
wards, that one or the other of these systems must probably be 
surrendered to a salt of nickel. 
The assumed powers of Dr Brewster's optical system of mine- 
ralogy are very remarkable. We have already seen the facility 
with which it professes to unfold the chemical and architectural 
splendours of ‘ tesselite. In page 363. of No. XVIII. of this 
Journal, Dr Brewster assures us, that the optical phenomena on 
which this system is founded, are the necessary residts of a me- 
chanical structure; and that their indications must infallibly har- 
monise with the sound deductions of crystallography . Yet in 
page 364, Dr Brewster asserts, that the tesselite possesses a 
structure which defies all the laws of crystallography. It is not 
easy to perceive how phenomena which necessarily result from 
