to his Observations on the Optical System of Mineralogy. 195 
stein, which I have ever been possessed of, were a few frag- 
ments of crystals without summits, and with very imperfect na- 
tural planes, for which I am indebted to Dr Brewster. All I 
could do with these was to cleave and measure them. And on 
applying the minute fragments obtained from cleavage, to the 
reflective goniometer, the inclination of the cleavage planes ap- 
peared to be the same as that of the common needlestone. 
The case of the mineral from Aix-la-Chapelle, now Hopeite , 
is different. On turning to a letter of Dr Brewster's, I perceive, 
that our last communication relative to this mineral was in De- 
cember 1821. At that time I had seen only one crystal of it, 
which was in a deep and narrow cavity erf the matr ix , and pre- 
sented only one of its sides to the observer. This, I believe, I 
mentioned to Dr Brewster at that time. In such a state, it is 
plain it could not be particularly examined. Some months af- 
terwards, my friend Mr Heuland gave me a detached crystal of 
this substance, and all the mighty resources which were called 
forth for its examination, were the measurement of two angles 
by the goniometer, and the resolution of a single spherical tri- 
angle. The result gave a difference from the Stilbite in the di- 
mensions of the prism, supposing it to be rectangular, and in 
the inclination of its lateral planes, supposing it to be rhombic ; 
and this was communicated to Mr Heuland a few days after I 
received the crystal. I should not have taken the trouble of 
relating these very unimportant circumstances, but for the pur- 
pose of correcting Dr Brewster's inaccurate statement. 
Dr Brewster alludes to a chemical examination of these mi- 
nerals, which, he says, has completely established his determi- 
nations. May I request him to state the precise result of this 
examination, and when, and by whom, it was made, as I cannot 
find any analysis of either of them published. 
Having so far disposed of the preceding points, I have now 
only to offer a few remarks upon the present state of Dr Brew- 
ster's Optical System. The double refraction of crystals had 
been ascertained to belong to many substances long before Dr 
Brewster’s researches began ; but Dr Brewster ascertained its 
existence in many others ; and that it might, in numerous in- 
stances, be observed in more than one position of the crystal ; 
and the term axis of double refraction, has been introduced to 
