B23 
called The Jeffersoiiite, 
teresting instance, that of fluor-spar, Professor Mohs has de- 
scribed, and I have seen in his possession in Freyberg, specimens 
of the Saxon fluor, which cleaved in the direction of the cube 
and the dodecahedron, but the octahedral cleavage was very 
distinct. Before we change our opinion on this point, we must 
change all our ideas of cleavage, and of its high importance in 
the determination of minerals. 
In the hardness there is also a remarkable difference, the Py- 
roxene being decidedly harder. The specific gravity js likewise 
different : the highest specific gravity of Pyroxene recorded by 
Haiiy, is that of a large crystal from Vesuvius, which gave 
3.3578. The highest specific gravity indicated by Mohs is 3.5, 
while that of The Jeffersonite has, in every instance which I have 
seen, exceeded this limit. 
The chemical analysis offers another important difference, in 
the absence of magnesia, which appears to be essential to Pyro- 
xene. 
For these and other reasons, I conceive that there can be no 
doubt as to the necessity of considering this mineral as a distinct 
species. I am inclined to believe, that a closer study of the 
Diopside and Fassaite, and of the Pyroxene analogique^ might 
lead to their separation from the Pyroxene and union with The 
Jeffersonite. This is a subject which appears to me fraught 
with interest, but upon which I am not able to offer any thing 
but conjectures, as my specimens of these minerals are not as 
good as would be necessary, to enable me to decide this point. 
I shall close these remarks merely by observing, that a similar 
opinion is, I believe, entertained by Mr Vanuxem. 
Art. ^KlL.-^Description of a New Reflecting Telescope, By 
David Brewster, LL. D. F. R. S. L. & Sec. R. S; Edin. 
The only improvements on the Reflecting Telescope which 
have been proposed during the last century, are those of Le- 
maire and M. Burckhardt. The first of these authors suggested 
the idea of employing what is called the Front View in New- 
tonian telescopes of a large size ; and this method was found of 
