158 
Mr Brooke's Remarks on Mr Haidinger s Paper 
in the Transactions of the Royal Society of Edinburgh, I ob- 
serve, that the lateral planes which, in fig. 6. of this Journal, 
are denoted by 5, are marked by the letter e> in Part I. of Vol. X. 
of the Transactions, and, according to Mr Haidinger 's theory of 
the structure of these crystals, the planes in question are those 
marked and not b, of fig, 2, in this Journal. I shall there- 
fore consider these planes as if they were marked e , although I 
shall refer to them as planes b , from their being so marked in 
fig. 6. 
It is not easy to understand Mr Haidingers own ideas of the 
manner in which this theoretical crystal, fig. 6. is formed, from 
the explanation he has given in the 12th page of his paper, in 
the Transactions of the Royal Society of Edinburgh ; but I think 
I do not misrepresent his meaning by stating, that in this figure 
the 44 faces of crystallisation on the other (under) side, are paral- 
lel to those considered above,"” so that e on the adjacent upper 
plane shall measure the same as e / on the adjacent plane below. 
This being so, it follows that b on a will measure the same as 
V on the plane below adjacent to e , and the measurements of the 
crystals would thus be 
a on e - - 112° 
a" e' - - 112 
e' adjacent plane below, which may be called a 2 112 
a b - - 90 14' 
a" b" - 90 14 
b' a 2 below - . 90 14 
a a' or a" - - 1 T9 1 0 
b b” a re-entering angle of - 180 20 
and consequently measuring by the gonio- 
meter - . 179 40 
the left hand b’ on b - 119 40 
the upper b" on the right hand W 119 40 
The lines on the upper and lower terminations are supposed 
to be edges at which the planes a intersect each other at angles 
of 179° 10'. This angle Mr Haidinger says it is 44 very easy to 
overlook."” From what experience Mr Haidinger entertains this 
idea is not expressed, but it appears to me that such an angle 
could not well escape the notice of even a moderately close ob- 
server. 
If the reader will take the trouble of examining any speci- 
3 
