THE 
EDINBURGH 
PHILOSOPHICAL JOURNAL. 
Art. I. — On a New Optical and Mineralogical structure^ 
exhibited in certain specimens qf Apophpllite and other 
minerals.'^ By David Brewster, LL. D. F. li. S. Lond. 
and Edin. Communicated by the Author. 
About the end of the year 1816, I received from Major 
Petersen some fine crystals of the Apophyllite surcomposee of 
Haiiy, from Fassa in the Tyrol. Upon exposing them to po- 
larised light, I found that they had one axis of Double refrac- 
tion ; but I was surprised to observe, that the system of colour- 
ed rings, with which this axis was surrounded, was composed of 
unusual tints, the only colours of the first orders being hlinsh- 
violet and greenish-yellow^ separated by a ring of white light. 
In order to ascertain the origin of these anomalous colours, I 
collected all the Faroe Apophy Hites which could be procured, and 
upon subjecting them to a minute examination, I discovered the 
very extraordinary structure of this mineral, and was led to the 
results which it is the object of the present paper to describe. 
The Apophy Ihtes from Faroe, crystallize in quadrangular prisms, 
with fiat summits, having an almost imperceptible truncation 
upon the angles, and also in single and double four-sided pyra- 
mids, the planes of which correspond with the small trunca- 
tions upon the prism, and form angles of 60® with those on the 
opposite side. The greater proportion of’ the quadrangular 
prisms have their planes highly polished, and very much stri- 
* This papei- was read before the Royal Society of Edinburgh on the 1st of 
February 1819. 
VOL. I, NO. L JUNE 1819^ 
A 
