30 G. J. Brush on Ralstonite. 
tographic plates see Pie subjected - a careful examination in 
may be made of the extent of error to 
which they are liable. an the case of a solar eclipse, or of a 
transit of a planet over the sun’s disk the photographic method 
has very great advantages over the observations of contact in 
many respects, and the errors to which it is subject are worthy of 
the most thorough investigation. The observation of a contact — 
is uncertain on account of irradiation, and it is momentary a aleag 
as to the soeaibility of ais measuring and oT a the ’ 
photographic observations of the transit of Venus. 
Mar, 25, 1871. 
Art. VL—On Ralstonite, a new Fluoride hse Arksut-Fiord; by | 
Go. J. BrusH | 
THE recent exploitation of the Greenland cryolite has igi : ] 
only led to the discovery of crystallized cryolite, but has given 
to mineralogical science several new fluorides, two of which, 
thomsenolite and pachnolite, are found in beautiful crystallized 
fe 
rms. 
I now call attention to another fluoride observed, a few 
months ee by Rev. J. Grier Ralston of Norristown, 
ound a mineral in minute octahedrons associated with 
Begehate, and being unable to identify it, he sent it to Pro 
Dana, by whom. the specimens were passed over to me for é 
aminatio 
