52 &. L. Penfield — Bertrandite from Colorado. 



The advantage of the method arises mainly from the natural 

 and easy use of the eye ; to secure this it is desirable that the 

 scale should be sufficiently open to be read without straining 

 the eye. 



The fixed mirror should be as large as possible, especially in 

 the horizontal direction, to facilitate the bringing of the eye 

 into the proper position without effort. 



It is also important if the movable mirror is round that the 

 line of division between the silvered and unsilvered surfaces 

 shall cross it nearly in a diameter in order that the position of 

 the eye may not be too" much restricted, as is the case if the 

 line is very short in which the fixed and movable mirrors 

 overlap. Where it is practicable a rectangular mirror is to be 

 preferred. 



Jefferson Physical Laboratory, April, 1888. 



Art. VII. — Bertrandite from Mt. Antero, Colorado; by 

 Samuel L. Penfield. 



This rare mineral was first identified as a new species by M. 

 E. Bertrand* from the study of a few small crystals collected from 

 a pegmatite vein at Petit Port, near 2s antes, France. M. Des 

 Cloizeauxf has also identified the mineral at the gneiss quar- 

 ries at Barbin, near Nantes, while M. A. DamourJ has anal- 

 yzed it and determined its composition to be H 2 Be 2 Si 2 9 ; he 

 also gave to it the name Bertrandite. The mineral has since 

 been identified by R. Scharizer§ at a feldspar quarry near 

 Pisek, Bohemia, where it occurs lining cavities left by the 

 decomposition and disappearance of beryl crystals. At all of 

 these localities the crystals are minute and are found only in 

 small quantities. The crystalline form determined by Ber- 

 trand and Des Cloizeaux is orthorhombic, while Scharizer finds 

 grounds for believing that the crystals are monoclinic with 

 close approximation in form and optical properties to ortho- 

 rhombic symmetry. 



The single hand-specimen in the author's possession was 

 selected by Mr. W. B. Smith, of Denver, Col., from a lot of 

 material collected during the past summer at Mt. Antero in 

 the search for specimens of phenacite. The crystals of ber- 

 trandite are attached to quartz which is associated with beryl. 



* Bull. Soc. Min. de France, iii, 1880, p. 96. 

 f Bull. Soc. Min. de France, v, 1882. p. 176. 

 % Bull. Soc. Min. de France, vi, 1883. p. 252. 

 § Zeitschr. Kryst.. ariv, 1888, p. 33. 



