62. 
70. 
a1. 
vee 
73: 
bo 
mn 
tions impose. This is especially useful in studying the 
optical properties of minerals in thin sections, as illus- 
trated by the present arrangement of the instrument and 
the first cut accompanying it. 
The other part, the ‘‘ Universal Drehapparat,” consists 
of a small glass vessel for holding a highly refracting fluid 
in which the section or crystal is immersed after being 
fastened on a small movable holder which permits adjust- 
ment to the axes of revolution of the apparatus. . The 
microscope is placed horizontally, as shown in the second 
cut, and the observations made with the same possibility 
of revolution on three axes, whether for optical study or 
the measurement of the angles of crystals. A special long 
focus condensing lens and similar objective accompany 
the instrument, which allow space for the revolution of 
the object. 
Exhibited by the Department of Mineralogy and Petrog- 
raphy of Harvard University through Prof. J. E. Wolff. 
DANBuRITE, Russell, N. Y. Twelve specimens showing 
the variation in form. 
TOURMALINE, @, white, DeKalb, N. Y. 
6, yellow, DeKalb, N. Y. 
c, brown, Gouverneur, N. Y. 
d, blacks Pierrepont, N. Y. 
WERNERITE, Pierrepont, N. Y. 
WERNERITE, Bedford, Ontario. 
WERNERITE, Burgess, Ontario. 
APATITE, Burgess, Ontario. 
APATITE, Eganville, Ontario. 
APATITE, Bedford, Ontario. 
TREMOLITE, Gouverneur, N. Y. 
