Character of Mineral Plates. — J J \7dsu 'ortli. 1 7 1 
In giving instruction in the use of the petrographical 
microscope as a polariscope, I have found a few directions of 
value to my students, — directions which I do not remember 
of having ever seen pubHshed. Thinking that they might be 
of some vahie to some of our readers who are interested in 
optical mineralog}-, these directions are published here. Since 
by varying the powers, the petrographical microscope can 
be used with mineral plates of any standard thickness, the 
directions here given can be used with the ordinary polar- 
iscope plates as well as those thinner ones prepared expressly 
for use with the microscope. 
L Uniaxial Minerals. 
When the mineral plate shows the common imiaxial cross 
in converging light its positive or negative character can be 
ascertained by means of the gypsum plate or quart2 wedge. 
;is well as by the ordinary mica plate. 
(/) (he of the Gypsujii Plate, 
Examine the mineral plate, which, in converging polarized 
light, between crossed nicols, shows a dark cross or part of a 
cross w'lth or without colored rings or arcs. Insert the gyp- 
sum plate in the slot in the bod}' of the microscope above the 
objective. The cross is then resolved into colored hyperbolas. 
The central portion is red terminated on the ends with yellow 
and bordered on the side by blue. If the blue that borders the 
red lies on a line parallel to the axis of least elasticity, the 
mineral is positive, but if it lies on opposite sides of this 
line the mineral is negative. The gypsum plate is often 
more satisfactory in its use than the mica plate for these de- 
terminations. 
(2) Use of the Quartz Wedge. 
Insert the quartz wedge thin end forward. When the 
wedge is gradually pushed in the cross resolves ifself into 
colored arcs that cross the field of view from two opposite 
sides of the field and pass out of sight on the other two sides. 
These arcs follow each other in succession as the wedge is 
pushed in. If these colored arcs advance towards the center 
of a line parallel to the axis of least elasticity the mineral is 
POSITIVE. But if the\ march toward the center from o}> 
posite sides of that line the mineral is negative. 
