174 Tlic American Geologist. March, i898 
before. Push in the wedge with its axis of least elasticity per- 
pendicular to the arcs or parallel to the line joining the hyper- 
bola eyes. If the lemniscate arcs move in towards the center 
of the field with their convex side forwards the mineral is 
POSITIVE. 
Push in the wedge with its axis of least elasticity tangent 
to the arcs or perpendicular to the line joining the vertices. 
If the arcs then move in with their convex side forwards the 
mineral is negative. If the arcs move outwards with their 
concave side forwards the mineral in the first position of the 
wedge is negative, and in the second position positive. 
(c) If the distance between the hyperbola eyes is not so 
great but that they lie within the field of view, the mica and 
gypsum plates can both be employed to determine the positive 
and negative characters when the lemniscate figure is placed 
as before, with the line joining the hyperbola eyes forming an 
angle of 45° with the cross hairs of the eye piece. Insert 
either the mica plate with its axis of least elasticity parallel to 
the chosen line, or else insert the gypsum plate with its axis of 
least elasticity perpendicular to the chosen line. With either 
plate in this position the arcs on one side of the hyperbola eyes 
will enlarge and those on the other side contract. If the arcs 
that lie on the inside of the eyes, or nearest the center of the 
figure, enlarge, and those on the outside contract, the min- 
eral is positive. On the other hand, if the arcs nearest the 
center contract and the outside arcs expand the mineral is 
negative. This method can be used with plates that have 
too great an axial divergence to admit of their determination 
when the unsynimetrical cross is placed with its arms parallel 
to the cross hairs. 
III. Chromatic Scale. 
Many students find it difficult to follow the color scales 
given in most text books of petrography owing to the numer- 
ous subdivisions of the scales. This difficulty can be obviated 
in part by each student making for himself a color scale suited 
to his eyes and experience. It is found that many students 
mistake their ignorance of the names of color tints for color 
blindness. The scale is made by placing the quartz wedge on 
the stage of the microscope with the nicols crossed. Then 
