1 72 The American Geologist. March, isas 
The use of the quartz wedge is less liable to error than 
either of the preceding; and besides it can be used in many 
cases where the others give no results. 
(a) If the uniaxial plate is cut so that it shows arcs of 
rings, its positive or negative character can be determined 
by placing the arcs so a line perpendicular to them shall make 
an angle of 45° with the cross hairs. By use of the quartz 
wedge, colored arcs or rings can often be brought into the 
field, when otherwise none are seen. Push in the quartz 
wedge with its axis of least elasticity tangent to the arcs. If 
the rings then move outwards with their convex side forwards, 
and, in time, a black or partially black arc appears, the min- 
eral is POSITIVE, but if the arcs move with their concave 
sides forwards the mineral is negative. 
As a check against any error, turn the wedge over and 
push it in. so its axis of least elasticity will be perpendicular to 
the arcs. If then the arcs move with the concave side for- 
ward, the mineral is positive, but if they move with the 
convex side forwards, and a black or partially black ring or 
rings show, the mineral is negative. 
(b) A uniaxial plate cut parallel to the vertical axis can 
have its positive or negative character shown in converging 
polarized light as follows: Place the plate at an angle of 45° 
with the cross hairs so as to show the colored arcs or imper- 
fect hyperbolas. Push in the quartz first with its axis of least 
elasticity perpendicular to the vertical or optic axis of the 
plate. If on pushing along the quartz wedge a dark hyperbola 
is seen to pass over the field the mineral is positive. 
Again, push in the quartz wedge with its axis of least elasticity 
parallel to the vertical axis of the plate. If then a dark hyper- 
bola is seen to traverse the field, the mineral is xeciative. 
II. Biaxial Minerals. 
In order to render intelligible the directions later given, 
there is here stated the method published in the text books for 
determining the positive or negat'ive character of a biaxial 
mineral plate. 
If a line of extinction of a biaxial plate properly cut is 
placed parallel to one of the cross hairs, it shows a cross with 
unequal arms; but if the line of extinction makes an angle of 
