604 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
(6) Miscellaneous. 
Some Methods of Determining the Positive or Negative Character 
of Mineral Plates in Converging Polarised Light with the Petro* 
graphical Microscope.* — Dr. M. E. Wadsworth gives the following 
instructions in the use of the petrographical Microscope as a polari- 
scope. 
“ 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 polariscope plates, as well as those 
thinner ones prepared expressly for use with the Microscope. 
I. Uniaxial Minerals . — When the mineral plate shows the common 
uniaxial cross in converging light, its positive or negative character can 
be ascertained by means of the gypsum plate or quartz wedge, as well 
as by the ordinary mica plate. 
(1) Use of the Gypsum Plate. — Examine the mineral plate, which, 
in converging polarised light between crossed nicols, shows a dark cross 
or part of a cross with or without coloured rings or arcs. Insert the 
gypsum plate in the slot in the body of the Microscope, above the ob- 
jective. The cross is then resolved into coloured 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 
determinations. 
(2) Use of the Quartz Wedge. — Insert the quartz wedge thin end 
forward. When the wedge is gradually pushed in, the cross resolves 
itself into coloured 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 
coloured arcs advance towards the centre of a line parallel to the axis of 
least elasticity, the mineral is positive ; but if they march toward the 
centre from opposite sides of that line, the mineral is negative. 
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 
that a line perpendicular to them shall make an angle of 45° with the 
cross-hairs. By use of the quartz wedge, coloured arcs or rings can 
often be brought into the field, when otherwise none aro 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 forward, and, in 
time, a black or partially black arc appears, the mineral 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 
that its axis of least elasticity will bo perpendicular to the arcs. If then 
the arcs move with the concave side forward, the mineral is positive; 
* Journ. of Applied Microscopy, i. (1898) pp. 20-1. 
