ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
605 
but if they move with tlie 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 polarised light as 
follows. Place the plate at an angle of 45° with the cross-hairs so as 
to show the coloured arcs or imperfect hyperbolas. Push in the quartz 
first with the 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 hyperbola is seen to traverse 
the field, the mineral is negative. 
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 negative 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 45° with that cross-hair, it shows 
two dark hyperbolas, whoso vertices or eyes mark] the position of the 
vertical axes. Accompanying the cross and hyperbolas are coloured 
lemniscate figures. Oftentimes the hyperbolas are wanting, and only 
the coloured lemniscata can be seen ; but by the insertion of the quartz 
wedge, the hyperbolas can frequently be brought into the field. 
(a) The positive or negative character of this biaxial plate can then 
be determined by placing the plate on the stage in such a position that 
a line joining the hyperbola eyes, or bisecting the lemniscata through 
their longest direction, shall form an angle of 45° with the cross-hairs. 
Push in the quartz wedge with its axis of least elasticity parallel to the 
line joining the hyperbola eyes. If the hyperbola eyes open and move 
toward the centre of the lemniscate figure, the mineral is positive. 
Push in the quartz wedge with its axis of least elasticity perpen- 
dicular to the line joining the hyperbola eyes. If these eyes open and 
move toward the centre of the lemniscate figure, the mineral is negative. 
Of course, if in either case the eyes contract and move outwards, this 
is proof, when the axis of least elasticity of the quartz wedge is perpen- 
dicular to the line joining the hyperbola, that the mineral plate is 
positive ; but if they move outward when the axis of elasticity is parallel 
to the chosen line, the mineral plate is negative. 
This method is less satisfactory in practice than the one where the 
eyes open and move inwards. 
(b) The above method given in our text-books can be supplemented 
by one that can be employed in numerous cases when both of the hyper- 
bola eyes cannot he seen, but only one of them or only the lemniscate 
arcs. In either of these cases the positive or negative character of the 
mineral plate can be ascertained ; if one can determine the position of 
the line joining the hyperbola eyes or optic axes, by the form of the 
interference figures, by the position of the larger arm of the cross, or by 
any other means. When this direction is observed, place the arcs so 
that the direction of the line joining the hyperbola vertices shall be 
perpendicular to, or shall bisect them ; also have this line make an angle 
