Cliaracter of Mineral Plates. — WadszvortJi . 173 
45° with that cross hair, it shows two dark hyperbolas, whose 
vertices or eyes mark the position of the optic axes. Accom- 
panying the cross and hyperbolas are colored lemniscate fig- 
ures. Oftentimes the hyperbolas are wanting and only the 
colored lemniscates 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 bi- 
secting the lemniscates 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 join- 
ing the hyperbola eyes. If the hyperbola eyes open and move 
toward the center of the lemniscate figure the mineral is 
POSITIVE. 
Push in the quartz wedge with its axis of least elasticity 
perpendicular to the line joining the hyperbola eyes. If these 
eyes open and move toward the center of the lemniscate fig- 
ure, the mineral is negative. 
Of course, if in either case the eyes contract and move out- 
wards, this is proof, when the axis of least elasticity of the 
quartz wedge is perpendicular to the line joining the hyper- 
bola, that the mineral plate is positive ; but if they move 
outward when the axis of elasticity is parallel to the chosen 
line, the mineral 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 hyperbola eyes cannot be 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 join- 
ing the hyperbola eyes or optic axes, by the form of the inter- 
ference 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 bisect, them; 
also have this line make an angle of 45° with the cross hairs as 
