8 BULLETIN 97, U. S. DEPARTMENT OF AGRICULTURE. 
the center. Upon rotating the stage, the cross dissolves into two 
hyperbolae. The convex sides of the hyperbolae are always toward 
the acute bisectrix. The smaller the axial angle, the nearer together 
are the loci of the optic axes; and finally, for very small angles, the 
interference figure approaches the form of a uniaxial interference 
figure. When the axial angle is so large that neither of the loci 
of the optic axes show, the biaxial character is indicated by the 
movement of the bars. 
Sections lying perpendicular to the obtuse bisectrix, Bxq, show 
similar figures to those of sections lying perpendicular to the acute 
bisectrix, provided the axial angle is not too large. 
Should the section be inclined to the bisectrix, one of the dark 
spots may be completely outside of the field of the microscope. 
The other spot, however, will show, and this is sufficient to determine 
the biaxial character. 
Sections at right angles to an optic axis show a single straight 
dark bar surrounded by almost circular concentric curves whenever 
the bar is parallel to the principal planes of the nicols. This bar 
changes to a hyperbola on rotating the stage, and the convex side is 
toward the acute bisectrix. 
In sections parallel to the plane of the optic axes the interference 
figure is similar to that of uniaxial crystals parallel to the optic 
axis. In this case it is advisable to apply a slight pressure with 
the finger upon one side of the cover glass which causes the crystal 
to move about into some other orientation which will show one of the 
interference figures described above. 
OPTICAL CHARACTER. 
The optical character of biaxial crystals may be determined by 
means of the mica plate, gypsum plate, and quartz or mica wedge. 
To use the mica plate, the interference figure is first obtained and the 
stage rotated until the hyperbolae form a cross. Upon the insertion 
of the quarter undulation mica plate, the phenomena are then the 
same as in uniaxial substances. 
Sections perpendicular to the acute bisectrix, the interference fig- 
ure of which shows no colored curves, are rotated until the hyperbolae 
form a cross. The gypsum plate is inserted; and, if its a direction 
is parallel to the elongation of the plate, the center of the field is 
red and the JNTE. and SW. quadrants are colored blue while the 
NW. and SE. quadrants are yellow in positive crystals. The reverse 
phenomena ta,ke place in negative crystals. 
If the stage is so rotated that the hyperbolae form a cross, the 
phenomena presented on the insertion of a quartz or mica wedge 
are similar to those of a uniaxial crystal. By remembering that the 
convex sides of the hyperbolae are always toward the acute bisectrix, 
