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 NE. and SW. quadrants are colored blue while the 

 NW. and SE. quadrants are yellow in positive crystals. The reverse 

 phenomena take 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, 



