IDENTIFICATION OF FERTILIZER MATERIALS. 7 



cross. When the crystal is so oriented that the section viewed is 

 perfectly perpendicular to the optic axis the center of the cross re- 

 mains stationary during a complete rotation of the stage. Should the 

 section be slightly inclined, the center will revolve around the inter- 

 section of the cross hairs. It may happen that the section is so much 

 inclined that the center of the cross does not appear in the field at all. 

 In this case, a dark bar is seen moving across the field, as the stage is 

 rotated, occupying successive positions parallel one to another and 

 the principal sections of the nicols, followed on further rotation of 

 the stage by a bar perpendicular to the first and also occupying suc- 

 cessive parallel positions. In cases where the section viewed is par- 

 allel to the optic axis, hyperbolas are shown which are somewhat 

 similar to those of biaxial figures. However, they do not appear 

 until the stage is nearly in the 90° position. Then they move in, 

 form an indistinct cross, and move out again very rapidly. The 

 hyperbolas leave the field in the direction of the principal axis. 



OPTICAL CHARACTER. 



The optical character of uniaxial substances, that is, whether they 

 are optically positive or negative, may be determined by means 

 of the quarter undulation mica plate, the gypsum plate giving red 

 of the first order, quartz or mica wedge, etc. In general the quartz 

 or mica wedge or plate serve the purpose when the center of the 

 interference figure is seen in the field. The interference figure 

 is first obtained and then one of these accessories placed between 

 the objective and the analyzer in the 45° position. In optically 

 positive crystals the colored rings in the NE. and SW. quadrants 

 contract while those in the NW. and SE. quadrants expand. With 

 negative crystals the reverse takes place. It is of course necessary 

 to know the directions of vibration of the plates used. This can 

 always be determined by the same means as those given for crystals 

 by using the plate as an object to be examined. Should the center of 

 the cross be beyond the field of the microscope, a gypsum plate 

 giving red of the first order with the & direction parallel to the 

 elongation of the plate is used. Positive crystals show blue in the 

 NE. and SW. quadrants and yellow in the NW. and SE. quadrants. 

 The phenomena are reversed in the case of negative crystals. 



BIAXIAL SUBSTANCES. 



Biaxial interference figures in sections lying perpendicular to 

 the acute bisectrix, Bx a , provided the axial angle is less than the 

 field of the microscope, show, when the plane of the axes and the 

 cross hairs coincide, a black cross and a series of colored curves. 

 The points of emergence of the optic axes are indicated by dark 

 spots with colored lemniscates surrounding them and joining at 



