OPTICAL IDENTIFICATION OF ALKALOIDS. 7 



Class^ may be uniaxial, possessing but one optic axis, or hiaxial, 

 possessing two. If biaxial, the angle between tbe two may be 

 measured, for which purpose an apertometer or a micrometer eye- 

 piece is needed. For the determinative work on alkaloids, however, 

 it is necessary only to class the angle as small or large. 



A uniaxial interference figure consists of a dark cross surrounded 

 by rings which are colored if white light is used, or alternately dark 

 and hght in monochromatic hght. It may appear in the center of 

 the field if the crystal is lying on face c, the base, or it may lie more 

 or less to the side. Even when the cross is not visible the dark arms 

 wiU, on revolving the stage, move across the field parallel to each 

 cross hair in turn, remaining perfectly straight. 



A hiaxial interference figure consists, in the most favorable case, of 

 two dark spots, where the two optic axes emerge, surrounded by 

 concentric oval or lemniscate curves, colored as in the uniaxial inter- 

 ference figure, and traversed by two dark bars which form a cross 

 when the vibration directions of the crystal are parallel to those of 

 the nicol prisms, and open out into hyperbolas when the stage is 

 rotated 45°. The apparent angle, 2 E, is proportional to the dis- 

 tance separating these dark spots; the acute bisectrix hes midway 

 between them. Two or three sheets of mica should be used for 

 forming an idea of the appearance of the interference figures with 

 different values of 2 E. Muscovite mica has 2 E from 60 to 70°; 

 while phlogopite usually has 2 E much smaller, about 15°. In some 

 positions only portions of the figure may be visible, but, in general, 

 on revolving the stage the dark bars move across the field, neither 

 parallel to the cross hairs nor remaining straight. 



Optical sign- {positive or negative), — Uniaxial crystals are positive 

 if CO is less than e; negative if w is greater than e. Biaxial crystals 

 are positive if Z is the acute bisectrix and negative if X occupies 

 that position. This is determined in the same way as is the sign of 

 elongation (page 6). That is, the selenite plate is inserted, and the 

 change of color of the interference figure observed; when the figure 

 shows, on opposite sides of the central cross, areas of blue, marking 

 an increase in the order of the color in a direction parallel to c of the 

 plate, the sign is positive; when the color is yellow in that direction, 

 indicating a decrease, the sign is negative. 



Dispersion. — Dispersion is the relative refraction of the red end of 

 the spectrum as compared with the violet end, abbreviated to r and 

 V, respectively. This can be observed in various ways, but, as it has 

 in general httle or no significance in the identification of alkaloids, 

 it will not be further discussed here. 



> strictly "ellipsoidity." 

 'Strictly "cliaracter." 



