OPTICAL IDENTIFICATION OF ALKALOIDS. 



II. IN PARALLEL POLARIZED LIGHT. 



Parallel polarized light is produced by inserting the crossed nicol 

 prisms. 



Extinction {the directions in which the crystal becomes darlc). — 

 Extinction is said to be parallel when it occurs on bringing a long 

 axis or side of a grain into parallelism with one of the cross hairs^ and 

 inclined when it occurs in some other position. When inclined, the 

 angle between the axis or side and the cross hair is measured and 

 recorded as the extinction angle. Several measurements are made, 

 and the maximum value used. Crystals of the tetragonal, trigonal, 

 hexagonal, and rhombic systems show normally parallel extinction, 

 as do also monoclinic crystals elongated parallel to axis b. Crystals 

 of the cubic system are dark in all positions, and are said to be 

 isotropic: they show none of the phenomena hereafter listed. 



Double refraction (birefringence). — This is the difference between 

 the highest and lowest indices of refraction. It is described as weak, 

 moderate, strong, etc., and is estimated by the order of the polari- 

 zation colors shown when the grain is midway between two extinction 

 positions. 



Determination of the double refraction by order of polarization colors. — 

 The polarization color obtained by placing any doubly-refracting 

 crystal grain between crossed nicols is dependent on its thickness 

 and the strength of its double refraction. Table 1 gives the colors 

 with different thicknesses and double refractions. 



Table 1. — Colors observed with various thicknesses and double refractions. 



Color 



First order: 



Black 



Gray 



White 



Yellow 



Orange 



Red 



Red- violet ' . 

 Second order: 



Violet 



Blue 



Green 



Yellow 



Orange 



Red 



Red- violet . . 



Double 

 refraction 

 Xthiclmess, 



Mm. 



0. 00000 

 . 00005 

 .00020 

 . 00035 

 .00045 

 .00052 

 .00055 



.00057 

 .00065 

 .00080 

 .00090 

 .00095 

 .00105 

 .00110 



Color. 



Third order: 



Light violet 



Light blue 



Light green 



Light yellow 



Light orange 



Light red 



Light red- violet . 

 Fourth order: 



Pale violet 



Pale blue 



Pale green 



Pale yellow 



Pale orange 



Pale red 



Pale red- violet ^ . 



Double 

 refraction 

 X thickness. 



Mm. 

 0.00115 

 . 00120 

 .00135 

 .00140 

 . 00145 

 . 00155 

 . 00160 



.00170 

 .00180 

 .00190 

 .00200 

 .00210 

 .00230 

 .00240 



• Sensitive tint, shown by the selenite plate. 



2 Fifth order colors are extremely faint, and higher orders are white. 



The selenite plate may be used to determine the order of the color, 

 provided it is not too high. The plate is inserted, and the crystal is 

 observed when parallel and again when perpendicular to its direc- 

 tion c. In one direction the color of the grain will be of lower order, 

 in the other of higher order, than the selenite, which itseK yields red- 



