OPTICAL. IDENTIFICATION OF ALKALOIDS. 3 



hairs in corresponding positions must then be used. It is a great 

 convenience if one of these prisms can be rapidly inserted and with- 

 drawn without changing the position of its vibration plane. A revolv- 

 ing stage, an Abbe condenser, and a substage iris diaphragm are 

 necessary. One 4-millimeter (^-inch) and one 8-millimeter (J- 

 inch) or 16-milUmeter (§-inch) objective are adequate for every pur- 

 pose. A slot should be cut in the microscope tube between the top 

 of the objective and the bottom of the upper nicol prism for the 

 insertion of a selenite plate yielding the '^ sensitive tint," red- violet of 

 the first order. This plate, which may be purchased from any dealer 

 in petrographic microscopes, is inserted so that its direction c lies at 

 45° from the vibration planes of the nicols. Finally, hquids of known 

 refractive index are needed, directions for preparing which are given 

 on page 4, 



OBSERVATIONS. 



Samples are prepared by gently crushing the dry substance. A 

 minute amount is immersed in a drop of a liquid in which it is insolu- 

 ble, on a glass slide, and protected by a small cover glass. The obser- 

 vations are made as follows : 



I. IN ORDINARY LIGHT. 



A 16-millimeter (§-inch) objective is usually most satisfactory for 

 the prehminary examination of a substance. 



Color or hue. — If the substance is pleochroic, showing different 

 colors in different crystallographic directions, the several colors and 

 their directions are recorded. 



Crystal habit. — This observation includes the outline of crystals, etc. 



Angles. — Interfacial angles are measured by revolving the stage of 

 the microscope until the trace of each face in turn lies parallel to one 

 of the cross hairs, and reading the angular position of each. This 

 process is repeated several times, and the results averaged. 



Cleavage —Cleavage refers to the directions in which splitting 

 along definite planes occurs. 



Refractive indices. — These are inversely proportional to the rates 

 at which light is transmitted in different crystallographic directions. 

 They are determined by immersion in liquids of known indices, 

 trying one after another until the boundaries in each direction 

 successively disappear. The following special instructions are im- 

 portant: In crystals of the rhombic, monoclinic, and triclinic sys- 

 tems there are three indices, the smallest being called a, the inter- 

 mediate jS, and the largest 7. The directions in which these values 

 hold are respectively X, Y, and Z. In substances of the tetragona , 

 trigonal, and hexagonal systems there are two indices, one called co 

 and the other e. In the cubic system there is but one index, called n. 



Determination of refractive indices hy the immersion method. — A 

 series of liquids of known indices should first be prepared, the com- 



