MICROSCOPIC TECHNIQUE AND REAGENTS. 773 



as one of the bases of classification is coming to be recognized. 

 The best illustration of this is to be found in the monograph 

 of the genus Eucalyptus by Baker and Smith, 4 in which they 

 have utilized the chemical properties and physical characters of 

 the oils, coloring principles, tannins, etc., in establishing differ- 

 ences of affinities or species. There is a growing tendency on the 

 part of investigators to study micro-chemically some of the char- 

 acteristic plant constituents, as alkaloids, etc. As a rule, how- 

 ever, the descriptions are superficial and the identification is by 

 means of color reactions. No real scientific progress will be 

 made until the botanist employs the petrographical microscope 

 and is fairly well grounded in the principles of physical and 

 chemical crystallography. The work is by no means so simple 

 as in ordinary microscopic work, but when the principles governing 

 the optical study of crystals are mastered, the study will appeal to 

 botanists not only as a fertile field for research but also as a 

 subject of importance in both morphological and taxonomic work. 



The study of microscopic crystals is accomplished by means 

 of the petrographical microscope. Brown (loc. cit.) has stated 

 succinctly the nature and use of this instrument : 



" The necessity of studying small crystals, . . . has re- 

 sulted in the evolution of a form of microscope which is at once 

 a goniometer, a polariscope, and an instrument for measuring 

 optic axial angles in short, for determining the physical crys- 

 tallographic constants of small crystals. . . . The polari- 

 scope portion of the petrographical microscope enables the ob- 

 server to determine the position and relative value of the elasticity 

 axes of crystals, to observe the position of the optic axes, and 

 to determine their inclination to each other and to the elasticity 

 axes. From these data the optical character of the crystal is 

 determined. These OPTICAL REACTIONS may be studied by this 

 instrument with as much ease, and in general with as much 

 accuracy, as with the larger and better graduated polariscope; 

 and the data thus obtained are quite as accurate in most cases 

 as those obtained by the use of the larger instruments. The. 

 use of the special eye-pieces arranged with artificial twins of 

 calcite or quartz enables the observer to determine the extinction 



