820 BOTANY AND PHARMACOGNOSY. 



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

 to determine their incHnation 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 

 angles of the crystals with as much accuracy as can be done with 

 any form of polariscope. 



" From such observations made witli the aid of this form of 

 microscope, the following constants may be determined : 



"(i) The plane angles of the crystals, in most cases the 

 interfacial angles, giving the data from which the axial ratios 

 are computed in other words the morphological constants of 

 single crystals. 



"(2) The relation of the composite crystals or twins to each 

 other, their angles, and the position of the twin plane, twin axis, 

 composition plane, and other constants of the twin crystals. 



"(3) The pleochroism of the crystals, the character of the 

 colors of the light vibrating parallel to the elasticity axes in 

 the crystal. This is effected by the use of the single polarizing 

 prism below the stage. By analyzing this light with the micro- 

 spectroscope the differences of tint and color may be given 

 quantitative values in wave lengths. 



"(4) The position and relative values of the light elasticity 

 axes in the crystals, upon which depend the angles of extinction 

 of the crystals, measured from certain crystallographic axes or 

 planes or edges. In uniaxial crystals (tetragonal and hexagonal 

 systems) there are two such elasticity axes the ordinary ray des- 

 ignated as CO, and the extraordinary ray, designated as e. Either 

 one of these may be the axis of greater or less elasticity ; and 

 according as the extraordinary ray is the axis of less elasticity 

 or of greater elasticity the crystal is called optically positive 

 or optically negative. In biaxial crystals (orthorhombic, mono- 

 clinic and triclinic systems) there are three elasticity axes at 



