774 A TEXT-BOOK OF BOTANY. 



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

 any form of polariscope. 



" From such observations made with 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 crystals. 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 to, 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 

 right angles to each other, and these are designated as fl, the 

 axis of greatest elasticity ; ft, the axis of mean elasticity ; and 

 C, the axis of least elasticity. 1 



" (5) The position and angle of inclination of the optic axes 

 or lines of single refraction through the crystals. These always 

 lie in the plane of the elasticity axes 01 and C and the angles 

 between the optic axes are bisected by the axes ft and t. Accord- 



1 Elasticity in the optical sense is the reciprocal of refractive index ; 

 hence a, b, t, are the axes of least, mean and greatest refractive index. 



