( 520 ) 



Very feeble inclined dispersion with q^>v; double refraction nega- 

 tive. The Longitudinal axis of the parallelograms and the shortest 

 dimension (breadth) of the more needle-shaped individuals are the 

 directions of a smaller optical elasticity. 



a-Thiophenic acid crystallises from solvents or from the fused 

 mass in long more or less broad needle-shaped individuals, which 

 usually exhibit only a rudimentary limitation and cannot therefore 

 be properly determined morphologically. Although the optical proper- 

 ties seem to point to a monoclinic symmetry one might also feel 

 inclined to conclude to a triclinic symmetry on account of the form 

 limitations occurring here and there. The extinction of the needles 

 is, however apparently orientated perpendicularly to their longitu- 

 dinal direction. The smaller optical elasticity axis coincides with the 

 longitudinal direction of the needles. The optical axial plane is 

 orientated perpendicularly to the longitude of the needles ; in con- 

 vergent light a single very chaiacteristically coloured hyperbole is 

 visible at about 2 /a °f the diameter of the fields of vision. 



Enormously strong dispersion with q<^v; the sign of the double 

 refraction around the correlated bissectrix* is positive. 



The two isomers are, therefore, readily distinguished microscopic- 

 ally by the following properties : 



@-Thiophenic acid 



Parallellogrammic limitation, or 

 short rods of rectangular form. 



Very high interference colours. 



One optical axis with elliptical 

 rings, very weak dispersion q <C t\ 



Negative double refraction. 



Monoclinic symmetry ; angle of 

 the parallelograms 42° — 43°. 



Optical axial plane for the paral- 

 lelograms // to the longest dia- 

 gonal for the needles perpendicular 

 to the longitudinal direction. 



The largest elasticity-axis is 

 parallel to the longitudinal direc- 

 tion of the needles or to the 

 shortest diagonal of the paralle- 

 lograms. 



a-Thiophenic acid 



Long, very slender needles, 

 mostly with rudimentary limita- 

 tion. 



Grey, or unconspicuous colours. 



One coloured hyperbole ; very 

 strong dispersion: Q<^v. 



Positive double refraction. 



Triclinic or monoclinic sym- 

 metry. 



The optical axial plane is per- 

 pendicular to the longitudinal 

 direction of the needles. The latter 

 coincides with the shortest elas- 

 ticity-axis of the crystals. 



