278 SENARMONT ON THE OPTICAL CHARACTERS OP 



Determination of the Angle of the Optical Axes, 



The bisecting line is the axis of major elasticity. 

 Oblique Prismatic System. 



Dextro-tartaric acid and laevo-tartaric acid. 



These two acids possess the form described by M. Pasteur ; 

 and I shall employ the same designation for the faces as he 

 adopts in his memoir*. 



Their characters of double refraction are absolutely identical ; 

 for when two faces of cleavage are cemented upon the same plate 

 of glass in a parallel position, and the refracting angles cut in 

 any direction, the double refraction produced is precisely the 

 same in both crystals. 



The horizontal diagonal of the base is parallel to the bisecting 

 line of the obtuse angle of the two optical axes, and the plane of 

 these latter makes an angle of 20° 27' with the normal of the 

 face M ; it follows from this that it is difficult to perceive the 

 rings through the natural faces, and above all, that the means 

 of measuring with accuracy the reciprocal inclination of the 

 optical axes are wanting. 



The rings may be seen when a plate, whose parallel faces are 

 cut artificially in a direction perpendicular to the horizontal di- 

 agonal of the rhombic base, is cemented between two appropri- 

 ately adjusted equilateral prisms of glass. By this means I found 

 the obtuse angle of the optical axes to be about 120°; but it is 

 evident that this method does not ensure any great accuracy. 



The direction of the bisecting line I determined according to 

 the first method, by making a divided plate hemitropic round 

 the normal of the plane of cleavage ; and this means of observa- 

 tion is more accurate the greater the refractive power. Ten cor- 

 responding observations gave me the above angle, which is in- 

 scribed on the figure. Plate IV., fig. 6. 



The bisecting line is the axis of maximum elasticity. 

 * Annales de Chimie et de Physique, 3 ser. t. xxviii. p. 56. 



