DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 275 



the apparent optical axes must form with the normals of the 

 planes of emergence an angle of 55° 13', but these normals form 

 with the bisecting line an angle of only 53° 16', consequently 

 the apparent axes intersect at double the difference or 3° 54', an 

 angle which is tolerably near to that obtained by direct obser- 

 vation. 



The bisecting line of the optical axes is in sulphate of potash 

 the axis of minimum elasticity, and in chromate of potash that 

 of maximum elasticity, a result which was confirmed by obser- 

 vation with plates cut in various directions. 



Lsevo- and dextro-tartrates of soda and potash. 

 „ „ „ ammonia. 



(NaO + RO) CSH^Oio+SH^O. 



These salts, which I was enabled to examine through the 

 kindness of M. Pasteur, present a peculiar interest, for the com- 

 plete isomorphism in almost all parts of the form of laevo- and 

 dextro-tartrate of soda and ammonia does not only fail to pre- 

 vent their crystallizing out separately from a state of common 

 solution, but this tendency even suffices as a means of sepa- 

 rating the laevo- and dextro-acids, notwithstanding the fact that 

 their previous combination took place with such energy as to 

 develope a sensible amount of heat. 



These four salts crystallize, absolutely in the same manner, in 

 rhomboidal prisms, modified by numerous facettes parallel to 

 their height. Among the most definite and well-developed are 

 those which tend towards a first rhombic prism of about 100° 30', 

 to a second rhombic prism of about 135° which truncate the 

 obtuse edges of the former, and finally, those of a rectangular 

 prism tangential to the obtuse and acute angles of both these 

 prisms. 



The laevo-tartrate and the dextro-tartrate of soda and of 

 potash possess characters of optical double refraction, which 

 are absolutely identical. I convinced myself of this fact in the 

 following manner: — 



1. Two crystals cemented upon the same plate of glass by 

 the same planes and in a parallel position, presented exactly the 

 same refraction when afterwards cut into prisms in any direction. 



2. It is impossible to discover the slightest difference between 



