DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 279 

 Dextro-tartrate and laevo-tartrate of ammonia. 



Both salts present the form described by M. Pasteur*; and I 

 shall designate the faces by the letters which he employs. 



The optical characters are perfectly identical ; this may be seen 

 when two crystals are cemented by their planes of cleavage, or 

 their generally very definite faces, M, upon a glass plate. When 

 the two crystals are in a parallel position, the prisms cut from 

 them in any manner possess exactly the same refraction. The 

 identity may be confirmed by a still more delicate proof. One 

 of the two optical axes is, as will be shown further on, almost 

 normal to the easy cleavage ; if therefore a plate of cleavage is 

 separated fromboth the laevo- and dextro-tartrate, and cemented 

 together in a parallel direction, the rings coincide completely, 

 and the combination acts like a single plate, of a thickness equal 

 to the sum of the partial thicknesses. 



The plane of the optical axes is normal to the horizontal di- 

 agonal of the rhombic base ; and I have determined the direc- 

 tion of the bisecting line by the second method. 



Determination of the Indices of Refraction, 



In order to determine the position of the bisecting line, I cut 

 a plate, the faces of which were parallel to the horizontal di- 

 agonal of the rhombic base, and whose normal made an angle 

 of 18° 22' with the normal of P, and an angle of 79° 4(y with the 

 normal of M, as the angle comprised between the latter two 

 was equal to 88° 2{. Through this plate I could see both 



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



t Mr. Miller has found another index for the same salt (Transactions of 

 the Phil. Soc. of Cambridge, vol. v.). He measured the minimum deviation 

 through the faces M and b, the angle of which he estimates at 39° 53', and 

 found it equal to 25® 17'. I experimented with the same faces, slightly po- 

 lished, and with an angle of 39° 27', I found the ordinary deviation to be 22° 51 ', 

 the extraordinary deviation 25° 1'. Mr. Miller appears, therefore, to have mis- 

 taken one ray for the other. 



