280 SENARMONT ON THE OPTICAL CHARACTERS OF 



systems of rings, and thus found the apparent angles which the 

 first and second optical axes make with the normal, are re- 

 spectively equal to 27° 3' and 32° 32', corresponding to the ex- 

 terior angles 17° 22' and 20° 40' ; the interior angle of these axes 

 is therefore 38° 2'. It follows from these data, that the bisecting 

 line is inclined at an angle of 16° 43' to the normal of the face 

 P, and at an angle of 71° 19' to the normal of M. 



Figure 7 represents an ideal section of the crystal parallel 

 to the plane of the optical axes ; it shows the direction of the 

 normals to the faces, those of the optical axes and of their bi- 

 secting line. 



This result is moreover susceptible of verification ; for as each 

 of the optical axes makes with the bisecting line an angle of 19° 1', 

 the former makes with the normal of P an angle of 2° 18' in the 

 interior, and of 3° 31' al the exterior. 



If, then, two plates, separated by cleavage in a direction 

 parallel to P, are cemented upon a glass plate parallel to each 

 other, but in inverse positions, the rings proper to each will be 

 seen, and the corresponding optical axes will be found to form 

 in the air an angle of 6° 59', almost twice as great as in the pre- 

 ceding experiment. 



The bisecting line is the axis of minimum elasticity. 



Neutral phosphate and arseniate of soda. 

 NaO,P2 0H24H2 0. 



These salts, whose form has been described by M. Mit- 

 scherlich*, have their optical axes in a plane normal to the 

 horizontal diagonal of the rhombic base, and appear to possess 

 optical characters which are perfectly identical. 



It is difficult to determine with accuracy the direction of the 

 optical axes and the indices of refraction, on account of their 

 tendency to effloresce ; but, by cementing two crystals, one of 

 phosphate and the other of arseniate, base to base in a parallel 

 position, it is possible to ascertain that the refraction and the 

 direction of the axes of optical elasticity are not appreciably dif- 

 ferent in either one or the other. 



I likewise cemented together two crystals of arseniate and two 



Annahs de Chimie et de Physique, 3rd ser. vol. xix. p. 419. 



