DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 28? 



niacal Seignette salt, whose optical axes are open and dispersed 

 in a certain diametric plane, and gradually increasing quantities 

 of potash Seignette salt*, whose optical axes are open and di- 

 spersed in the diametric rectangular plane. The biaxial double 

 refraction and the considerable dispersion of the optical axes for 

 different colours in this case give to the phaenomena a character 

 of marked peculiarity. 



The angular distance of the optical axes in the potash salt, 

 amounting originally to 76°, decreases at the same time as the 

 red axes, which are at first wider apart than the violet, approxi- 

 mate to each other, because they approach more rapidly, so that 

 they ultimately unite. At this moment the dispersion of the 

 optical axes for different colours ceases, and with it the remark- 

 able anomalies in form and coloration which Sir John Herschel 

 has pointed out in the isochromatic curves. 



The violet and red optical axes then separate again ; but the 

 latter, from always moving more rapidly than the others, are less 

 wide apart. The dispersion is the reverse of that originally pre- 

 sented, although still sufficiently strong to cause a considerable 

 contortion in the rings, which have then the red inwards and 



* The complex crystals are generally smaller than the simple ones, and it is 

 necessary to take care that the solution is kept strongly amnioniacal in order 

 to prevent the precipitation of bitartrate of potash. 



One of the most perfect of the mixed crystals, in which the two constituents 

 must have been nearly in atomic proportions, possessed the form represented 

 in fig. 10 Plate IV; and the normals of the planes fojmed the following angles : — 



90°=l' = ^.=^ = -=&c. i=90°, 

 h h M a g 



Two other vertical prisms are likewise found where the normals of the planes 

 H and p are respectively inclined to the normal of g by angles of 16° 50' and 

 31° 10'; m', 71, and k are situated in the same zone, as are likewise m', e" and 

 m'l. 



The symbolic notations for all these faces are — 



h(lOO), ^(010), P(OOl), /i(410), v(210), M(llO), N(120), 



f'(102), e"(101), e"'(201), /«'(111), w"(331), a(012), w(I21). 

 A :B : H=1'205 : 1:0-627. 



