DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 285 



axes corresponding to different colours. The latter salt, which 

 may be called ammoniacal Seignette salt, presents all those 

 remarkable characters which Sir John Herschel has pointed out 

 as belonging to the true potash Seignette salt. The divergence 

 of the optical axes, the dispersion of the axes for different 

 colours, the contortion of the isochromatic curves are in both 

 cases the same ; but the optical axes extend in two diametric 

 rectangular planes. 



It is then sufficiently proved that the elasticity of the aetherial 

 medium in certain isomorphous substances presents a perfect 

 inversion of relative magnitudes in the same direction. But as 

 this elasticity in crystals assumes, under the influence of the 

 forces emanating from the molecul&r network, unequal values in 

 dissimilar directions, the resultants acting in these directions 

 must present the same inversion of magnitude without the forms 

 suffering any essential alteration in their angles or in the arrange- 

 ment of their faces. 



If this species of mobility of the optical characters belonging 

 to the same geometrical envelope requires any further demon- 

 stration, this would undoubtedly be found in the remarkable 

 phaenomena presented by crystalline alloys of substances en- 

 dowed with opposite optical characters. 



Crystals of Hyposulphates of Strontia and Oxide of Lead, 



In the first instance I allowed hvposulphate of strontia, whose 

 optical axis is that of maximum elasticity, to crystallize in various 

 and progressively-increasing proportions with hyposulphate of 

 lead, whose optical axis is that of minimum elasticity ; or rather 

 1 combined hyposulphate of strontia, in which the ordinary ray 

 is always more refracted than the extraordinary ray, with hypo- 

 sulphate of lead, in which the contrary obtains. 



The rings of colours which the mixed crystals present in con- 

 vergent polarized light enlarge at first, on account of the dimi- 

 nution of birefractive power resulting from the combination of 

 opposite optical characters ; and they subsequently contract as 

 soon as this power reappears with an opposite sign, so that it is 

 not difficult to obtain, in the series of salts containing these two 

 bases, crystals whose birefractive power possesses an almost 

 equal absolute value, and in which the optical axis is sometimes 



