276 SENARMONT ON THE OPTICAL CHARACTERS OF 



the two systems of isochromatic curves presented by two plates 

 of equal thickness ; every circumstance is identical, angle and 

 dispersion of the optical axes, but this absolute identity may be 

 demonstrated by a still more decisive proof. 



It is known that the bisecting line is normal to the rhombic 

 base of the prism of 100° 30'. If then two plates, one of laevo- 

 tartrate, the other of dextro-tartrate, are cut without injuring 

 that face, and then joined together by their natural bases with 

 Canada balsam, it will be possible after a few trials to bring the 

 crystals into perfect parallelism, and then a single system of 

 isochromatic curves may be seen through the complex plate the 

 same as with a simple plate of a thickness equal to the sum of 

 the partial thicknesses. Nothing is altered, either in the singular 

 form of the rings or in the disposition of the colours. 



The angle of the optical axes in dextro-tartrate of potash and 

 soda was found by Sir J. Herschel to be = 76° for the red and 

 56° for the violet rays ; and further, the index of refraction of the 

 red rays =1*4929, and that of the green =1*4985, when both 

 lie in the plane of the optical axes and are polarized in it. The 

 same data w^ere likewise obtained for the laevo-tartrate. If, 

 however, plates perpendicular to the optical axes of the inter- 

 mediate ray are cut, in accordance with these data, from Sei- 

 gnette salt, it is easy to perceive that the inclination of the optical 

 axes is greater than is stated by Sir J. Herschel. 



I have observed the isochromatic curves through several plates 

 of one or other salt parallel to the rhombic base, and cemented 

 between the hypothenuses of two rectangular isosceles prisms of 

 glass. By this means, the axes, which are too divergent to be 

 seen directly through a plate with parallel faces, are rendered 

 more convergent at their emergence, and I have thus convinced 

 myself that in both salts the plane of the optical axes is parallel 

 to the minor diagonal of the rhombic base. 



The bisecting line is the axis of major elasticity*. 



* As this latter result is not in accordance with the statement of Sir D. 

 Brewster, which appears to have been confirmed by other physicists, I will de- 

 scribe the method by which it was obtained. 



From large and perfectly pure crystals of Seignette salt, the form of which 

 was previously determined, three large thick plates were cut, one parallel to 

 the rhombic base, the other two parallel to the very well developed faces of the 

 rectangular prism ; all these plates were bounded laterally by the natural faces. 



