Royal Society. 461 



deduced by Landolt and the author. This is more particularly 

 the case with H, CI, O, S, and Q, while as regards P there is a 

 considerable divergence. Of the refractive equivalents determined 

 by Schrauf from compound bodies, agreement is only met with 

 in the case of Br and I, and to a less extent in Si. As and Sn 

 exhibit marked divergence, which is to be ascribed to the cir- 

 cumstance that Schrauf has calculated the corresponding num- 

 bers from arsenic-ethyle and stannethyle, two substances which 

 are difficult to observe and to obtain pure. 



LV. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 398.] 



April 23, 1868.— Dr. William Allen Miller, Treasurer and Vice- 

 President, in the Chair. 



rpHE following communications were read : — 

 -*- " New Researches on the Dispersion of the Optic Axes in Har- 

 motome and Wohlerite, proving these Minerals to belong to the Cli- 

 norhombic (Oblique) System." By M. A. L. O. Des Cloiseaux. 



"We are already acquainted with a considerable number of crys- 

 tals, natural as Avell as artificial, the forms of which have only been 

 determined with precision by the examination of their optical pro- 

 perties as doubly refracting bodies. Harmotome and Wohlerite fur- 

 nish two fresh examples of this ; and they afford all the more im- 

 portant proof of the necessity of appealing to these properties, inas- 

 much as the crystals of these substances would appear certainly to be 

 derived from a right rhombic prism, so long as we consider only the 

 apparent symmetry of their external forms, or the orientation of the 

 plane containing their optic axes. The different sorts of dispersion 

 which these axes might be capable of presenting are so feeble, and 

 so difficult of appreciation on account of the slight transparency of 

 Wohlerite, and the complex structure of the crystals of Harmotome, 

 that the determination of these dispersions has hitherto been too in- 

 complete to allow of any conclusion being drawn as to the crystalline 

 type they might otherwise serve to characterize. 



It was a remark of M. Axel Gadolin that induced the author to 

 resume the attentive study of the phenomena of dispersion, first in 

 Harmotome, and then in Wohlerite, and as a consequence to modify 

 the crystallographic type to which these minerals have been in general 

 referred. 



Harmotome. 



Several years ago the author showed that simple crystals of Har- 

 motome did not exist, and that those of Strontian in Scotland (Morve- 



