On Material Molecules and the Etherial Medium. 255 



tially connected the possibility of discovering such a formula 

 must be with the state of our knowledge of the laws of sym- 

 metry of crystalline forms, I was led into some rather extended 

 researches on this latter subject, the result of which was to 

 induce me to take a different view of the laws of symmetry 

 of those uniaxal crystals which belong to the rhombohedral 

 system from those previously held by crystallographers. 

 Having discovered that such crystals were in reality sub- 

 ject to the same laws of symmetry as crystals of the pris- 

 matic system, — one of the angular elements being always 

 equal to 60°, — I was led almost immediately afterwards 

 to a formula, expressing the angle between the optic axes 

 of a crystal of the prismatic system in terms of the angular 

 elements,* which agrees remarkably with observation for 

 those crystals for which both the angle between the optic 

 axes and the angular elements have been most accurately 

 determined. In the case of aragonite, the angle between 

 the optic axes, as calculated by the formula from its angular 

 elements, differs from KirchhofTs very accurate measure- 

 mentsf for mean rays by only 15', in the case of chryso- 

 beryl by 19'. In the case of other minerals, as karstenite 

 (anhydrite), nitre, cerussite, the calculated agrees with the 

 observed angle, by making slight changes in the generally 

 received values of the angular elements, which are quite 

 within the limits of errors of observation, or of the varia- 

 tions in the angular elements observed in different speci- 

 mens of the same mineral. 



I am at present engaged on further investigations con- 

 nected with this formula; but in the meantime, although 

 perhaps it may require some modification, I am impressed 

 with the conviction that it contains the elements of 

 truth. Such a formula would, of course, be a most import- 

 ant guide in all theoretical researches in which the 

 action of the material molecules is attempted to be taken 

 into account. 



It is needless here to dilate upon the admitted defects of 



* " On the Connection between the Form and Optical Properties of 

 Crystals," Proceedings Royal Society, Edinburgh, 2d May 1864. 

 t Kirchhoff, Pogg. Annalen, cviii. (185D) \>. AT I. 



