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 Kirchhoff's 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 tlie Form and Optical Properties of 
Crystals," Proceedings Koyal Society, Edinburgh, 2d May 1864. 
t Kirchhoff, Pogg. Annalen, cviii. (1859) p. 574. 
