242 Proceedings of the Boyal Society 
17. The angular elements of Calcite are — 
100,110 = 60°; 010, 011 = 45°23'-4; 001, 101 = 29° 40'. 
Those of Aragonite are — 
100, 110 = 58° 5'; 010, Oil = 40° 50'; 001, 101 = 35° 47'. 
These angular elements differ considerably. Aragonite and 
Calcite will therefore continue to form distinct mineral species. 
18. The author hopes soon to be able to publish the results of 
investigations which appear to show that the crystals included in 
the oblique and anorthic systems are formed by the combination of 
hemihedral and tetartohedral forms of the prismatic system. 
8. Preliminary Note on the Connection between the Form 
and Optical Properties of Crystals. By Alfred K. Catton, 
B.A., F.C.P.S., Scholar of St John’s College, Cambridge. 
1. It is the object of this note to give an account of the results 
of investigations, which have had for their object the discovery of 
the connection between the form and optical properties of crystals. 
It is believed that in the results here given, some of the prin- 
cipal difficulties of this important problem have been overcome. 
2. The first step towards the solution of this problem was made 
by Sir David Brewster in 1818. He discovered that crystals be- 
longing to the prismatic, oblique, and anorthic systems, are biaxal ; 
those belonging to the pyramidal and rhombohedral systems uniaxal, 
while crystals of the cubic system do not possess double refraction 
(a fact which had been previously stated by Hauy). 
In the prismatic system, the optic axes always lie in a plane 
containing two of the crystallogTaphie axes, and make' equal angles 
with each of these axes. 
Now the theoretical investigations of Freknel have shown that 
the optic axes lie in a plane containing the axes of greatest and 
least elasticity, and make equal angles with each of these axes. 
Hence it follows, that, in the prismatic system, the axes of elas- 
ticity coincide with the crystallographic axes. 
In the oblique system, one axis of elasticity coincides with that 
crystallographic axis which is at right angles to each of the other 
two. 
