OF OPTICAL ELASTICITY IN OBLIQUE-PRISMATIC CRYSTALS. 215 



(12). In Sulphate of Soda, Na iSu H w , hp = 72°. 16', pc=W-\5' 

 km m 40° . 12', kl = 22° . 54', ke = 49° . 54'. The 

 symbols of the simple forms are, k J01 Oj, 

 h {100}, p {001}, / {120}, e {Oil}, 

 »i {11 0}. 



The apparent directions of the optic axes 

 seen in oil through the faces h lie in a plane 

 making an angle of 78°. 30' with the face //, and 

 make with each other an angle of 97°. 30'. 

 n = 1.44 nearly. Hence h% = 12°. 24', a/3 = 80°. 26' nearly. 



(13.) In Hydrous Oxalate of Lime, Ca C H, a new mineral species 

 described by Mr Brooke in the Philosophical Magazine for June 1840, 

 b being the face parallel to which a very distinct cleavage exists, 

 cm = 50°. 18', cf = 65°. 28, ca = 37°. 24', 5, cu = 31°. 3', cs = 28°. 41', 

 pm = 76°. 46', pb = 70°. 33', pern = 72°. 41'. The symbols of the 

 simple forms are, c {0 1 0}, p {0 1}, j» {11 0}, 

 a \0 1 1}, b {1 1}, u {1 2 0}, / {1 1 2}, s \\ 3 2}. 



The optic axes could not be seen ; the position of 

 the axes of elasticity was however determined approx- 

 imately by placing the crystal in a polarizing apparatus, 

 having the planes of polarization and analyzation at 

 right angles to each other, with the face c perpen- 

 dicular to the axis of the instrument, and observing the position of the 

 face p when the crystal ceased to transmit light. In this manner it was 

 found that J| = 8°. 



W. H. MILLER. 



