210 



PROFESSOR MILLER ON THE POSITION OF THE AXES 



of the simple forms are expressed, and of the method of representing 

 the form of a crystal by its " sphere of projection," will be found in 

 the Cambridge Transactions, Vol. V. p. 433. The velocity of light in 

 air divided by its velocity within the crystal, for a ray in the plane 

 of the optic axes, and polarized in the same plane, is denoted by ft. 

 I being the refracting angle of a prism having its edge perpendicular 

 to the plane of the optic axes, and D the minimum deviation of a ray 

 refracted through it, polarized in the plane of the optic axes, n sin 1 / 

 = sin ^ (D + I). The index of refraction of the oil used in some of 

 the observations is 1.4706 for the brightest rays of the spectrum, a, /3 ; 

 £, £, denote the extremities of radii of the sphere of projection drawn 

 parallel to the optic axes and axes of optical elasticity respectively. 



(1). In Oxalic Acid, C H 3 , the cleavages being 

 parallel to the faces m, mm' = 63°. 5', ee' = 34° . 32, 

 pa = 50°. 40', cp' = 76°. 45', ac m 52°. 35', pm 

 = 81°. 34', am = 6l°.13',5, cm = 62°.55',5. The 

 symbols of the simple forms are, p {0 1}, 

 m {1 1 0}, e {Oil}, a {10 1}, c {10 1}. 



The apparent directions of the optic axes seen in oil through the faces p 

 lie in a plane perpendicular to the faces p, e, and make with each other an 

 angle of 115°. 30'. ^ = 1.499- Hence a/3 m 68°, and the axis of optical 

 elasticity £ coincides with the axis of the zone peep'. 



(2). In Sphene, the faces being denoted by the same letters as in the 

 treatises of Mohs and Naumann, and the principal cleavages being parallel to 

 the faces I, y, #/=66 .54', y/=131°.21', fy> = 85°.33', 

 yx = 21°.5', xp = 39°.19', pql = 85°. 10', pqt = 53°.36', 

 pqn = 28°. 6'. The symbols of the simple forms are, 

 q {0 1 0}, p {0 1}, / {1 1 0}, m {1 3 0}, r{0 1 1}, 

 y {1 1}, x {1 02}, o {0 1 3}, t {1 2 1}, d {1 1 3}, 

 n {I 2 3}, u {16 3}, / {T 1 2}, s {14 1}. 



The apparent directions of the optic axes seen in water through the 

 faces x lie in a plane perpendicular to the faces xp, and make angles of 



