Note on the Optical Characters of Luallamite. 135 



tained by moans of the needles of 0*553 millim. diameter. It 

 is seen that it presents the same general characters as the curve 

 P, which relates to the permanent magnetism. The portion 

 of the curve, li 1? R 2 , comprised between the foot of the tan- 

 gent to the point of inflection and the point where this tan- 

 gent cuts the horizontal asymptote of the curve is the region 

 of rapid magnetization. The extent of this region, and the 

 coordinates of the two curves corresponding to the extreme 

 points, very well characterize, in default of the complete curves, 

 the magnetic qualities of the steel employed. 

 [To be continued.] 



XVIII. Note on the Optical Characters of Luallamite. 

 By N. S. Masxelyne, M.A., F.R.S* 



IN my contribution to Mr. Field's paper on the above 

 mineral I gave a very brief and inexact notice of its op- 

 tical characters, not having the time or the material to work 

 them out. The preliminary experiments had been made upon 

 fragments of rough crystal on which only the cleavage-plane 

 could be identified, and the form of which was misleading as 

 to the position of the face 10 0. I have since had the oppor- 

 tunity of working upon another crystal, which, though very 

 full of divisional planes, has nevertheless enabled me to obtain 

 the following results. 



On looking through a plate of a crystal of Ludlamite parallel 

 to a cleavage-plane in a Noremberg's polarization-apparatus, 

 one of the optic axes may be discerned near the edge of the 

 field ; and it is also seen that the optic axes lie in the principal 

 section parallel to the plane of symmetry. 



Accordingly two faces were ground on the last-mentioned 

 crystal as nearly as possible parallel to the plane of symmetry. 

 One of the planes thus ground was cemented to a plate of glass 

 and laid upon the revolving stage of a microscope, and by the 

 aid of a goniometer eyepiece, one wire of which was accurately 

 adjusted parallel to the plane of polarization of the light em- 

 ployed, the directions were determined of the principal sections 

 of the crystal relatively to the trace or edge of the cleavage- 

 plane, 01. This process gave an angle of 12° 22' for the 

 inclinations of the two principal sections upon the trace of the 

 face 01 and on its normal respectively, the greater angle of 

 102° 22' for one* of the principal sections lying on the positive f 



* Supplementary notice to Mr. Field's memoir read before the Crystal- 

 lological Society on December 15, 1876. Communicated by the Author. 



t In my first communication the contour of the fragment led me to the 

 erroneous conclusion that this angle was on the negative side of the normal. 



