Mr. W. J. Lewis's Crystallographic Notes, 455 



one in which the axes have been projected in the usual manner. 

 To avoid confusion, the small planes s have been omitted in the 

 latter figure. The crystal is elongated in a direction nearly 

 coinciding Avith the normal to a face of the tetrahedron. An 

 oval ring seems to have been first deposited ; and on this the 

 crystal has grown, vaulting itself on the lower surface so as 

 not to cling to the bottle. There were no definite crystal- 

 faces to be seen on this concave portion. The top is also irre- 

 gular and indefinite. The crystal introduced into the polari- 

 zing microscope between crossed Nicols depolarized the light. 

 Exact experiments on the rotation of the plane of polarization 

 could not be made without destroying the crystal. 



On some crystals of barium nitrate crystallized out of solution 

 during the course of a few weeks, the forms k{1 1 1}, tc{l 1 1}, 

 {101}, and k{1 2 2} were found. The planes k{1 1 1} seemed 

 to be smoother and brighter than those of /c{lll} ; and the 

 edges of the former carried the planes /e{l 2 2}. 



Sphene. 



On a crystal from the Tyrol, obtained by me some years 

 ago, two rough ill-developed planes are situated on the quoin 

 formed by the two planes n = {l 2 3} and the base c={0 1}. 

 They look almost like the result which would be produced by 

 slightly grinding down this quoin. The exact symmetry of 

 the two planes, as also the frequency of their occurrence, show 

 them, however, to be really planes. Hessenberg, who devoted 

 considerable attention to this mineral, has noticed similar faces 

 on the crystals from the Zillerthal, described in his Min, 

 JSfotizen, vi., and has introduced them in two of the figures of 

 these crystals. As this part of the Notizen is out of print, I 

 have copied one of these figures (fig. 34), in which the small 

 triangular dotted planes are those under consideration. Hes- 

 senberg says that he found them more or less clearly deve- 

 loped on almost all the crystals from this locality. He ex- 

 presses, however, his conviction that the rough portion is only 

 a continuation of the plane n. 



Lately I obtained several crystals on which these planes 

 were very fairly developed, of one of which fig. 4 is a projec- 

 tion. They give such excessively bad reflections, that it was 

 only by observing with a ray of sunlight thrown into the room 

 by a mirror, and by slightly oiling the surfaces, that reliable 

 measurements were obtained. The form calculated from these 

 measurements is {3, 3, 10}, adopting the axial system given 

 in Miller's ' Mineralogy.' The following are the angles ob- 

 served and calculated : — 



