108 PROCEEDINGS OF THE AMERICAN ACADEMY 



between the octahedral and the prismatic faces, on which we had chiefly 

 to depend for determining the position of the vertical axis. The value 

 of the angle Tl2 on 110, which we selected as one of the fundamental 

 data, was obtained by comparing a number of crystals on which the 

 condition of the faces was especially favorable, and is the most proba- 

 ble value deduced from many observations. Nevertheless, this is the 

 one doubtful element, and may be in error to the extent of a few 

 minutes. The result was checked by measuring the angle of Tl2 on 

 110, which, although not favorably situated as a measure of the funda- 

 mental dimensions of the crystal, was useful as a proof of the accuracy 

 of the work by which they were deduced ; and the table below shows 

 how well it bears this test. As in this last measurement, the reflected 

 image of the signal crossed the striations at nearly 90'*, the signal was 

 comparatively well defined ; and the same was true in the measurements 

 of the prismatic angles, which agreed very closely on different crystals. 

 The hemioctahedral planes {112} were by far the best developed of 

 all the planes of this class ; but we selected in preference, as the funda- 

 mental octahedron, a subordinate form {Til}, to which the associated 

 planes bore a simpler numerical relation. The planes ^Tl3| were so 

 small that the reflection of the signal was seen only by flashes. The 

 planes ^331^ were the best defined of an indefinite zone between 

 ^Tll^ and ^TlO^ ; among which others, with still higher parameters, 

 might have been doubtfully distinguished. We have previously called 

 attention to similar zones of planes * with high but yet definite numeri- 

 cal ratios. They are by no means an exceptional phenomenon, and 

 their crystallographic interpretation seems to us worthy of more atten- 

 tion than it has received. The result of our measurement is given in 

 the following table : — 



Crystalline Form of Antimonious Oxichloride (Sb^O^Clg). 



Prepared by Schaeffer's Method. 



MoNOCLiNic System. 



Forms |T10^, |Tll^, JT12|, |T13|, \SS\\, a jlOl^, a {I00| 



Fig. 8, Plate I. 



Fundamental Angles. 



Between planes TTO and TlO 86° 49' 

 „ „ T12 „ TlO 156^^ 42' 



„ „ TT2 „ T12 112'' 7' 



* These Proceedings, vol. iii., page 87. 



