288 Mr. L. Fletcher's CrystallograpMc Notes. 



chosen for its approximate homogeneity, this excess of FeS 2 

 may be almost wholly due to mere admixture of iron pyrites. 



Since the above determinations "were made, the collection 

 has been enriched by the accmisition of a superb Freiberg 

 specimen of very much the same character as the one just 

 described, but on a relatively colossal scale ; for the length of 

 side of the triangular face o o x o 2 (and this is the smallest) is 

 here not 1*5 but 24 millim. Owing to the absence of reentrant 

 angles, the growth very much resembles a regular octahedron 

 truncated by the faces of a cube, one of the octahedron-faces, 

 however, showing in a very marked way the threefold com- 

 position of that part of the crystal. 



A further examination of the collection has resulted in the 

 finding of a specimen (from Pool mines near Redruth) which 

 confirms the parallelism of the planes of composition and of 

 twinning in the most satisfactory way. The crystals of copper 

 pyrites have been here deposited on quartz crystals which they 

 partly enclose: they thus have not an all-round development; 

 in fact, there is a practical difficulty in determining more of the 

 growth than is shown in fig. 11; but this, so far as it goes, falls 

 little short of perfection (the dotted lines indicate the twin 

 in theoretical completeness). The images from co co 2 were so 

 well defined that the largest magnifier, a, of Fuess's instru- 

 ment could be used, three different measurements at nearly 

 normal incidence giving respectively for the value of this angle 

 1° 23', 1° 23y, and 1° 23', while a measurement at almost 

 grazing incidence, when the images were broader, gave 1° 23^'. 

 The faces o o 2 were not so perfect ; but still, with the S 

 eyepiece, good images were seen, from which two consecutive 

 measurements of the angle gave 1° 23' and 1° 23£'. Two 

 measurements of oaf gave respectively 108° 39' 30" and 

 108° 39' 45". The following table renders more evident the 

 close correspondence of the observed and calculated angles. 

 Observed, 





Angles of 

 one individual 



... 70° 71' 





-* , Calculated 



Symmetrically disposed from 

 angles of the other. 001.101 



o,co 2 70° 1\' 70° 7' 32" 



co. 2 eo 35° 4' 35° 3' 46" 



<o. 2 o f 2 .... 70° 12', 70° 4i' 70° 7' 32" 

 o' 2 e' 2 .... 35° 4' 35° 3' 46" 

 o 2 o' 2 108° 37' 108° 40' 4" 



we... 

 coo f . 



0' .. 



... 35° 3|' 



... 70° 4f 



.. 108° 39f 



Also 



CO (0. 2 



0.-, 



1- 



23'. 1° 23V, 1° 23', 1° 23V 1° 23' 30" 

 ' 23', 1° 23V 1° 23' 30" 



The linear faces e{l 01} and the minute faces of another 



