286 Mr. L. Fletcher's CrystallograpUG Notes. 



however, gave 20° 9'; and closer examination rendered it clear 

 that on the crystal measured the faces belonged not to the 

 form z{201\, but to another less common form h{302\, 

 whilst smaller almost linear faces of z{2 1} were to be seen 

 lower down in the reentrant angle. The angle between (3 2) 

 and (1 1) being 11° 20^', the reentrant angle for this form 

 should be, according to Haidinger's law, 22° 41/ ; on the 

 other hand, according to Naumann's law, it should be 20° 59' 

 — that is, the difference of the inclinations of (3 2) and its 

 parallel (3 2) to the plane (1 I). The measured angle was 

 thus 50' less than the angle calculated from Naumann's law; 

 and the latter angle was itself 1° 42' less than the one calcu- 

 lated according to that of Haidinger. 



The difference between the calculated and measured angles 

 is large, and in fact is so considerable that it can only be 

 attributed either to the growth being not strictly a regular 

 twin, or to a deviation from the fundamental angle as deter- 

 mined by Haidinger. As the extreme accuracy of Haidinger's 

 measurement of the fundamental angle of copper pyrites had 

 been confirmed both in the memoir of Sadebeck and in the 

 Catalogue of the Strassburg collection, and also by Kokscha- 

 row*, and as, further, this particular crystal did not lend 

 itself to a precise determination of any other angle than the 

 reentrant one above mentioned, the result seemed very unsatis- 

 factory, and for some time the examination was discon- 

 tinued. Two years later it was resumed ; and, happily, another 

 crystal from the same specimen was found to give reflections 

 so good that a precise measurement of the angle between two 

 octahedron-planes belonging to the same individual and on 

 opposite sides of c could be obtained. This was found to be 

 108° l?y, a deviation of 22\' from the angle as determined 

 from the specimens previously measured. A less precise deter- 

 mination of the angle of a terminal edge gave as mean 

 69°59±', the limiting values being 69° 54f and 70° 0|'. From 

 the same crystal the angle o o x was found by help of the S eye- 

 piece of a Fuess's goniometer (as improved by Y\ r ebsky) to be 

 2° 3', instead of zero according to Haidinger, and 1° 23|' 

 according to Naumann and Sadebeck. 



The whole difficulty had, however, now disappeared, as is 

 shown by the following table of calculated and observed 

 angles: — 



* Butt. Soc. St. Pet. 1874, xix. p. 562. 



