240 Prof. Lewis, On a crystal of Stephanite [May 1- 



(2) On a crystal of Stephanite from Wheal Newton. By Prof. 

 W. J. Lewis. 



Last summer Prof. Warrington Smyth was good enough to lend 

 me for examination a remarkably fine crystal of stephanite, which he 

 had obtained some years previously from Wheal Newton in Cornwall, 

 and which is, I believe, the only specimen so far known from an 

 English locality. It is now placed in the Jermyn Street Museum. 

 It is implanted on a large fragment of matrix of chalybite and 

 quartz about four inches long by about two and a half wide. 

 The specimen was fixed for the observations in the apparatus 

 described by Prof. Miller in a paper on his goniometer in the Phil. 

 Magazine, 5th series, vol, II., p. 281, 1875. From the size of the 

 matrix, and the weight of the specimen, considerable difficulty was 

 experienced in the accurate adjustment of the zones, and no great 

 attention was paid to centreing the edges when once the zone- 

 axis was placed vertical. Most of the faces gave good definite 

 reflexions; but various artifices had to be resorted to for the 

 purpose of obtaining some of the measurements. I was quickly 

 struck with the smallness of the errors in the angles observed 

 when compared with the angles given in Miller's Mineralogy. 

 These errors are mainly due to two causes : (1) the error arising 

 from inaccurate coincidence of the two signals, (2) that due to the 

 eccentricity of the crystal. The first was probably, from the 

 difficulties of the observations, at times considerable, and seemed 

 likely to be sufficient to account for the whole error. It would 

 also vary greatly in the measurement of different angles as the 

 matrix would come more in the way at one time than at another. 

 The latter I had not up to that time carefully estimated, but it 

 seemed capable of approximate determination. I have thought 

 the examination of this error of sufficient interest to append to this 

 paper, and the more so as it is an error which has called forth a 

 good deal of criticism in Germany. Schrauf in his Lehrbuch der 

 Phys. Mineralogie, vol. I. p. 210, has given an expression for it which 

 gives quite an erroneous idea, and seems applicable only to an 

 utterly impossible method of observation. 



The forms, and the relation of the principal zones, observed on 

 the crystal are shown on the stereographic projection (fig. 1). The 

 crystal is a combination of the forms 



a(100), 6(010), m(110), tt(310), /)(111), z{IV1), n(113), c(O01), 



*(203), 6(101), rf(201), h (412). *(314), and X(0L1). 



