THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



SEPTEMBER 1841. 



T 



XXVII. On the Form and Optical Constants of Anhydrite* 

 By W. H. Miller, M.A., F.R.S., Professor of Mineralogy 

 in the University of Cambridge*. 



HE following values of the angles between normals to the 

 faces of anhydrite were obtained from the measurements 

 of a single crystal, — the only one exhibiting any forms, ex- 

 cept those of cleavage, which I have been able to procure. 

 The faces being extremely dull, the only signal that could be 

 used in observing with the reflective goniometer was a small 

 opening in a screen, through which sun-light was thrown by 

 reflexion from a plane mirror. The results obtained under 

 such circumstances cannot be very accurate ; they are, how- 

 ever, certainly much nearer the truth than those which Haiiy 

 has given, and which appear to have been adopted by all suc- 

 ceeding mineralogical writers. 



The most perfect cleavages are parallel to the faces m, t ; 

 a cleavage a little less perfect than the preceding exists par- 

 allel to the face p, which is usually rough ; and cleavages still 

 less perfect may be obtained parallel to the faces r. The 

 face s is common to the zones m /, o p. 



• Communicated by the Author. 

 Phil. Mag. S. 3. Vol. 19. No. 123, Sept. 1841. N 



