bowen: anthophyllite 



413 



is very sensitive. The first trial, with a Hquid of index 1.622, 

 showed that a. was distinctly lower and on making up a liquid of 

 index slightly less than 1.620, no difference in refraction of liquid 

 and mineral was observed. The determination therefore checks 

 with the calculated value <x = 1.6 195. A rough determination 



/,6¥o 



/.62.0 



'/t^FeSCOj 



Fig. I. Reflection indices of anthophyllite. 



of the birefringence of the plate which gave the value 0.021 is 

 also in accord with this lower value of a. From these determi- 

 nations it may be stated, then, that the indices of anthophyllite 

 from Franklin, North Carolina, are 



7 = 1.6404, |S = 1. 6301, a = 1. 6195. 



The optic axial angle is 88° 46' and the birefringence 7 — a = 

 0.0209. Ill the text-books the erroneous value a « 1.6288 is 

 given as well as the correspondingly erroneous value of the bire- 

 fringence 7 — a = 0.0116. The new values of the indices as 



