MINERALOGY. 15 3 



transparent substance is placed over it. This measurement 

 is obtained by determining with an appropriate scale (to 

 nnny f au inch) the distance which the body of the micro- 

 scope must be moved to bring the original object into focus. 

 When this distance is known, and also the thickness of the 

 object in hand, the index of refraction may be calculated by 

 a simple equation. Mr. Sorby describes the method in full, 

 showing how readily it may be applied and with what use- 

 ful results. He also discusses some phenomena which were 

 observed in the course of his experiments, and which led to 

 the discovery of "a new class of optical properties." These 

 phenomena are fully explained, in accordance with the known 

 laws of refraction, by Professor Stokes in the Proceedings of 

 the Royal Society, to which it is here only possible to make 

 reference. 



Des Cloizeaux has continued his optical examination of the 

 feldspars, and has come to the interesting result that there ex- 

 ists a triclinic potash feldspar, having identically the same 

 composition with the monocluiic potash feldspar ortho- 

 clase. The new species is called by him microcline. He 

 shows, moreover, that these two feldspars generally occur 

 crystallized together, the one interpenetrating the other, 

 often in successive parallel bands. The beautiful green 

 feldspar called amazonstone is conspicuous for this associa- 

 tion of the two species : it is also true of the chesterlite of 

 Pennsylvania, and specimens from many other localities. 

 On the other hand, the feldspar enclosing regirite from Mag- 

 net Cove, Arkansas, is found to be pure microcline. 



Another extended work on the feldspar group has been 

 published by Professor Szabo, of Buda-Pesth, Hungary. His 

 object is to make it possible to distinguish between the dif- 

 ferent species as they occur in rocks. The method is based 

 upon Bunsen's "Flame-reactions." It consists in part upon 

 the accurate determination of the degree of fusibility, and in 

 part upon the degree of coloration given to the flame, under 

 different conditions, from which the amounts of sodium and 

 potassium are estimated. The system is an ingenious one, 

 and has been most carefully elaborated by the author; but 

 it may be well questioned whether in hands less skilful than 

 his it would yield any valuable results. 



The important group of minerals embraced under the gen- 



G2 



