1880.J NATURAL SCIENCES OF PHILADELPHIA. 241 



P R O C E E D 1 N a S 



OP THE 



MiNERALOGICAL AND GEOLOGICAL SECTION OF THE ACADEMY 



OF Natural Sciences of Philadelphia. 



187 7-18 7 9. 



May 28, 1877. 



A New Polariscope — Mr. H. C. Lewis remarked that a cheap 

 and accurate polariscope for the measurement of the optic-axial 

 diveroence in minerals had Ions:: been a desideratum amona; min- 

 eralogists. He wished to direct attention to an instrument for this 

 purpose, lately made for him by Queen & Co., of this cit}', which 

 had proved very satisfoctory. The light was polarized b}^ reflec- 

 tion from a plate of black glass, converged upon the rotating stage 

 by two sets of adjustable lenses, and analyzed by a Nicol's prism. 

 A graduated circle of steel, having through its axis a sliding 

 forceps, is fastened at right angles to the stage. A pointer records 

 the amount of rotation of the forceps. The mineral to be examined 

 is either held in the forceps or is attached by a drop of oil to a 

 piece of thin glass which is held in the same way. Cross-hairs are 

 fixed below the eye piece, and the measurement of the divergence 

 of the optic axes is performed in the usual way. The instrument 

 was found to work admirably and could be recommended. The 

 adjustments were made quickly and the axial divergence could 

 be determined to within 30'. It is simple, absorbs but little light, 

 and gives good results even with very small fragments of minerals. 



A Garnet with Inverted Crystallization. — Mr. Lewis ex- 

 hibited a garnet which he had found in Germantown, and stated 

 that it showed a very perfect example of inverted cr^'stallization. 

 Its form was a perfect trapezohedron except that one octant was 

 depressed, its apex lying within the crystal, one-halfway towards 

 the centre. The re-entrant angles corresponded in position with 

 the trihedral edges on the opposite octant of the crystal. The 

 garnet was an isolated one found in a matrix of gneiss. Atten- 

 tion was called to the fact that such inverted cr3'stallization was 

 apparently more common in the isometric than in other S3'stems 

 of crj'stallization and comment was made upon the cause of such 

 phenomena. 



June 25, 1877. 



Change of Serpentine into Quartz. — Mr. Theodore D. Eand 

 described and presented specimens showing the change of ser- 



