1896.] NEW YORK ACADEMY OP SCIENCES. 51 



tion. If the liquid is of the mean refractive index of the crj'stal 

 the border will for one position move towards the liquid and for 

 the other towards the grain. 



A final proof of the correctness of the preparation is that if 

 n=^ 2 Ha + 2ffo = 180°, but if on measurement 2Ha + 2ffo> 

 180°, then- < 1 and too much diluent has been added or if 



2 ^, + 2 ZTo < 180° then " > 1 and more diluent should beadded. 



4. Determination of Pleochroism. 



If the plane of the optic axes is made horizontal and the line 

 of sight besects the axial angle, then either a or p is normal to 

 the line of sight and horizontal, the other can be made so by a 

 rotation of K through 90° and 6 is normal to the line of sight 

 and vertical. 



Therefore, with analyzer out and polarizer vibration vertical 

 record color for vibrations in direction h. Then with polarizer 

 vibration horizontal record color for vibrations in direction a 

 or t and b^^ rotation of K through 90° that for vibrations 

 parallel to the remaining axis of elasticity. 



III. Klein's Apparatus for the Orientation or Thin 

 Sections.* 



The determination of the position of axial plane and of the 

 character of the double refraction in the crystals of a rock sec- 

 tion require either the finding of properly oriented crystal sec- 

 tions or some device by which the orientation may be readily 

 nltered so that the light shall pass through the crystal in the 

 ilesired direction. 



The different types of " Universal Table " of von Federowf ef- 

 fect this reorientation, yet even to the later improved types the 

 great objection exists that the sections must be mounted on small 

 round object glasses of high index of refraction, n = 1.523. 

 This means that the innumerable sections mounted on square or 

 rectangular slides of common glass can not be studied with this 

 apparatus. The cost of the special glasses is also considerable. 



The new apparatus of Professor Klein substitutes for the glass 

 of high refractive index a hemispherical vessel filled with some 

 liquid of high refractive index, usually glycerine, w = 1.46, 

 or some oil, or if necessary the still more strongly refracting 

 liquids. The section mounted on ordinary glass slides is sup- 



*C. Klein, Sitzungsber. Berlin. Akad., 1895, 1151. 



t E. V. Federow, Zeit. fur Krystallographie, etc., v. 25, p. 3ol. 



