1896.] NEW YORK ACADEMY OF SCIENCES. 45- 



SOME NEW APPLIANCES AND METHODS FOR THE 

 STUDY OP CRYSTALS. 



By Alfred J. Moses, Ph. D. 



The appliances herein described illustrate well the recent de- 

 velopment of exact methods in the microscopic stad> of small 

 detached crj^stals, and crj'stals in thin sections. This develop- 

 ment has been of such a nature that it may now almost be said 

 that a good microscope, a good goniometer and their attach- 

 ments^ suffice for all determinations of optical properties and 

 geometric form. 



The appliances herein described are : 



I. The Fuess microscope model No. YI. 

 II. The Klein universal rotation apparatus for crystals. 

 III. The Klein universal rotation apparatus for thin sections. 

 lY. The von Federow mica wedge. 

 Y, The Traube arrangement for darkening the field of a 

 goniometer. 

 YI. The Fuess goniometer adapted for the measurement of 

 angles between optic axes. 



I. The No. YI. Fuess Microscope. 



This latest microscope for crystallographic and petrographic 

 work shown in Fig. 1 was described in 1895,* and is in size and 

 build simply model No. If with certain important improvements.. 

 The essentials^ of any petrographical microscope : polarizer, ana- 

 lyzer, condenser, rotating stage, objective and eye piece may be 

 variously modified and supplemented and the Fuess No. 1 in- 

 strument with its man}' adjustments has hitherto held a rarely 

 disputed preeminent position. 



The principal improvement in the new No. YI. model is a de- 

 vice for simultaneous rotation of polarizer and analyzer, the ob- 

 ject remaining at rest, necessarily the directions of greatest and 

 least elasticity' in the crystal under examination, undergo just the 

 same relative change with respect to the vibration directions of 

 the nicols that they would if the stage were revolved and the 

 nicols were at rest and this is accomplished without it being 

 necessary that the eye should follow the changes in the relative 

 position of the parts of the field. There is also a marked gain 

 in the fact that no centering is required. 



*Neues Jahrbuch f. Mineraloqie Beilage Bd. X. 180 by C. Leiss. 



ilbid. Beil Bd. VII. 55 1891' R. Fuess. 



JFor description see School of Mines Quarterly V. 17, p. 43 by L. McI. Luquer. 



