597 



THE DETERMINATION OF MINERALS UNDER THE 

 MICROSCOPE BY MEANS OF THEIR OPTICAL 

 CHARACTERS.* 



By J. W. Evans, D.Sc, LL.B. (London), 



OF THE IMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY, 

 AND BIRKBECK COLLEGE, UNIVERSITY OF LONDON. 



(Read June 22nd, 1915.) 



Communicated by the Hon. Editor. 



Plates 35-37. 



A petrological microscope is not merely employed for the 

 study of details too small to be seen by the unaided eye, it is 

 also an instrument for the investigation of the optical properties 

 of minerals, by means of which they may be distinguished 

 from one another. 



Kotation of Nicols or Stage. For this purpose the microscope 

 must be so constructed that the minerals can be examined 

 between crossed nicols. A Nicol's prism or nicol permits only 

 light vibrating in a particular direction to pass. Two nicols 

 are said to be crossed when these directions of vibration are at 

 right angles to each other. It is also necessary that either the 

 stage or the nicols shall be capable of rotation round the micro- 

 scope axis. For many reasons the rotation of the nicols, while 

 the stage remains stationary, is to be preferred ; and when an 

 immersion lens is employed with loose material, it is essential 

 that there should be no relative movement between the stage 

 and objective. The mechanical difficulties of construction in 

 instruments of this type add, however, considerably to the 

 expense, with the result that in the majority of petrological 

 microscopes in use the nicols are fixed, while the stage rotates. 

 I have, accordingly, assumed throughout that such an instru- 



* A brief communication to the Geologists' Association on similar lines 

 was made by the author in 1909 (see Proc. Geol. Assoc, vol. xxi., 1909, 

 pp. 79-94). The Quekett Microscopical Club is indebted to the 

 Geologists' Association for their courteous permission to use the blocks 

 illustrating this paper. 



