600 J. W. EVANS ON THE DETERMINATION OF MINERALS UNDER 



Means should be provided to close the slot, when not in use. 



Fine Adjustment. The fine- adjustment screw should be 

 graduated on its circumference so as to show the number of 

 microns by which the microscope is raised or depressed. A 

 micron is the thousandth part of a millimetre and is the most 

 convenient unit of length for microscopical purposes. A com- 

 plete turn of the screw will usually correspond to 500 microns, 

 and in that case a scale parallel to the axis should be provided, 

 divided into half millimetres, so that by means of the double 

 graduation comparatively large movements may be accurately 

 measured. 



Illumination. The best illumination is that from the sky. If 

 artificial light must be resorted to, a gas mantle provided with 

 a cylinder of ground or milk-white glass, or a small arc light 

 similarly treated, should be employed. If, however, the illumi- 

 nation is very strong, the lower nicol may be injured by over- 

 heating. If there is any danger of this, a suitable glass vessel 

 containing water may be interposed. 



Objectives. Although a 1-in. objective is used for most pur- 

 poses, a lower power is convenient in the case of rocks of coarse 

 texture, while for very fine structures and minute crystals and 

 inclusions a l/4th-in. or still higher power must be employed. 

 I have myself found a twelfth very useful. 



These close objectives are also required for the simultaneous 

 examination of different directions in a crystal, a subject I shall 

 deal with later. 



Rock-slices. A good rock-slice should range between twenty 

 and thirty microns in thickness, but with comparatively large 

 transparent minerals much thicker sections may usefully be em- 

 ployed, while those with fine structures or which are comparatively 

 opaque should be as thin as they can be made. It is important 

 that a section should be as uniform in thickness as possible, 



It is preferable that a rock-slice intended for research should 

 have no cover- glass and that its surface and sides should be free 

 from Canada balsam. It may then be covered in turn by liquids 

 with different refractive indices (see p. 626) or subjected to 

 microchemical tests. 



