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



may be employed.* In that case all that is necessary is to focus 

 the microscope on the object, and after nearly closing the dia- 

 phragm lower the condenser till the aperture in the diaphragm 

 appears in focus. The glass slip is then adjusted, if necessary, 

 so that the mineral to be observed is in the centre of the field 

 and the diaphragm opened or closed till the maximum area of 

 that mineral, but no portion of any other, is illuminated, f It is 

 scarcely necessary to add that the greatest care must be taken 

 to see that the nose-piece is exactly centred so that the object 

 remains in the illuminated area during the rotation of the stage. 

 The Bertrand or Becke lens is now placed in position and the 

 directions-image can be studied. 



The same result can be obtained by placing a diaphragm at 

 any point above the object where a real object-image is formed, 

 provided of course that it is not affected by the conversion of 

 the microscope into a hodoscope. One of the following methods, 

 preferably the second, may be employed : 



1. If the eye-piece be removed, an object-image can be formed 

 exactly at the upper end of the microscope tube by operating the 

 coarse or fine adjustment. The mineral selected for examination 

 is then brought into the centre of the field and a cap with a 

 central perforation, not larger than the image of the mineral, is 

 placed on the end of the tube. If the eye be now placed close 

 to the aperture, the directions- image will be seen low down in 

 the tube in the position already described, illuminated only by 

 light which has traversed the mineral. 



2. The eye-piece may be retained and the mineral to be studied 

 isolated by means of a diaphragm in the focus of the eye-piece. 

 The Becke lens is then placed in position and the directions- 

 image of the mineral, unmixed with other light, is seen. 



3. If the Bertrand lens be employed, an object-image is 

 formed above it and below the eye-piece, and can be seen if the 

 eye-piece be removed. A diaphragm may be inserted here, 

 but the low magnification of the image is a drawback. A 

 diaphragm is frequently placed just below the Bertrand lens. 



* In other instruments the iris diaphragm is so close to the condenser 

 that the latter cannot be lowered sufficiently to bring it into focus. 

 Another diaphragm must then be provided. 



f Light traversing glass or other isotropic substances will not, 

 however, affect the result, if the nicols be crossed. 



