The MicKo.sroi'E. 117 



ing the diaphragm. Here the observer will have an opportunity 

 to exercise his judgment, for this is one of those things that 

 cannot be described ; it must be taught personally, or discovered 

 by the worker unaided. 



To the use of the mirror in this connection certain rules apply. 

 With very low power objectives where the actual field is com- 

 paratively large, the apparent field cannot be evenly lighted by 

 either mirror. With all low powers, and without a substage con- 

 denser to modify the intensity, it is better to use the plane mirror. 

 This will sufficiently illuminate the field of all below the one inch, 

 until we reach those very low powers now used, such as Zeiss's 

 variable A* which may be used as a three or five inch, with all 

 intermediate amplifications, by simply turning an adjustment 

 collar. The field of these lenses cannot be lighted throughout 

 its whole extent by either mirror. Tlie concave gives only a 

 central bright spot while the plane surface does little better. To 

 light this large region fully, the plane mirror should be covered 

 with a disk of white card-board, and parallel rays thrown on it 

 by the Bull's eye condensing lens. The mirror and card are 

 then manipulated as the mirror alone should be. Or a more 

 successful result may perhaps be attained by taking the light 

 with the mirror from the brightly illuminated inner surface of a 

 white lamp-shade. With the two inch or the one inch the plane 

 mirror alone is sufficient. All higher powers demand the use of 

 the concave surface unless the Bull's eye lens is placed between 

 the lamp and the mirror, with a substage condenser between the 

 mirror and the object; or unless some form of microscopical 

 lamp is used with a permanent Bull's eye lens in which cases 

 the plane surface should be turned toward the condenser. 



It sometimes happens that if the microscope has travelled for 

 some distance by railroad, the owner will have trouble to get rid 

 of a broad, cresentic shadow in an otherwise well lighted field. 

 No manipulation of the diaphragm or other substage appliance 

 serves to remove the annoyance ; it remains partly eclipsing the 

 field by its region of darkness, and the beginner may think that 

 something serious has happened. The trouble is caused by the 

 jarring of the diaphragm in the body or in the draw tube, the 

 journey having shaken it too far downward. Remove the body, 

 first of course taking off the objective and the eye-piece, and 

 push the diaphragm up a little way. repeating the operation if not 

 at first)|successful. 



