188 THE USE OF THE MICROSCOl'E 



26. With a binocular, use only covers of the correct 

 thickness, if the tube, length cannot be altered by pulling 

 out the eyepieces slightly. 



27. Objectives with correction collars are advantageous 

 on a binocular when the tube length cannot be altered 

 much. 



28. Have a diaphragm in the drawtube of the monocular, 

 about 14 millimeters wide, below the longest eyepiece. 



29. Choose a lower objective usually, rather than a low 

 eyepiece (which often shows curvature of the field). 



30. Use as a rule no higher eyepiece than the maximum 

 useful magnification of 1,000 times the working aperture 

 will allow (except for drawing). 



31. Employ dark-ground illumination for fine details 

 which are due to differences of refraction. 



32. Red-stained bacteria and spirochsetes show well 

 when examined dry on a dark ground (Coles). 



33. Some objects are better observed in water than in 

 balsam, when either can be used. 



34. Use low Huyghenian or high orthoscopic eyepieces 

 with low achromatic objectives; and compensating eye- 

 pieces with high-power achromatic, or fluorite objectives. 



35. Use compensating eyepieces with apochromatic 

 objectives. 



36. Employ projection eyepieces or homals for photog- 

 raphy. 



37. Get the interocular distance exact in a binocular, and 

 note its measurement. In the Jentzsch form of binocular 

 the tube length may be corrected for different interocular 

 distances. 



38. Compensate the unoccupied eye, in a monocular, 

 with a disc of ground glass. 



39. Keep the upper surface of the eyelens optically 

 clean. 



40. Use a condenser cone up to nine-tenths of the aperture 

 of the objective, with well-stained objects, if possible. 



41. Mount the objects in as close contact with the cover- 

 glass as possible. 



