156 THE USE OF THE MICROSCOPE 



18. Time may be saved by using a 50 or 60 oil-immersion 

 objective of low aperture and longer working distance, 

 where the full power of the 90 or 100 of 1.3 aperture is not 

 necessary. 



19. Time is saved by the use of the binocular microscope 

 instead of the monocular for long-continued work. 



20. Covering the microscope (like covering the type- 

 writer) saves time in cleaning, dusting and repairing. 



Necessary Apparatus for Optimum Work. 



1. A binocular microscope, or a monocular with binocular 

 eyepiece attachment, is needed for long-continued work. 



2. A powerful electric lamp with a disc of ground glass 

 and a circular diaphragm of about 3 millimeters (for high 

 objectives) is required. A Pyrex low-voltage lamp with 

 tungsten ribbon is good. 



3. Yellow-green screens (Wratten), such as Nos. 66, 56, 

 and 57A are required on the binocular. 



4. A 25- or 30-millimeter silvered reflecting prism is 

 useful instead of a plane mirror. 



5. A water-immersion aplanatic achromatic condenser 

 of 8.5 to 12 millimeters focal length and 1.25 true aperture, 

 corrected for lamp at 25 centimeters and slides 1.0 miUi- 

 meter thick, is essential. A similar dry condenser is suited 

 for routine work. 



6. A stock of cover-glasses, 0.16 to 0.17 millimeter thick, 

 is much to be desired; and slides 1.0 millimeter thick must 

 be used for optimum results. 



7. Necessary achromatic objectives are the 10 of 0.3 

 or less aperture (or, better, the corresponding apochro- 

 matic) and the 100 fluorite; as well as, in some cases, the 90 

 water-immersion objective of 1.2 aperture, with collar. 

 A 12.5 pair of compensating eyepieces is best with these 

 two high-power objectives. Objectives 20 of about 0.4 

 aperture, and 50 oil-immersion of nearly 1.0 aperture, are 

 useful for routine work. 



8. Apochromatic objectives regarded by the writer as 

 best are the 10 of 0.3 aperture and the 60 oil-immersion 

 of 1.3 (or 1.4) aperture used with compensating eyepieces 



