CH. H] DARK-GROUND ILLUMINATION WITH HIGH POWERS 75 



when it is in the focus of the condenser, one should know how far 

 above the condenser the focus is formed, and select slides of a thick- 

 ness that will allow the hollow light cone to focus on the objects 

 mounted on the upper face of the slide. In the original dark-field 

 condensers, 1856-1877, and in the more recent ones now manufactured, 

 the focal distance is carefully planned, and the manufacturers state 

 on the instruments the 

 thickness of slide that should 

 be used with it. This thick- 

 ness varies with different 

 manufacturers, ranging from 

 0.95 mm. to 1.55 mm. It is 

 a good plan to select several 

 of the proper thickness and 

 mark the thickness with a 

 writing diamond on the end, 

 then one can proceed with 



(Magnified two diameters.) 



FIG. 5oa. UPPER FACE OF THE FULLY 

 LIGHTED PARABOLOID; (A) WITH CLEAN 

 FACE, (B) WITH STARCH GRAINS DUSTED 

 UPON IT. 



confidence. 



Cover-glasses should be of 



the thickness designated by the makers for dry objectives, but for 

 oil immersion objectives the thickness does not matter provided 

 they are not so thick that they exceed the working distance of the 

 objective ( 76-81). 



The tube-length of the microscope for all objectives should be 

 adjusted strictly in accordance with the length given by the manu- 

 facturers, as the aberrations are more evident and troublesome in 

 dark-field than in bright-field microscopy. 



128. Lighting for dark-field microscopy. As the objects 

 must be seen by the light they deflect into the microscope, and not 

 by the light directly from the source, it is evident that the original 

 light must be very brilliant for only a small fraction of it is finally 

 utilized for rendering the object visible. From the earliest use of 

 the dark-field method direct sunlight was found the most satisfactory 

 but as direct sunlight is available only a part of the time in the most 

 favored regions, and is constantly shifting when available, artificial 

 light is mostly used. Of all the artificial lights available, the arc 



