70 DARK GROUND ILLUMINATION WITH HIGH POWERS [Ch. II 



image of the ring diaphragm will be seen in the air bubbles. The 

 spherical air bubbles act like concave lenses in the surrounding 

 medium of greater refractive power, and by changing the direction 

 of the rays passing through them turn some of them into the 

 objective, hence the appearance. If one uses saliva as liquid, the 

 irregular gray bodies seen are epithelial cells from the mouth. 



(2) Objects having a greater refractive power than the surround- 

 ing medium: Most objects studied belong to this group. For object 

 use milk diluted four or five times with water or make an oil-globule 

 preparation by beating in a large drop of water a small drop of oil. 

 The oil globules are more refracting than the surrounding Uquid, hence 

 will act as convex lenses. But the difference in refraction is not so 

 great as with air and water ; hence the dark center wiU be wider and 

 the bright ring narrower. One can also see the image of the ring 

 diaphragm, but the central stop is relatively larger than with the air 

 bubbles. 



§ 124. Infusoria with dark-ground illumination. — A very striking 

 and instructive preparation for dark-ground illumination may be 

 made by taking water well supplied with living infusoria and other 

 micro-organisms as object. These, under the microscope when prop- 

 erly lighted as indicated above (§ 123), appear like shining creatures 

 swimming in black ink (§ 211). 



Dark Ground Illumination with High Powers 

 § 125. For this a special condenser which gives a very oblique 

 beam is required. The aperture of the objective must be rather 

 low also; that is, less than i.oo. A numerical aperture not to exceed 

 0.95 will be found most satisfactory. For oil immersion objectives 

 with their high numerical aperture (i. 20-1 .40) a diaphragm must be 

 employed to reduce the aperture to less than i.oo (fig. 48). The 

 proper diaphragm is inserted by the manufacturers. With some 

 objectives the diaphragm is easily inserted and removed by the ob- 

 server, depending on his needs, but with some objectives only the 

 manufacturer should insert and remove the reducing diaphragm. 

 This involves either having a special oil immersion objective for dark- 

 ground work, or being subjected to much inconvenience to have the 



