CH. II] DARK-GROUND ILLUMINATION WITH HIGH POWERS 71 



will shine like globules of silver in a dark field. By focusing care- 

 fully the 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 objec- 

 tive, 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 liquid, 

 hence will act as convex lenses. But the difference in refraction is 

 not so great as with air and water; hence the dark center will 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. As was pointed out by Wenham 1850-1856, re- 

 fracting condensers, like those considered above ( 100-113), are not 

 so well adapted for obtaining a suitable hollow cone of light for dark- 

 field work as a reflecting condenser. This is due to the difficulty 

 in getting rid of the spherical and chromatic aberration in the re- 

 fracted rays of light at such great apertures. In the best reflecting 

 condensers the aperture of the rays in the hollow cone of light ranges 

 from i. oo N.A. for the lower limit (fig. 48) to 1.40 N.A. for the upper 

 limit. The rays at an aperture below i.oo N.A. are cut out by a 

 central stop (fig. 48). 



