Ch. n] DARK GROUND ILLUMINATION WITH HIGH POWERS 73, 



objective and a sx ocular for centering the condenser. For actual 

 study the lox ocular should be used also. 



Focus down until the top of the condenser appears and one or two 

 circles will be seen on the top of the condenser. If these circles are 

 not in the center of the field, use the centering screws and get them in 

 the center, that is, so that there will be an 

 equal margin of the field all around. 



§ 127. Slide, cover-glass, and specimen. 

 — Select a perfect and clean sUde of the 

 thickness required by the special condenser 

 used. This varies with different makes 

 from I to 1.5 mm. The thickness can be 

 measured by a fine rule or by the cover- 

 glass measurer (Ch. X). The reason for 

 having the slide of a definite thickness 

 that each condenser is designed to bring its 

 very obHque Ught to a focus a definite dis- 

 tance above its face, and unless the slide is 

 of the correct thickness the object will be 

 too high or too low and thus be inade- 

 quately lighted. 



It is best also to use a cover-glass of the 

 correct thickness for the objective used (see 

 Ch. IX). One 0.15 mm. thick wiU answer 

 for almost all objectives. 



For object use fresh unstained things like 

 ciliated epithelium from the frog's mouth, 

 fresh human or animal blood, sahva with 

 some of the substance scraped from the teeth of some person, or some 

 of the water and scrapings from the surface of the hay in a hay in- 

 fusion (§ 211). The purpose in all of these preparations is to show 

 fresh hving things which move, and the structural details without 

 staining, and in still living substance (see Ch. X for mounting). 



Put on a cover-glass (fig. 70). Put a drop of homogeneous liquid 

 on the top of the cover, and another drop on the slide immediately 

 below or opposite the cover. Place the slide under the microscope. 



1 



Fig. 50. Condenser 

 FOR Dark-ground Il- 

 lumination. 



(Bausch & Lomb, from 

 Chamot). 



The illuminating beam 

 is composed of parallel 

 light, the central part 

 being stopped out by thfr 

 stop (C). 



The slide must be of a 

 thickness to admit of the 

 focus on the objective just 

 above it. 



The condenser must be 

 joined to the slide by 

 water or homogeneous im- 

 mersion fluid (see under 

 Critical Angle). 



