66 EXPERIMENTS WITH A SUBSTAGE CONDENSER [Cn. II 



the condenser is centered, fill the aperture of the objective about 

 3/4 full of light ( no). Study the preparation with the central 

 light and note the appearance of the markings. Cover a part of the 

 diaphragm opening by putting the finger or some other opaque object 

 between it and the mirror (fig. 46). Note that the markings come out 

 more strongly. Hold the finger in position and open the diaphragm 

 widely and see if the markings can still be made out. Now remove 

 the finger so that the object is lighted by the full aperture of central 

 light. Probably the markings will not appear at all. Put the finger 



Objectly* 7? Bach of C Objective 



FIG. 45. APERTURE OF THE SUBSTAGE CONDENSER AND OF THE OBJECTIVE. 

 (From Nelson, Jour. Roy. Micr. Soc.). 



A The cone of light from the condenser fills the aperture of the objective (B) 

 D The cone of light of the condenser only partly fills the aperture of the 



objective (C). 



In A and D the condenser and objective are shown in section; in B and C, 



the back lens of the objective is shown in face view as when looking down upon 



it. with the ocular removed. 



back in position to give oblique light and the markings will again be 

 seen. Remove the finger and slowly close up the diaphragm. When 

 the proper aperture is reached the markings will again appear. 



For histological preparations the oblique light is not a help in bring- 

 ing out details of structure. There the end is reached by using the 

 proper aperture, regulating the source of light, and by differential 

 staining (Chs.X-XI). 



116. Lateral swaying of the image. Frequently in studying 

 an object, especially with a high power, it will appear to sway from 

 side to side in focusing up or down. A glass stage micrometer or fly's 

 wing is an excellent object. Make the light central or axial and focus 

 up and down and notice that the lines simply disappear or grow dim. 



