4« LIGHTING AND FOCUSING [CH. II 



§ 88. Mirror and Light for the Abbe Condenser. — It is best to 

 use light with parallel rays. The rays of daylight are practically par- 

 allel ; it is best therelore to employ the plane mirror for all but the 

 lowest powers. If low powers are used the whole field might not be 

 illuminated with the plane mirror when the condenser is close to the 

 object ; furthermore, the image of the window frame, objects outside 

 the building, as trees, etc., would appear with unpleasant distinctness 

 in the field of the microscope. To overcome these defects one can 

 lower the condenser and thus light the object with a diverging cone of 

 light, or use the concave mirror and attain the same end when the con- 

 denser is close to the object (Fig. 48). 



§ 89. Artificial Light. — If one uses lamp light, it is recommend- 

 ed that a large bull's eye be placed in such a position between the 

 light and the mirror that parallel rays fall upon the mirror or in some 

 cases an image of the lamp flame. If one does not have a bull's eye 

 the concave mirror may be used to render the rays less divergent. It 

 may be necessary to lower the illuminator somewhat in order to illum- 

 inate the object in its focus. 



ABBE CONDENSER : EXPERIMENTS 



§ 90. Abbe Condenser, Axial and Oblique Light. — Use a dia- 

 phragm a little larger than the front lens of the 3 mm. (j/£ in.) objec- 

 tive, have the illuminator on the level, or nearly on the level of the 

 upper surface of the stage, and use the plane mirror. Be sure that 

 the diaphragm carrier is in the notch indicating that it is central in 

 position. Use the Pleurosigma as object. Study carefully the appear- 

 ance of the diatom with this central light, then make the diaphragm 

 eccentric so as to light with oblique light (§78). The differences in 

 appearance will probably be even more striking than with the mirror 

 alone. 



§91. Lateral Swaying of the Image. — Frequently in study- 

 ing 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. Now make the light oblique, either by making the diaphragm 

 opening eccentric or if simply a mirror is used, by swinging the mirror 

 sidewise. On focusing up and down, the lines will sway from side to 

 side. What is the direction of apparent movement in focusing down 



