5 6 LIGHTING AND FOCUSING [ CH. II 



§ ioo. Artificial Light. — If one uses lamp light, it is recom- 

 mended 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 condenser somewhat 

 in order to illuminate the object in its focus. 



ABBE CONDENSER : EXPERIMENTS 



§ ioi. Abbe Condenser, Axial and Oblique Light. — Use a 

 diaphragm a little larger than the front lens of the 3 mm. (}4 in) 

 objective, 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 care- 

 fully the appearance of the diatom with this central light, then 

 make the diaphragm eccentric so as to light with oblique light 

 (§ 89). The differences in appearance will probably be even more 

 striking than with the mirror alone. 



§ 102. 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. 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 with reference to the illumi- 

 nating ray ? What in focusing up ? If one understands the experi- 

 ment it may sometimes save a great deal of confusion. (See under 

 testing the microscope for swaying with central light § 130.) 



§ 103. Dark-Ground Illumination. — When an object is 

 lighted with rays of a greater obliquity than can get into the front 

 lens of the objective, the field will appear dark (Fig. 57). If now 

 the object is composed of fine particles, or is semi-transparent, it 

 will refract or reflect the light which meets it, in such a way that a 



