46 LIGHTING AND FOCUSING. [CH. II. 



§ 84. 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, therefore, 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 ob- 

 ject ; 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 condenser is 

 close to the object (Fig. 48). 



§ 85. Artificial Light. — If one uses lamplight, it is recommended 

 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 con- 

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

 necessary to lower the illuminator somewhat in order to illuminate the 

 object in its focus. 



ABBE CONDENSER : EXPERIMENTS. 



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

 phragm a little larger than the front lens of the 3 mm. (}i in.) objec- 

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

 per 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 Plenrasig?na as object. Study carefully the appearance of the 

 diatom with this central light, then make the diaphragm eccentric so as 

 to light with oblique light. The differences in appearance will probably 

 be even more striking than with the mirror alone (§ 74). 



§ 87. 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 ec- 

 centric or if simply a mirror is used, by swinging the mirror side wise. 

 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 illuminating ray ? What in focusing up ? If one understands 

 this experiment it may sometimes save a great deal of confusion. (See 

 under testing the microscope for swaying with central light § 104). 



