68 



DARK-GROUND ILLUMINATION 



[CH. II 



118. Dark-ground illumination by reflected light. This is the 

 simplest method of getting the appearance of shining or white objects 

 in a dark field. Use a i6-mm. or lower objective. For object use a 

 glass slide with particles of lint, flour or starch, or other powder dusted 

 on the slide. Put a piece of black velvet, or other dull black surface, 

 over the opening in the stage and then put the slide in position. Place 

 the microscope near a well-lighted window or use artificial light and 

 let it shine on the slide. If now one looks into the microscope, the 

 particles of dust on the slide will appear bright and the field will be 

 dark. In the subsequent experiments ( 119, 121) remember that 

 the appearance may be due to light falling on top of the specimen, 

 as here, and not that passing up from below. The top light can 

 be eliminated by shading the stage with the hand or a black card. 



119. Dark-ground illumination 

 with light from below the stage. 

 Swing the substage aside, use a i6-mm. 

 or lower objective. For object dust 

 some starch grains on a glass slide. 

 Swing the mirror far to the side and 

 Lam W direct the ^ght very obliquely upon 

 FIG. 4 6a. DARK-FIELD Iixum- the object. If the light is sufficiently 

 NATION BY LIGHT FROM BELOW THE oblique the grains will shine, as if self- 



luminous in a dark field. Use central 

 soIbX'S^ofUct'^r Hght and the grains will be dark on a 



the objective directly, and the ob- white field. 



deflected rays are indicated by (fig. 46a) Read's original experiment 



can be tried with striking resu lts. 



broken lines. 



DARK-GROUND ILLUMINATION WITH THE CONDENSER 



120. Put the condenser back in place and close up under the 

 stage. As with the mirror alone the light striking the object must 

 be so oblique to the axis of the microscope that it will get into the 

 objective only when reflected or refracted by the object. This may 

 be accomplished in two ways. 



121. By lighting from one side as with the mirror only. Use 

 a i6-mm. or lower objective; for object, a glass slide with dust particles. 



