68 DARK-GROUND ILLUMINATION CCh. H 



§ 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 sUde in position. Place 

 the microscope near a well-Ughted window 'or use artificial hght and 

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

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

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

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

 as here, and ncft that passing up from below. The top hght can 

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

 card. 



§ 119. Dark-ground illumination with transmitted Ught. — Lower 

 the condenser if one is present and swing it out to the side, or remove 

 it and its mounting entirely, so that only the stage with its large open- 

 ing remains. 



Use a 16 mm. or lower objective. For object take a microscopic 

 slide which has been standing face up for some time. It wiU be cov- 

 ered with fine dust. Or one can add a Uttle dust, flour, starch, or other 

 fine powder. Swing the mirror bar to the side away from the hght, 

 and then turn the mirror so that the hght will strike the object very 

 obliquely to the optic axis of the microscope (fig. 35). If the light is 

 sufficiently oblique the dust particles will appear as shining specks on 

 a dark background. Compare the appearance with that given by 

 central light. Swing the mirror bar back in a vertical position, and 

 reflect the hght directly upward. Now the particles will appear black 

 on a white field. 



Dark-ground Illtimination with the Condenser 

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

 stage.' As with the mirror alone the hght 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 accompHshed in two ways. 



