XI. WHEN TO USE SPECIAL MICKOSCOPES 



359 



design. A simpler, less efficient construction uses a paraboloid mirror 

 and is useful for less critical microscopy. For elementary work a 

 center stop having an opaque region of proper diameter {ca. 17 mm.) 

 can be inserted into the lower part of the microscope condenser. 



B 



C D 



Fig. 6. Comparative cones of light: condenser without 

 (A) and with (B) immersion oil contact to the sHde; (C) ob- 

 lique lighting; (D) darkfield lighting. 



B 



Light from ^Objective 



specime 



Light from 

 condenser 



Blue 



'Objective 



Red 

 Red light from specimenT^-. 3; 

 Blue light from bockgroundXIi^ 



Light 



Fig. 7. Light path through bispheric, darkfield condenser (A) and through 

 condenser with optical staining disc (B). 



More elaborate condensers have means for focusing the light and 

 even changing from bright- to darkfield (59). The darkfield con- 

 denser must have an immersion fluid contact with the under surface 

 of the slide. With high aperture objectives, it is often necessary to 



