128 DARK-GROUND ILLUMINATION 



certain amount of diffused light, but this soon gives place to complete dark- 

 ness ; by continuing carefully to lower the tube, the back-ground will suddenly 

 become lit up in places and dotted with bright points ; the preparation is 

 then focussed. 



2. With an oil-immersion lens. Place a drop of cedar-wood oil on the 

 cover-glass and lower the tube until the lens touches the oil. Then with the 

 mechanical adjustment gently raise and lower the tube until the back-ground 

 is illuminated with bright spots. 



If the field be unequally lighted or if it be narrowed by shadows, the 

 centering is at fault and must be corrected by careful manipulation of the 

 dark-ground condenser (p. 126). 



E. Appearances seen in the field under dark-ground illumination. 



When the lighting and centering are satisfactory, and the object focussed, 

 luminous points and spots of different appearances motile or non-motile 

 will be seen corresponding to the microscopical objects (micro-organisms, 

 cells, particles of colloid matter, etc.) in the preparation. Certain non-motile 



FlG. 116. Preparation showing spirochaetes, leucocytes and red cells (after Gastou). 



spots, generally taking the form of rosettes or flocculent masses, may be 

 seen ; these are merely flaws in the glass and must not be confused with 

 the objects in the preparation. [This generalization of course only applies 

 when the size of the objects is greater than the resolving power of the com- 

 bination of lenses employed. Any objects in the field which are beyond the 

 resolving power of the combination of lenses will appear as bright spots 

 with light and dark diffraction rings and the size of the objects which will 

 appear as such will depend upon the intensity of the illumination. It has 

 already been pointed out that the so-called ultra-microscope or dark-ground 

 illuminator does not increase the resolving power of the microscope, hence 

 whatever the shape of the object if it be so small as to be below the resolving 

 power of the system of lenses used it will appear as a bright dot surrounded 

 by rings.] 



It will be found easy to study the movements (Brownian movements, move- 

 ments of propulsion, etc.) of the different corpuscles. In interpreting these 

 it must not be forgotten that an universal movement of the illuminated 



