164 A MANUAL OF BACTERIOLOGY 



two carbons. By this arrangement the positive crater 

 maintains a constant position. 



Small metallic filament lamps, run from a storage 

 battery, and attached directly under the condenser, can 

 be obtained from Messrs. Hearson. 



The most satisfactory illuminant is the " Pointolite " 

 lamp already mentioned (p. 159). For use with gas the 

 Barnard incandescent lamp, in which the mantle is made 

 into a roll, is very suitable and is supplied by Messrs. 

 Hawkesley, of Oxford Street, London. An inverted 

 incandescent gas mantle lamp may be used with the 

 paraboloidal type only. An acetylene bicycle lamp may 

 also be employed. If we can choose our illuminant, we 

 should select as the best the " Pointolite," next the Barnard 

 incandescent gas lamp with rolled mantle. 



Objectives. A variety of objectives may be used ; an 

 oil-immersion one is not essential. Any of the following 

 may be employed 



8-mm. apochromatic dry. 



4-mm. apochromatic dry with correction collar. 



3-mm. apochromatic oil-immersion. 



2-mm. apochromatic oil-immersion. 



J-in. achromatic dry. 



J-in. achromatic dry. 



^-in. Zeiss achromatic oil-immersion. 



^-in. achromatic oil-immersion. 



No objective of indifferent quality is satisfactory for 

 dark-ground illumination, as any defects tend to be 

 accentuated, and the ordinary J-in. is usually unsatis- 

 factory. 



An important point is the numerical aperture of the 

 lens ; this must not exceed 0-95. The N.A. of the 4-mm, 

 apochromat is, therefore, the utmost permissible, and 

 even this may have to be reduced with many dark -ground 

 condensers. As all oil-immersions have a N.A. of 1-25 or 



