THE CONDENSER 95 



seiirchiiig can be done well with a corrected immersion 

 bright-field condenser with center stop (45). Filter-passing 

 organisms, flagella on living bacteria, and cilia on diatoms 

 have been demonstrated with these special condensers, 

 which are excellently fitted to show up living bacteria in 

 thin layers of water. A special chamber is useful, with a 

 slide of the proper thickness, having an island so arranged 

 as to hold a definite thickness of liquid (less than 10 microns 

 deep) when the cover-glass is pressed down. In the 

 writer's experience, these condensers can be conveniently 

 centered by first centering a small diaphragm on the source 

 of light by means of the bright-field condenser. Then this 

 spot of light is used to center the dark-field condenser, which 

 has been put in the place of the bright-field condenser, 

 neither mirror nor microscope being moved. A piece of 

 lens paper may be mounted in balsam on a special slide, 

 and used as a screen to catch the image of the source. Since 

 these special dark-field condensers are reflecting (not 

 refracting), they are achromatic. They are also nearly 

 free from spherical aberration. 



The lower limit of aperture of certain specially made 

 double-reflecting condensers has been increased until it 

 exceeds that of the oil-immersion objectives of 1.3 aperture. 

 Such condensers (Cassegrain and Leuchtbild) are useful 

 for viewing small objects mounted in balsam or in immer- 

 sion oil. These condensers must be oil immersed. They 

 require accurate centering, and close attention to the 

 appropriate thickness of the slide and the position of the 

 light. When everything is right, they give remarkably 

 good pictures of lightly stained bacteria in balsam or immer- 

 sion oil. The color of such bacteria is complementary to 

 that of the stain, when white light is used. Thus bacteria 

 stained with gentian violet appear golden-yellow, while 

 those stained with fuchsin show greenish. This is due 

 to the complementary color, to which the dye is opaque, 

 being diffracted (Berek, 38). A 6- volt, 108-watt lamp, 

 with coil or ribbon filament, may well be used with these 

 condensers; the incandescent filament itself being focused 



