94 



THE USE OF THE MICROSCOPE 



achromatic condenser of 1.3 aperture. Thus an oil- 

 immersion objective of that aperture can be used for dark 

 field. Water immersion is most convenient for this con- 

 denser. Such a condenser is superior to the special 

 reflecting condensers, in that it is already centered; it can 

 be used through a greater range of shde thicknesses; the 

 change from bright field to dark field is made immediately 

 by inserting the stop; and this condenser concentrates 

 more light on the object than do the special reflecting 

 condensers, all of which have a smaller range of aperture. 



Fig. 21. — Sectional views of the ciirdioid (Zeiss) and bicentric (Leitz) con- 

 densers. Note that the light is totally reflected, first at a convex, and secondly 

 at a concave surface. The black part represents an air space. 



The bicentric condenser, the cardioid condenser (Fig. 21), 

 and similar double-reflecting condensers made by all 

 optical firms, give an excellent dark field up to objective 

 apertures of 0.85 (paraboloid), 1.05 or higher (cardioid), 

 and 1.15 or more (bicentric). They are used mainly on 

 objects in watery media, and are all water or oil immersed. 

 The condensers with large lenses are to be preferred to those 

 with small lenses, for the former do not require such exact 

 centering. These dark-field condensers demand, however, 

 a special slide thickness ; and give the best results only with 

 special oil-immersion objectives, usually of about 60 or 

 more magnification, and 0.85 to 1.15 aperture. (Doubtless 

 water-immersion objectives could be used advantageously.) 

 For apertures of 0.85 to 0.95 an oil-immersion objective 

 is preferable to a dry objective, because there are less 

 errors from wrong cover-glass thickness, and more light. 

 Such double-reflecting condensers are used in searching for 

 delicate spirochsetes. Coles, however, has shown that such 



