124 



COLLOIDS IN BIOLOGY AND MEDICINE 



they dip into a small trough containing the fluid for examination. 

 The illumination intensity of the " immersion ultramicroscope " is 

 much greater than the original, and particles are made visible which 

 formerly had quite eluded observation; the contrast effect in the 

 intensely dark field is quite perfect. For biologists, the ultramicro- 

 scope with a cardioid condenser is at present the most important 

 instrument. It permits the use of twenty times as much light as the 

 slit-ultramicroscope, "practically the maximum available from the 

 source of light." 



The construction of the apparatus is shown in Fig. 29. c con- 

 tains an electric arc lamp with a perforated sleeve cap d to cut out 



FIG. 32. Holder for the quartz chamber em- 

 ployed with the cardioid ultramicroscope. 



FIG. 33. Flasks for storing 

 ultrawater. (A. Haak.) 



interfering light. An illuminating lens e passes the light sharply 

 downward, through a glass trough filled with water, to the center of 

 the microscope mirror. 



The water trough serves to remove the heat rays or when neces- 

 sary acts as a color filter. The microscope mirror throws the light 

 perpendicularly through the cardioid condenser, which replaces the 

 ABBE condenser in the microscope. It is evident from the diagram 

 of the cardioid condenser (Fig. 30) that the various ascending rays 

 strike the slide e obliquely by reason of the double reflection from the 

 two spherical surfaces and that thus, all the light is utilized for 

 illumination; only the rays reflected by the object take the usual 

 path through the objective and the ocular to the observer's eye. 

 Water is used for immersion. 



With the cardioid ultramicroscope the object is placed between 

 slide and cover glass as in ordinary microscopy. For reasons we 

 shall revert to later, a slide with a special quartz chamber (Fig. 31) 

 is used, which is held in the holder (Fig. 32). 



