METHODS OF COLLOID RESEARCH 



123 



rays in the path of the object examined are lost by refraction. Very 

 small objects, such as bacteria are too faintly illuminated to be 

 visible by his "dry system." On this account, a highly refractive 

 fluid (water or cedar oil) is placed between the object and the objec- 



FIG. 29. Illumination of the cardioid ultramicroscope. 



tive (of wide aperture), which permits many more rays to pass from 

 the object into the objective. It was impossible to use immersion 

 in the earlier slit-ultramicroscope because the illuminating (Bi) and 



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FIG. 30. Course of the light rays 

 through the cardioid condenser. 

 (H. Siedentopf .) 



FIG. 31. Quartz chamber for the 

 cardioid ultramicroscope. 



the examining objective (J5 2 ) could not be brought sufficiently close 

 together (see Fig. 3 la). This difficulty was overcome through an 

 improved method of construction by the optical works of R. WINKEL 

 of GOTTINGEN (see Fig. 30). A drop of the fluid to be examined is 

 placed between the two immersion objectives of wide aperture or 



