METHODS OF COLLOID RESEARCH 127 



staining. In the description of the investigations of N. GAIDUKOW, 

 E. RAHLMANN, etc., we shall return to this topic. The optical system 

 depends on the fact that the central rays of light reflected from the 

 mirror of the microscope are cut out by a disc, whereas the lateral 

 rays which strike the object obliquely are utilized. 



The simplest and cheapest arrangement is the one by which a 

 central blind is placed in the diaphragm carrier of the ABBE illumi- 

 nating apparatus (see Fig. 35), yet this arrangement is not recom- 

 mended on account of the faint illumination and the difficulty in 

 centering. 



Much to be preferred, because of the strong illumination, is SIEDEN- 

 TOPF'S paraboloid condenser (see Fig. 36). It is adapted to the study 

 of living bacteria and especially for thin organized structures. 



The thicker the preparation the weaker must be the objective. 



Preparations must be made with greater cleanliness than for bright field 

 illumination, though such scrupulous care is not necessary as for the cardioid 

 condenser. 



The slide must have a definite thickness (not less than 1.1 mm. or more than 

 1.4 mm.). 



The object to be studied is placed on a slide moistened with a drop of physio- 

 logical salt solution and a cover glass adjusted so that there are no bubbles. The 

 water pressed out at the sides is absorbed and the rim is sealed tight with wax 

 (1 part wax, 2 parts rosin). A drop of water without bubbles is placed between 

 the slide and the condenser. Neither water nor oil is used between the slide 

 and the objective (dry system). 



[ Other ultramicroscopes have been devised by COTTON and MOUTON, and by 

 IVANOWSKI (made by E. LEITZ) . See also L' Ultra-microscope by PAUL GASTON, 

 Paris, 1910. Tr.] 



