16 



3. SPEC I AX DARK FIELD CONDENSER AMD MOIST CHAMBER. 



A new dark field condenser (fig. 11 a) has been especially designed for use with 

 the Chambers micromanipulator. This condenser is of the bicentric type. It has 

 a focal length of 10.7 mm and a working distance of 14.0 mm. The range of aper- 

 ture of the illumination rays is from 0,85 mm to 0.99 mm, a remarkably large 

 aperture, necessary for use with oil immersion objectives. The condenser has a 

 diameter of 49 mm, a height of 42.5 mm, and is provided with lateral slots 15.5 

 mm wide and 10 mm high to allow ample room for the vertical and horizontal move- 

 ments of the microneedles. A depression in the base may be filled with water to 

 prevent evaporation of hanging drops during operations. In order to use this 

 condenser, the opening in the microscope stage must be 50 mm in diameter. 



a 



C&&. 



Fioii 



A special moist chamber for use with this condenser is shown in figure 1*. b. The 

 base is an 80 mm square of glass, with a circular hole cut in the center. This 

 hole is bordered except at two opposite regions, x and y, by two curved glass walls, 

 18.5 mm high. The enclosed chamber is roofed with glass in such a manner as to 

 leave an exposed space across which a cover slip with hanging drops may be laid 

 as shown in the diagram. The walls should be lined with moist cotton or filter 

 paper, and the chamber fitted over the condenser. For the best results a strongly 

 concentrated light should be used and the condenser should be racked up and down 

 to bring to focus the most intense light on the tip of microneedles. Hauser, E.A, 

 1930, Ueber Mikromanipulationen an Latex im Dunkelfeld. Kolloid Zeitschrift, 52, 78. 



