The base, B, is a thin glass-slide about 2x3 inches in size. The sides, S, consist 

 of strips of plate-glass about 2 inches long and J inch wide, and of a height determined 

 by the available condenser, usually 3/8 inch. One end, E, of the chamber is closed with 

 a strip of glass of the same height as the sides and backed by another strip a fraction 

 of an inch higher. The trough of the chamber should be at least 7/8 of an inch wide. 

 Wet strips of blotting paper should be placed along the sides of the trough. This moist- 

 chamber is designed for cover slips of a size 22 x 40 mm. The cover slip, C, is sealed 

 on the chamber 7.-ith a thin layer of vaseline. 



The moist-chamber is open at one end to permit the entrance of microneedles or pipettes 

 for operating the tissue or cells which are suspended in hanging drops from the under 

 surface of the cover slip. See figure 2, y. 



MECHANICAL STAGE . A mechanical stage is used to clamp and move the moist chamber. Any 

 make may be used provided it be sturdy and accurate, and provided it racks back suf- 

 ficiently to permit complete clearance of the condenser lens by the open end of the 

 moist chamber. 



LENS SYSTEM . A. "Split" Condenser. (Three lens condenser with top lens removed). The 

 reason for Splitting the condenser is that operating in the hanging drop suspended from 

 the cover slip necessitates raising the focal point from the Stage level to the under 

 side of the cover slip. Therefore, the complete three lens condenser ordinarily supplied 

 would focus the light too far beneath the operating field; and the intensity of light 

 would therefore be insufficient. Hence the top lens is removed, giving a floodlight of 

 increased brilliance. This "split" condenser is raised until the top lens is flush with 

 the upper surface of the microscope stage. The large diameter of this "split" condenser 

 lens is the reason why a larger aperture in the microscope stage is needed. 



B. Objectives and Oculars. The objectives and oculars used, will, of 

 course, vary with the magnification desired. In general the rule is: for best illumina- 

 tion use the lowest ocular with the highest objective. For very low power work, such as 

 rapidly searching a field, etc., the #lb Leitz objective and lOx ocular are a serviceable 

 combination. A somewhat higher and very workable combination is a #3b Leitz objective 

 and a lOx ocular. For still higher power a #5 Leitz objective may be used. This #5 

 objective furnishes considerable depth of focus for its degree of magnification. High 

 dry and oil immersion objectives may also be employed when desired; and the magnification 

 may be further increased by using oculars of the highest power. The Leitz 8x Peri plan 

 ocular is recommended for critical definition of cell constituents. 



LIGHTING SYSTEM . It is very desirable that the lighting equipment of a laboratory be 

 uniform. This is especially necessary when color work, such as pH determination is being 

 done by a group. A simple, yet serviceable, lighting unit, (fig. 3) may be constructed by 



