30 BACTERIOLOGY 



ing consist of a rack and pinion which accomplish the coarse ad- 

 justment, and a more delicate fine adjustment. The stage, upon 

 which the objects to be examined are placed, has an opening in 

 the middle. In this opening an iris diaphragm and Abbe con- 

 denser are inserted. The iris diaphragm enables one to alter 

 the size of the opening as desired. Beneath the stage is a mov- 

 able mirror, of which one side is plane and the other concave. 

 All of these parts are supported on a short, heavy pillar which 

 is fixed in the horseshoe-shaped base. 



The essential parts of the microscope are, of course, the eye- 

 piece (German, Ocular), and the 

 objective. Objectives are given 

 various names by different makers, 

 for instance, A, B, C, etc., or i, 2, 

 3, etc.; or they are named accord- 

 ing to their focal distances, as f 



FIG. 19. inch, J inch, f inch, etc. In bac- 



Abbe Condenser. On the right side the, . , . , 



figure gives a sectional view. tenological work a rather low 



power" | or f inch objective, an 



ordinary "high power" J to f inch dry objective, and a high power 

 -jV inch oil-immersion objective are needed. The magnification 

 with the f or f inch objective is about 75 to 100 diameters; with 

 the J to | inch 400 to 700 diameters; with the yV immersion 

 750 to 1,000 diameters. The magnification varies according to 

 the eye-piece used, as well as with the objective. A i inch and 

 if inch eye-piece (Leitz No. 2 and No. 4) serve well for most 

 purposes. The eye-pieces are Usually named arbitrarily, like the 

 objectives. The oil-immersion objective is used in the exami- 

 nation of bacteria where a very high power is desired. A layer 

 of thickened oil of cedar-wood is placed between the lower sur- 

 face of the objective and the upper surface of the glass covering 

 the object under examination. The oil must be wiped away 

 from the surface of the objective when the examination is finished. 

 For this purpose the soft paper sold by dealers in microscopical 

 apparatus serves admirably. Care must be taken not to scratch 



