3° 



BACTERIOLOGY 



which can be raised or lowered for focusing. The objectives 

 should be three in number, and should be attached to the body by- 

 means of a triple nose-piece, which permits any objective to be 

 turned into the optical axis at will. The eye-piece slips into the 

 upper and opposite end of the body or tube. The arrangements 

 for focusing consist of a rack and pinion, which accomplish the 

 coarse adjustment, and a more delicate fine adjustment. The 

 stage, upon which the objects to be examined are placed, has an 

 o'pening in the middle. In this opening an iris diaphragm and 

 Abbe condenser are inserted. The iris diaphragm enables one 

 to alter the size of the opening as desired. Beneath the stage is a 

 movable 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 according to their focal 

 distances, as % inch, 3^ inch, ^^ 

 inch, etc. In bacteriological 

 work a rather "low power" % 

 or ^ inch objective, an ordinary 

 "high power" 3^ to 3^ inch dry 

 objective, and a high power }{2 inch oil-immersion objective are 

 needed. The magnification with the % or K inch objective is 

 about 75 to loo diameters; with the }i to }i inch 400 to 700 

 diameters; with the K2 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 i^ 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-im- 

 mersion objective is used in the examination of bacteria where a 

 very high power is desired. A layer of thickened oil of cedar- 



FlG. 19. — Abb6 Condenser. On the 

 right si de the figure gives a sectional 

 view. 



