PART I. 



CHAPTER I. 



EXAMINATION OF BACTERIA WITH THE MICROSCOPE, 

 INCLUDING METHODS OF STAINING. 



The Microscope. The microscope consists of a tubular 

 body which carries the optical parts, and which can be raised or 

 lowered for focusing. It is a matter of convenience to have 

 three lenses attached to the body of the instrument by means 

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

 into the optical axis at will. But a low-power dry lens and an 

 oil-immersion objective are all that are essential for studying 

 the bacteria. The eye-piece slips into the upper and opposite 

 end of the body or tube. The arrangements for focusing con- 

 sist of a rack and pinion which accomplish the coarse adjust- 

 ment, '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 

 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 horseshoes-shaped base. 



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

 piece (German, Ocular) and the objective. Objectives are 

 variously designated by different makers, for instance, some 

 use letters, A, B, C, etc., others use numbers, i, 2, 3, etc. 

 others again give the focal distance, as f inch, J inch, J inch, 



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