CHAPTER III. 



MICROSCOPIC METHODS — GENERAL BACTERIOLOGICAL 

 DIAGNOSIS — INOCULATION OF ANIMALS. 



The Microscope. — For ordinary bacteriological work a good 

 microscope is essential. It ought to have a heavy stand, with 

 rack and pinion and fine adjustment, a double mirror (flat on 

 one side, concave on the other), a good condenser, with an iris 

 diaphragm, and a triple nose-piece. It is best to have three 

 objectives, either Zeiss A, D, and -^^ inch oil immersion, or the 

 lenses of other makers corresponding to these. The oil immer- 

 sion lens is essential. It is well to have two eyepieces, say 

 Nos. 2 and 4 of Zeiss or lenses of corresponding strengths. 

 The student must be thoroughly familiar with the focussing 

 of the light on the lens by means of the condenser, and also 

 with the use of the immersion lens. It may here be remarked 

 "that when it is desired to bring out in sharp relief the mar- 

 gins of unstained objects, e.g. living bacteria in a fluid, a 

 narrow aperture of the diaphragm should be used, whereas, 

 in the case of stained bacteria, when a pure colour picture is 

 desired, the diaphragm ought to be widely opened. The flat 

 side of the mirror ought to be used along with the condenser. 

 When the observer has finished for the time being with the 

 immersion lens he ought to wipe off the oil with a piece of 

 silk or very fine washed linen. If the oil has dried on the 

 lens it may be moistened with xylol — never with alcohol, 

 which will dissolve the material by which the lens is fixed in 

 its metal carrier. ' 



Microscopic Examination of Bacteria, i. Hanging-drop Pre- 

 parations. — Micro-organisms may be examined: (i) alive or 

 dead in fluids; (2) in film preparations; (3) in sections of 

 tissues. In the two last cases advantage is always taken of the 

 affinity of bacteria for certain stains. When they are to be 

 examined in fluids a drop of the liquid may be placed on a slide 



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