CHAPTEK III. 



MICROSCOPIC METHODS. 



The Microscope. For ordinary bacteriological work a good 

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

 coarse and fine adjustments, 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 T \r-inch oil immersion, or the lenses 

 of other makers corresponding to these. The oil immersion 

 lens is essential. It is well to have two eye-pieces, 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 margins 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 coloured 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. 1. Hanging-drop 

 Preparations. Micro-organisms may be examined : (1) 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 

 and covered with a cover-glass. 1 It is more usual, however, to 



1 In bacteriological work it is essential that cover-glasses of No. 1 thickness 

 (i.e., *14 mm. thick) should be used, as those of greater thickness are not 

 suitable for a ^-inch lens. 



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