CHAPTER 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 advisable to have three objectives, 

 ordinary low and high powers, and a -J^-inch oil immersion, 

 which is essential. It is well to have two eye-pieces. The 

 student must be thoroughly familiar with the focusing 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 

 employ hanging-drop preparations. The technique of making 



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

 {i.e., '14 mm. thUk) should he used, as those of greater thickness are not 

 suitable for a ^-inch lens. 



