CHAPTER IV 



MICROSCOPIC EXAMINATION AND STAINING OF 



BACTERIA 



The Microscope. In order to study the structure and move- 

 ments of individual bacteria a good microscope is essential (Fig. 

 12). A complete instrument generally has four oculars or eye pieces 

 A numbered from 1 to 4. Number one gives the lowest magnifi- 

 cation and number four the highest. At the lower end of the 

 tube B there are usually three objectives C attached to a revolving 

 nosepiece; the objectives give the main magnification. For 

 the examination of groups of bacteria growing together in solid 

 media, when one wishes to see only the general appearance of the 

 assemblage, the lowest magnification is used, i.e. ocular 1 or 2 and 

 objective 4. For unstained preparations, when motility or serum 

 reaction is to be studied, ocular 2 or 3 and objective 7 is employed. 

 When the finer structures of individual organisms are to be noted 

 in stained preparations, ocular 4 and the oil immersion objective 

 T2 should be used; the latter combination gives a magnification 

 of about one thousand diameters. When the oil immersion lens 

 is employed a small drop of oil of the same index of refraction as 

 the glass lens (cedar oil is generally used) is placed on the prepara- 

 tion to be examined. The tube is lowered until the objective is 

 connected with the slide by means of the oil, thus all the rays of 

 light are held together and pass into the tube without the loss 

 through deflection which occurs when air fills the intervening space 

 between the slide and the dry objective. After using, the oil 

 should be gently wiped from the lens with Japanese lens paper 

 or a clean soft linen handkerchief. If absolutely necessary a little 

 zylol may be used but never alcohol ; the latter dissolves the ma- 

 terial by means of which the lens is fixed in its metal container. 



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