MICROSCOPIC METHODS 77 



in thickness twelve hours' immersion is sufficient, if larger twenty-four 

 hours is necessary. They should then be placed in pieces of gauze 

 and left in running water for from twelve to twenty-four hours, accord- 

 ing to the size of the pieces, to wash out the excess of sublimate. They 

 are then placed for twenty-four hours in each of the following strengths 

 of methylated spirit (free from naphtha): 30 per cent., 60 per cent., 

 and 90 per cent. Finally they are placed in absolute alcohol for twenty- 

 four hours and are then ready to be prepared for cutting according 

 to the usual histological methods. The paraffin sections of tissue having 

 been prepared and cut, they are ready for staining. t 



LOKFFLKR'S METHOD. The section is placed in Loeffler's alkaline 

 methylene-blue solution for 5 to 30 minutes, then placed for a few sec- 

 onds in 1 per cent, acetic acid. It is then placed in absolute alcohol, 

 xylol, and Canada balsam. The number of seconds during which the 

 preparation remains in the acetic acid must be tested by trials. The 

 bacteria are dark blue, the nuclei blue, and the cell bodies light 

 blue. 



Thionin solution, carbol-fuchsin solution, and gentian violet can be 

 used instead of Loeffler's methylene blue. Gram's method, with 3 

 per cent, hydrochloric acid in alcohol as a tissue decolorizer for ten 

 seconds, is also valuable. 



Preservation of Specimens. Dry unstained or stained preparations 

 of bacteria keep indefinitely, but if mounted in Canada balsam, cedar 

 oil, or dammar lac they tend to gradually fade, but may be preserved 

 for many months or vears. 



THE MICROSCOPE AND THE MICROSCOPIC EXAMINATION 

 OF BACTERIA. 



Different Parts of the Microscope. A complete instrument usually 

 has four oculars, or eye-pieces (^4), which are numbered from 1 to 4, 

 according to the amount of magnification which they yield. Numbers 

 2 and 4 are most useful for bacteriological work. The objective the 

 lens at the distal end of the barrel serves to give the main magnifica- 

 tion of the object. For stained bacteria the y 1 ^ achromatic oil-immer- 

 sion lens is regularly employed; except for photographic purposes the 

 apochromatic lenses are not needed. Even here they are not indis- 

 pensable. A T% may at times be useful, but hardly necessary; a No. 

 4 ocular and a -^ lens give a magnification of about 1000 diameters 

 (Fig. 55). For unstained bacteria we employ either the T V immersion 

 or y dry lens, according to the purpose for which we study the bac- 

 teria; for the examination of colonies where, as a rule, we do not wish 

 to see individual bacteria, but only the general appearance of whole 

 groups, we use lenses of much lower magnification (Fig. 56). 



The stage C the platform upon which the object rests should be 

 large enough to support the Petri plates if culture work is to be done. 

 The iris diaphragm D, which is now regularly used in bacteriological 

 work, opens and closes like the iris of the eye, and so controls the 



