34 GENERAL BACTERIOLOGY 



b. Practice making cover-glass preparations by staining speci- 

 mens from each of your cultures. Use Loeffler's methylen blue for 

 the gelatin and bouillon ; aqueous solution of gentian violet for agar, 

 and carbol-fuchsin for potato. Examine, mount permanently and 

 hand to instructor for inspection. 



EXERCISE 19. USE OF MICROSCOPE. 



GENERAL DIRECTIONS. For bacteriological purposes a microscope 

 with a magnifying power of at least 500 diameters is needed. There 

 should be a coarse adjustment (rack and pinion) as well as a fine 

 micrometer screw ; and the following accessories : Two oculars, one 

 1 in. (25 mm.) and one 2 in. (50 mm.) ; three objectives, one 2 in. 

 (16 mm.), one -J- in. (4 mm.), or | in. (3.5 mm.) and one oil 

 immersion -fa i* 1 - or TS" i n - (2 mm.) ; a triple nose-piece, and an 

 Abbe substage condenser with iris diaphragm mounting. 



In the use of the microscope the following points should be noted : 



a. LIGHT. The proper angle at which the mirror should be 

 placed is best determined by removing the ocular and so arranging 

 the mirror that the unobstructed light from the window covers the 

 whole field. The ideal light is that from a white cloud. Direct 

 sunlight should never be used. 



~b. ABBE CONDENSER. The purpose of the condenser is to furnish 

 a large cone of light, and as it is corrected for parallel rays the plane 

 side of the mirror should always be used, except when artificial light 

 is employed. When highly stained objects are to be examined, the 

 open diaphragm should be used, but when the structural rather than 

 the color picture is desired, it will be necessary to diminish the light 

 by closing the diaphragm. When the high powers are employed, 

 raise the condenser as high as possible ; for low powers a lower posi- 

 tion will give better definition. 



c. FOCUSING. Turn the proper objective in place and rack down 

 until the objective nearly touches the cover-glass. This should be 

 done while the eye is held at one side and directs the movement. 

 Then with the eye at the tube slowly move up with the micrometer 

 screw. Never rack down with the eye at the tube. 



d. USE OF OIL-IMMERSION. The oil-immersion objective is indis- 

 pensable to the proper study of bacteria. It is constructed upon the 

 principle that a drop of fluid having the same refractive index as the 



