EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 21 



that more depends upon securing a distinct picture than upon a 

 very high magnification of the object. 



The microscope should be placed in front of the observer 

 on a firm table. The observer should be able to bring the eye 

 easily over the eye-piece when the tube of the microscope is in 

 vertical position. Daylight should be employed if possible, 

 but not direct sunHght. When artificial illumination is, neces- 

 sary, an ordinary lamp, a Welsbach burner or an incandescent 

 electric light may be used. It is best to modify the artificial 

 light by inserting a sheet of blue glass between the light and the 

 mirror. 



In order to focus upon any object, having first secured a 

 satisfactory illumination with the mirror, it is best, beginning 

 with the low power and using the coarse adjustment for focusing, 

 to bring the objective quite close to the object, and then, with 

 the eye in position, to raise the tube until the object comes into 

 focus. The exact focusing is done with the fine adjustment. 

 The observer should keep both eyes open when using the micro- 

 scope, and should be able to use either eye at will. 



All measurements of microscopic objects are expressed in 

 terms of a micromillimeter. This is one-thousandth of a 

 milHmeter (o.ooi mm.), which is about 2TT7ro^ of an inch. It 

 is generally called a micron for short, and is denoted by the 

 Greek letter fi. For example, 5 /^ = 0.005 t^"^- = ttw^ inch. 

 The Preparation of Specimens of Bacteria for Exami- 

 nation with the Microscope. — The substance under ex- 

 amination is placed upon thin shps of glass called cover- 

 glasses. The material is spread over the cover-glass by 

 means of a platinum wire which has been fixed in a glass 

 rod about six inches long. Such a platinum wire is used con- 

 stantly in doing bacteriological work. It is the tool by means of 

 which one is able to handle bacteria with impunity. It serves, 

 in fact, as a kind of additional finger. The platinum wire must 

 be stiff enough not to bend too easily, and yet it should not be 



