32 MANUAL OF BACTERIOLOGY. 



less, and substituting the concave for the plane mirror. 

 It is to be remembered 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 micro- 

 scope is in vertical position. Daylight should be em- 

 ployed if possible. When artificial illumination is neces- 

 sary, an ordinary lamp, a Welsbach burner or an incan- 

 descent 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, begin- 

 ning 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 microscope, 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 millimeter (.001 mm.), which is about . 2 - t ( 1 MM) of an inch. 

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

 the Greek letter ft. For example, 5 IJL == .005 mm. == - (] l Q -$ 

 inch. 



The Preparation of Specimens of Bacteria for Exami- 

 nation with the Microscope. The substance under ex- 

 amination is usually placed upon thin slips 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 



