EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 21 



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 with the light in front. 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 sunlight. When artificial 

 illumination is necessary, 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 focus- 

 ing, 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 (o.ooi mm.), which is about a soo'o Q^ ^.n inch. It 

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

 Greek letter i^. For example, S a- = 0.005 mm. = -g^TTr inch. 



The Preparation of Specimens of Bacteria for Exami- 

 nation with the Microscope. — The substance under ex- 

 amination is 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 used constantly in 

 doing bacteriological work. It is the tool by means of which 

 one is able to handle bacteria with impunity. The platinum 



