20 Agricultural Bacteriology. 



Use of the microscope. The microscope of necessity 

 is a tool of vital importance to the bacteriologist. The 

 extreme minuteness of the bacteria require that an in- 

 strument magnifying several hundred diameters be 

 used. Even then the bacteria in an untreated condition 

 are very difficult to see on account of their transpar- 

 ency. In order to make them more easily visible a very 

 small amount of the substance in which they are grow- 

 ing is spread on a thin piece of glass and allowed to 

 become perfectly dry. The organisms are then killed 

 by heating gently and are treated with various stains 

 which impart a bright color, red, blue, or purple as the 

 case may be, to the bacteria. Just as a red glass is seen 

 more easily than a piece of perfect plate glass so the 

 stained bacteria are more easily seen than the unstained 

 forms, and, moreover their size, exact form and other 

 characteristics are more easily determined. 



Appearance of the bacteria under the microscope. 



The appearance of the individual bacterial cell under 

 the microscope can not vary widely because of the limi- 

 tation that the bacteria are one-celled structures. The 

 spherical forms always appear as tiny dots, the rods as 

 dashes, and the spiral forms as dashes or lines more or 

 less curved in various ways. The size varies somewhat, 

 as do the arrangement of the cells with reference to 

 each other, but the microscope rarely enables one to 

 tell the kind of bacteria present in a culture. The ap- 

 pearance of the growth on a large number of media 

 must be noted, as also the changes produced in these 

 different food substances. These serve as aids to the 

 appearance under the microscope in determining the 

 kind of bacteria one has at hand. 



