42 AGRICULTURAL BACTERIOLOGY 



that the organisms that one desires to study will develop, 

 and thus the plate-culture method of determining bacteria 

 is of great value in spite of its limitations. 



Microscopic examination. An object having dimensions 

 less than 0.2 micron can not be demonstrated even under a 

 high-powered microscope. Some of the bacteria approach 

 this limit. Indeed, the great majority of the spheres are 

 less than one micron in diameter. It is therefore essential 

 to employ instruments that will give the maximum degree 

 of magnification; but, since magnification is secured only 

 with a reduction in illumination, a practical limitation is 

 placed on the possible increase in the magnifying power of 

 the microscope. 



The two developments of the microscope that greatly ac- 

 celerated the development of bacteriology were concerned 

 with an increased illumination of the object. The first was 

 the immersion objective, which prevents the refraction or 

 bending of the rays of light as they pass from the glass on 

 which the object to be examined is placed, into the air. 

 This is accomplished by placing a drop of liquid having the 

 same index of refraction as glass between the lens and the 

 glass slide. Cedar oil dissolved in xylol is used for this 

 purpose. 



The light rays are reflected from the mirror through the 

 object into the lens of the microscope. The Abbe con- 

 denser is a series of lenses placed beneath the stage of the 

 microscope which concentrates the light rays coming from 

 a certain area of the mirror and focuses them on a much 

 smaller area of the object, thus illuminating it brightly. 



Examination of unstained bacteria. The motility of 

 bacteria can be determined only by examining a liquid that 

 contains the living cells. For this purpose, a drop of the 

 liquid containing the bacteria is placed on a very thin piece 

 of glass, known as a cover-glass, which is inverted over a 



