108 AGRICULTUBAL AND INDUSTEIAL BACTERIOLOGY 



cover glass. Evidently the light entering the lens is more 

 concentrated when oil is used. This results in a more 

 brilliantly illuminated field and more satisfactory observa- 

 tion. 



Microscopic Observation of Colonies and Cultures. — 

 Bacteria, yeasts, or molds, growing on agar or gelatin plates 

 are advantageously observed under the low power of the 

 microscope, either by inverting the plate and examining the 

 colony through the medium, or by removing the cover and 

 observing directly. This is particularly advantageous in 

 studying molds. Frequently the arrangement of spores on 

 conidiophores and the manner in which conidiophores 

 branch can be better determined in this fashion than by 

 mounting material under a cover glass. Observation of 

 bacterial colonies under the lower power will frequently 

 show structural peculiarities which may be useful in dif- 

 ferentiating the related species from each other. 



Unstained Preparations. — ^Microorganisms are not infre- 

 quently examined in the living condition by placing them in 

 a drop of suitable liquid such as physiological salt solu- 

 tion, water, or broth on a slide and covering them with a 

 cover glass. Such preparations are made for the purpose 

 of observing the appearance of the living organism, also to 

 determine motility, and in some cases to study method and 

 rate of grovrth. It is sometimes difficult to mount molds in 

 this manner without numerous air bubbles interfering with 

 satisfactory observation. This difficulty may be obviated 

 by a drop of alcohol followed by a drop of water. 



Most microorganisms (bacteria in particular) are com- 

 paratively transparent and can be seen under the micro- 

 scope unstained only because their refractive index is dif- 

 ferent from that of water. Considerable care is, therefore, 

 frequently necessary in the adjustment of the light, by 

 shifting the mirror and the iris diaphragm of the substage, 

 or of the Abbe condenser. 



