26 ESSENTIALS OF BACTERIOLOGY 



CHAPTER III 

 METHODS OF EXAMINATION 



WE divide the further study of the general characteristics of 

 bacteria into two portions: 



First, the examination of bacteria by aid of the microscope. 



Second, the continued study through artificial cultivation. 



They both go hand in hand; the one incomplete without the 

 other. 



Microscopic. The ordinary microscope will not suffice for 

 bacteriologic research. Certain special appliances must first 

 be added. It is not so much required to have a picture very 

 large, as to have it sharp and clear. 



Oil-immersion Lens. The penetration and clearness of a 

 lens are very much influenced by the absorption of the rays of 

 light emerging from the picture. In the ordinary dry system 

 many of the light rays, being bent outward by the air which is 

 between the object and the lens, do not enter the lens, and are 

 lost. By interposing an agent which has the same refractive 

 index as glass, cedar-oil or clove-oil, for example, all the rays 

 of light from the object enter directly into the lens. 



The "homogeneous system," or oil-immersion lens, consists 

 of a system of lenses which can be dipped into a drop of cedar- 

 oil placed upon the cover-glass, and which is then ready for use. 



Abbe's Condenser. The second necessary adjunct is a com- 

 bination of lenses placed under- 

 neath the stage, for bringing 

 wide rays of light directly under 

 the object. It serves to intensify 

 the colored pictures by absorbing 



Fig. 7 . Abbe's condenser. or hiding the unstained structure. 



This is very useful in searching 



a specimen for bacteria, since it clears the field of every- 

 thing that is not stained. It is called Abbe's condenser 

 (Fig. 7). Together with it is usually found an instrument 



