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 the same by aid of the microscope. 

 Second. The continued study through artificial cultivation. 

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

 other. 



Microscopical. The ordinary microscope will not suffice for 

 Bacteriological 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 be- 

 tween 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," as this lens is called, dips into 

 a drop of cedar-oil placed upon the cover-glass, and is then ready 

 for use. 



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

 bination of lenses placed underneath the stage, for bringing 



wide rays of light directly under 

 FIG. 7. the object. It serves to intensify 



the colored pictures by absorbing 

 or hiding the unstained structure. 



This is very useful in searching 

 a specimen for bacteria, since it 

 clears the field of everything that 

 is not stained. It is called Abbe's 



Abbe's condenser. Condenser. Together with it is 



usually found an instrument for 



