26 ESSENTIALS OF BACTRIOLOGY. 



light, giving to various objects which they inhabit a character- 

 istic glow or phosphorescence. 



Fluorescence. An iridescence, or play of colors, develops in 

 some of the bacterial cultures. 



Gas Formation. Many bacteria, anaerobic ones especially, 

 produce gases, noxious and odorless ; in the culture-media the 

 bubbles which arise soon displace the media. 



Odors. Some germs form odors characteristic of them: some 

 are pleasant and even fragrant; others, foul and nauseous. 



Effect of Age. With age, bacteria lose their strength and die. 



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. 



Microscopical. 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 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 dove-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. 



