CHAPTER VIII. 



METHODS OF OBSERVING BACTERIA THE USE OF THE 

 MICROSCOPE AND ACCESSORIES. 



Cultures. It is absolutely necessary in the study of pathogenic 

 bacteria to obtain each definite species free from any other live 

 organisms. When such a preparation is successfully made whether 

 of a pathogenic or other bacterium it is called a pure culture. It is 

 generally composed of small discrete masses called colonies, and these 

 in turn are formed by innumerable individual bacilli which in their 

 entity make up the pure culture. When bacteria have grown abund- 

 antly on a properly prepared artificial culture soil they often appear 

 as one continuous mass of the growth and the individual colonies are 

 no longer distinguishable. They were present very early in the course 

 of the development, but have become confluent. If the bacterium 

 has originally been inoculated into the culture soil in a very dilute 

 form individual colonies can always be seen. 



Microscopic Study. Several of the properties and characteristics 

 of pathogenic bacteria in pure cultures can be studied with the naked 

 eye, such as the varying degrees of moisture or dryness, the smooth 

 or granular surface of the growth, its color, its power to liquefy certain 

 culture media, etc. In order to observe the individual bacterium, 

 however, a microscope is required with one low and one high power 

 lens, and an especially powerful illuminating apparatus. The prepar- 

 ation of pure cultures is much more difficult and time-consuming 

 than the microscopic study of bacteria, after they are once obtained 

 in a pure state; hence, the student should first familiarize himself 

 with the use of the microscope in observing bacteria in the stained 

 and unstained condition. This study will, therefore, be considered 

 prior to the subject of culture media, pure cultures, and the sterili- 

 zation methods necessary to obtain them. 



Source of Light. In the microscopic study of bacteria, natural, 

 reflected, diffuse light is employed during the day, best obtained at 

 a window with a northern exposure. When artificial light is used, 

 an ordinary coal-oil lamp or gas burner will serve the purpose, an 

 Argand burner is better, and a Welsbach light is best of all. An 

 Edison incandescent lamp is not a good light, and if used at all 

 should have a bulb of frosted glass, to prevent the incandescent film 

 from disturbing the field of vision. 



The Microscope. The modern microscope consists of a stand 

 forming the foot or base and carrying the stage and a brass tube 



