CHAPTER XIX. 

 STUDY OF INDIVIDUAL BACTERIA-STAINING. 



When an organism has been obtained in pure culture by 

 any of the methods described in the preceding chapter the 

 next step is the study of its morphology as discussed in 

 Chapters II-IV. This involves the use of the microscope, 

 and since bacteria are so small, objectives of higher power 

 than the student has presumably used will be needed. 

 Doubtless only the two-third inch or 16 mm. and the one- 

 sixth inch or 4 mm. objectives are all that have been used 

 in previous microscopic work, while for examining bacteria 

 a one-twelfth inch or 2 mm. is necessary. It will have 

 been observed that the higher the power of the objective 

 the smaller is the front lens or object glass and consequently 

 the less is the amount of light which enters. With the use 

 of the one-twelfth inch or 2 mm. objective it is necessary to 

 employ two devices for increasing the amount of light enter- 

 ing it, with which the student is probablj' not familiar. One 

 of these is to place a drop of cedar oil between the front 

 lens and the object and to immerse the lens in this oil — 

 hence the term "oil-immersion objective;" the other is the 

 substage or Abbe condenser. The latter is a system of 

 lenses placed below the stage and so constructed as to bring 

 parallel rays of light — daylight — from an area much larger 

 than the face of the front lens of the objective to a focus 

 on the object to be examined, thus adding very greatly to 

 the amount of light entering the objecti^'e. Since the con- 

 denser brings parallel rays to a focus on the object the flat- 

 mirror is always used with the condenser when working 

 with daylight. With artificial light close to the microscope, 

 the concave mirror may be used to make the di^vergent rays 

 more nearly parallel and thus give better illumination. 



The function of immersion oil is to prevent the dispersion 



