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 |-inch or 16 mm. and the J-inch or 4 

 mm. objectives are all that have been used in previous 

 microscopic work, while for examining bacteria a yV^i^ch 

 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 yV-i'^ch 

 or 2 mm. objective it is necessary to employ two devices 

 for increasing the amount of light entering it, with which 

 the student is probably 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 objective. Since the con- 

 denser brings imrallel rays to a focus on the object the fiat 

 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 divergent rays 

 more nearly parallel and thus give better illumination. 



The function of immersion oil is to prevent the dispersion 



(197) 



