CHAPTER IX 



MICROSCOPIC EXAMINATION AND STAINING METHODS 



OBJECTIVES having a higher power than those commonly used 

 in other work are required for the examination of bacteria. A 

 ^inch or 1.8-2 mm. oil-immersion objective is most commonly 

 used. This lens differs from the low-power dry lenses in that it 

 requires a layer of cedar oil between it and the object to be exam- 

 ined. This oil is used upon the lens for the following reasons. In 

 general, the higher the power of the objective, the smaller the 



Fig. 54. Diagram showing the function of an oil-immersion objective (adapted 



from Gage). 



opening through which light may come to the eye. It is necessary, 

 therefore, that all the light possible shall enter the lens in order 

 that a well-illuminated field may result. The accompanying ex- 

 aggerated diagrammatic representation of the objective and the 

 stage of the microscope, may be helpful in understanding the use 

 of the oil. 



Let C represent the microscopic slide, H the drop of oil having 

 the same refractive index as glass, and L the tip of the objective 

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