198 STUDY OF INDIVIDUAL BACTERIA-STAINING 



of considerable light that would otherwise occur owing to 

 refraction as the light passes up through the slide and into 

 the air. The accompanying diagram will help to make this 

 clearer (Fig. 134). A ray of light (A B) coming through the 

 slide will be refracted in the direction B C li the medium 

 has a lower refractive index than the slide, as air has, and 

 hence will not enter the objective 0. If, however, there is 

 interposed between the objective and the slide a medium 

 which has the same refractive index as the slide, as immer- 

 sion oil has, then the ray will continue in the same direction 

 {B D) at the point B and hence enter the objective. Evi- 



^Front lens of objective 

 '^ \ -A Immersion oil 



Fig. 134. — Diagram of use of immersion oil. 



dently the immersion oil causes much more light to enter 

 the front lens and makes the field brighter and at the same 

 time prevents considerable refraction and dispersion of light 

 from the object seen and hence this appears more distinct 

 and sharply defined. The Abbe condenser and the oil- 

 immersion objective are practically always used in the 

 microscopic study of bacteria (Fig. 135). 



HANGING DROP SLIDE. 



It is sometimes necessary to examine living bacteria, and 

 for this purpose the device known as the "hanging drop 



