CHAPTER VI. 



THE MICROSCOPE. THE HANGING DROP. SIMPLE STAINING. 



In view of the fact that many of the students begin the 

 study of bacteriology without any previous experience in 

 the use of a microscope, it is very desirable to describe this 

 instrument, and the manner in which it should be employed. 



It is customary to speak of simple, and of compound mi- 

 croscopes. The former consist, usually, though not neces- 

 sarily, of a single lens, as in the case of an ordinary read- 

 ing glass. In the simple microscope, the rays of light 

 which enter the eye 

 come directly from the 

 object, and a virtual 

 image is produced. 

 Fig. 15 illustrates the 

 action of a simple mi- 

 croscope. It should 

 be observed that the 

 object is between the 

 principal focus P and 



FIG. 15. Virtual image, simple microscope (Carpenter). 



the lens. This figure 



also illustrates the action of the eye-piece in the compound 



microscope. 



If the object is placed beyond the principal focus (P), 

 as in Fig. 16, a real image will result, and can be received 

 on a screen. This corresponds to the action of the objec- 

 tive in the compound microscope. 



The compound microscope consists of two set of lenses, 

 the objective and the eye-piece. The former is placed near 

 the object, and gives rise to a real image (Fig 17 A B). This 



