DIFFERENT PARTS OF THE MICROSCOPE. 201 



state definitely which particular variety of organism is 

 responsible for the conditions. This is especially true 

 of the different varieties of bacillus dysenteria?, and of 

 the group of typhoid and paratyphoid organisms. Fur- 

 ther study will, no doubt, reveal variations in other 

 pathogenic bacteria, which varieties are to-day regarded 

 as a distinct species. 



MICROSCOPIC EXAMINATION OF PREPARATIONS. 



THE DIFFERENT PARTS OF THE MICROSCOPE. 

 Before describing the method of examining prepara- 

 tions microscopically, a few definitions of the terms 

 used in connection with the microscope may not be out 

 of place. (The different parts of the microscope referred 

 to below are indicated by letters in Fig. 35.) 



The ocular or eye-piece (A) is the lens at which the 

 eye is placed when looking through the instrument. It 

 serves to magnify the image projected through the ob- 

 jective. 



The objective (B) is the lens which is at the distal 

 end of the barrel of the instrument, and which serves 

 to magnify the object to be examined. 



The stage (c) is the shelf or platform of the micro- 

 scope on which the object to be examined rests. 



The diaphragms are the perforated stops that fit in 

 the centre of the stage. They vary in size, so that dif- 

 ferent amounts of light may be admitted to the object 

 by using diaphragms with larger or smaller openings. 



The " -iris " diaphragm (D) opens and closes like the 

 iris of the eye. It is so arranged that its opening for 

 admission of light can be increased or diminished by 

 moving a small lever in one or another direction. 



The reflector (E) is the mirror placed beneath the 



