FUNDAMENTAL CONSIDERATIONS 9 



THE COMPOUND MICROSCOPE 



A. Its Construction: 



The principal parts of a compound microscope are: 



1. The base, generally horseshoe shaped, which rests on the table. 



2. The pillar, an upright bar, which is attached to the base below 

 and supports the rest of the instrument. 



3. The stage, a horizontal shelf upon which is placed the prepara- 

 tion or slide to be examined. The stage is perforated in the center 

 for transmitting light reflected up by the mirror. On the stage are 

 two clips for holding the glass slide. 



4. The mirror, situated below thq stage, by which the light is 

 reflected upward through the opening in the stage. 



5. The diaphragm, inserted in the opening of the stage or attached 

 to its lower face, and used to regulate the amount of light reflected 

 by the mirror. 



6. The body tube, a cylinder which holds the draw tube and lenses 

 and moves up and down perpendicularly above the opening in the 

 stage. The tube is raised or lowered either by sliding it back and 

 forth with a twisting movement or by a rack and pinion mechanism. 

 The latter is called the coarse adjustment. 



7. The fine adjustment, a micrometer screw back of the tube, 

 which, on being turned, produces a very small motion of the entire 

 framework which holds the body tube. 



8. The oculars or eyepieces which slip into the upper end of the 

 draw tube. Each of these consist of two plano-convex lenses, the 

 lower one being the larger and known as the field lens because it 

 increases the field of vision. The upper or smaller lens is called the 

 eye lens. It magnifies the image formed by the objective. Midway 

 between the field and eye lens is a perforated diaphragm, the object 

 of which is to cut out edge rays from the image. 



According to the system adopted by the maker, oculars are desig- 

 nated by numbers, as i, 2, 3, 4, etc., or by figures which represent 

 focal lengths. 



9. The objectives, which screw into the bottom of the body tube 

 or nose piece. They consist of a' system of two, three or four lenses, 

 some of which are simple, others compounded of a convex crown 

 lens and a concave flint lens. Objectives like oculars are usually 



