20 INTRODUCTION. 



8. Stage the flat part on which is laid the object to be 

 examined, and which is perforated by a central hole to allow 

 illumination of the object from below. The stage may be 

 circular or square, stationary or movable, and mechanical. 



Underneath the Stage are Found the Parts: 



9. Flat mirror for low-power, and 



10. Concave mirror for high-power objectives. 



The mirror is so arranged as to allow motion in all direc- 

 tions. For ordinary histological purposes it is usually fixed 

 perpendicularly to the stage, and gives direct light; occasion- 

 ally it is placed in an oblique direction, giving oblique light. 



11. Diaphragm. Immediately below the stage and about 

 two inches above the mirror, though freely movable up and 

 down, is found the diaphragm or stop : used to prevent the 

 peripheral or diffuse rays of light from the mirror from 

 reaching the object, and to allow only the more central and 

 direct rays to illuminate the same. The holes in the dia- 

 phragm are of different sizes, the smaller ones being used 

 with the higher power and the larger ones with the lower 

 power class of work. 



12. Condenser. For very high powers, especially such as 

 are used in bacteriology, besides the foregoing parts, there 

 are on the substage the condenser, which is a lens, or system 

 of lenses, used to concentrate still further the light from the 

 mirror on the object. The condenser most commonly in use 

 is known by the name of its introducer as the Abbe. The 

 condenser should be exactly central, and, as a rule, it should 

 be brought almost into contact with the object on the stage. 



13. Iris Diaphragm. Immediately below the condenser, 

 instead of the ordinary diaphragm, what is known as an iris 

 diaphragm is used (so called from its peculiar variability, 

 like the iris of the eye). 



14. Nose-piece or Revolver. At the bottom of the tube 

 a mechanical piece, which enables one to attach two or three 

 objectives to the microscope at the same time, is known as 

 the nose-piece or revolver. 



