1 8 THE MICROSCOPE AND ITS ACCESSORIES. 



after the other opposite the opening in the stage. A much better 

 diaphragm, constructed on an entirely different principle, is the so- 

 called iris diaphragm. Although its opening is not exactly circu- 

 lar, yet it has the advantage of being easily enlarged or contracted 

 by manipulating a small handle controlling the metal plates sliding 

 over one another. 



The tube, which is contained in a close-fitting metal sheath, 

 is attached to the upright of the microscope. In the simpler forms 



Ocular or eyepiece. 



Draw-tube. 



Tube. 



Triple nose-piece. 

 Objectives, 



Stage. 



Iris diaphragm and 

 Abbe condenser. 



Screw for focusing 

 condenser. 



Mirror. - 



Rack and pinion for 

 coarse adjustment. 



Micrometer screw for 

 fine adjustment. 



Pillar. 



- Stand. 



Fig. I. Microscope. 



of microscopes the tube is raised, lowered, or twisted by hand. In 

 more complicated instruments the upward and downward move- 

 ments are accomplished by means of a rack and pinion coarse 

 adjustment. A micrometer screw fine adjustment situated 

 at either the upper or the lower end of the upright, controls the fine 

 adjustment. The tube possesses an upper and a lower opening, into 

 which lenses may be laid and screwed. The ocular, into the ends 

 of which lenses are inserted, fits into the upper opening. The 



