46 LABORATORY GUIDE IN BACTERIOLOGY 



The usual pattern of microscope consists of two 

 main parts: the stand, and the optical parts (Fig. 19) 

 which are attached to the stand. 



The stand consists of a body tube, draw tube, 

 coarse adjustment, fine adjustment (micrometer screw) 

 in a pillar, nose-piece, stage with clips for holding the 

 object, main pillar, and the horseshoe base. At the 

 junction of the main pillar and the fine-adjustment 

 pillar is the inclination joint. 



The draw tube, regulating the focal length, which 

 varies in different instruments, should be raised to 16 

 or 17 mm. If a nose-piece is attached, the width of 

 this must be deducted from the tube length. 



The optical parts are the oculars, the objectives, the 

 substage condenser, and the mirror. The ocular is a 

 combination of lenses, which slips into the top of the 

 draw tube and is nearest the eye. The objective is a 

 combination of lenses which is screwed into the nose- 

 piece and fits to the lower end of the draw tube. The 

 substage condenser fits under the stage. It concen- 

 trates the light on the object and is raised for high 

 powers or lowered for low powers. At the lower end 

 of this condenser is the iris diaphragm, which is regu- 

 lated by a small lever with a milled head, and serves 

 the purpose of regulating the light supply. The mirror , 

 has two sides, a concave and a flat one. 



In the manipulation of the compound microscope 

 the following points should be observed: 



1. Keep the instrument clean. When not in use, 

 lock it in the case or cover it with a bell-jar. 



2. When carrying the instrument, grasp it by the 

 main pillar underneath the stage, not by the fine- 



