146 Invertebrate Zoology. 



C. 



MICROSCOPES. 



THE microscopes required for ordinary biological work 

 should be " simple " and " compound." 



The simple microscope may be either a pocket lens or one 

 of the more elaborate dissecting microscopes figured in cat- 

 alogues of optical instruments, etc. Pocket lenses of a very 

 convenient form are made in German silver mountings. For 

 laboratory work each lens should be provided with a long- 

 arm support, that it may be held, if need be, over a dissect- 

 ing dish, allowing the free use of both the operator's hands. 



The compound microscope should be provided with eye- 

 pieces and objectives that will give a magnifying power 

 ranging from fifty to six or seven hundred diameters. The 

 mechanical and optical parts of the microscope, and their 

 uses, should be familiar to the student. The more impor- 

 tant parts are as follows : The heavy foot, or base, often 

 made in the shape of a horseshoe ; the upright pillar, which 

 may be supplied with a joint, allowing inclination of the in- 

 strument. The stage, generally black in color, is the hori- 

 zontal support upon which the object to be examined is 

 placed. The stage is often provided with a pair of com- 

 pressors or clips, which need not be used in ordinary work. 

 Immediately under the stage, and a part of it, is the sub- 

 stage. The sub-stage is generally provided with a perforated 

 apparatus the diaphragm for limiting the amount of 

 light. In the more expensive instruments the sub-stage has 

 also condensing glasses for increasing the intensity of the 

 light. The mirror is provided with a plane and a concave 

 surface. Though a more simple part of the instrument, the 



