CHAPTER II. 



LIGHTING AND FOCUSING; MANIPULATION OF DRY, 



ADJUSTABLE AND IMMERSION OBJECTIVES; CARE 



OF THE MICROSCOPE AND OF THE EYES; 



LABORATORY MICROSCOPES. 



APPARATUS AND MATERIAL FOR THIS CHAPTER. 



Microscope supplied with plane and concave mirror, achromatic and Abbe con- 

 densers, dry, adjustable and immersion objectives, oculars, tripple nose-piece. 

 Microscope lamp and movable condenser (bull's eye or other form (Fig. 52), 

 Homogeneous immersion liquid ; Betizin, alcohol, distilled water ; Mounted 

 preparation of fly's wing (§ 68) ; Mounted preparation of Pleurasigma. Stage or 

 ocular micrometer with lines filled with graphite (§ 73, 74) ; Glass slides and 

 cover-glasses (Ch. VII) ; 10 per ct. solution of salicylic acid in 95 per ct. alcohol 

 (§ 88) ; Preparation of stained microbes [\ 101) ; Vial of equal parts olive or cot- 

 ton seed oil or liquid vaselin and benzin (§ 105) ; Ward's and double eye-shade 

 (Figs. 59, 60) ; Screen for whole microscope (Fig. 58). 



FOCUSING. 



I 56. Focusing is mutually arranging an object and the microscope so that a 

 clear image may be seen. 



With a simple microscope (§9) either the object or the microscope or both may 

 be moved in order to see the image clearly, but with the compound microscope 

 the object more conveniently remains stationary on the stage, and the tube or 

 body of the microscope is raised or lowered (frontispiece). 



In general, the higher the power of the whole microscope whether simple or 

 compound, the nearer together must the object and objective be brought. With 

 the compound microscope, the higher the objective, and the longer the tube of 

 the microscope, the nearer together must the object and the objective be brought. 

 If the oculars are not par-focal, the higher the magnification of the ocular, the 

 nearer must object and objective be brought. 



§ 57. Working Distance. — By this is meant the space between the simple micro- 

 scope and the object, or between the front lens of the compound microscope and 

 the object, when the microscope is in focus. This working distance is always con- 

 siderably less than the equivalent focal length of the objective. For example, 

 the front-lens of a ^th in., or 6 mm. objective would not be j^th inch, or 6 milli- 

 meters from the object when the microscope is in focus, but considerably less than 

 that distance. If there were no other reason than the limited working distance of 



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