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 

 condensers, dry, adjustable and immersion objectives, oculars, triple nose-piece. 

 Microscope lamp and movable condenser (bull's eye or other form, Fig. 53), 

 Homogeneous immersion liquid ; Benzin, alcohol, distilled water ; Mounted prep- 

 aration of fly's wing(§ 70); Mounted preparation of Pleitrosigma ($ 77, 78). Stage 

 or ocular micrometer ($ 92); Glass slides and cover-glasses (Ch. VII); 10 per ct. 

 solution of salicylic acid in 95 per ct. alcohol ( \ 92 ) ; Preparation of stained bac- 

 teria (\ 108); Vial of equal parts olive or cotton' seed oil or liquid vaselin and 

 benzin ($ 112): Double eye shade (Fig. 60); Screen for whole microscope (Fig. 59). 



FOCUSING 



\ 60. Focusing is mutually arranging an object and the microscope so that a 

 clear image may be seen. 



With a simple microscope (| 11) 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 the object and objective be brought. 



\ 61. Working Distance. — By this is meant the space between the simple mi- 

 croscope 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 6 mm. or J/jfth in. objective would not be 6 millimeters or Xth 

 inch 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 

 high objectives, it would be necessary 7 to use a very thin cover-glass over the 



