30 INTERPRETATION OF APPEARANCES. 



(§ 31). Iyight the field well and focus sharply. The image will be 

 clear, but part of the field will be obscured by the irregular outline of 

 the particles of lint. Move the object to make sure this appearance is 

 not due to it. 



Grasp the ocular by the milled ring, just above the tube of the mi- 

 croscope, and rotate it. The irregular object will rotate with the ocu- 

 lar. Cloudiness or particles of dust on any part of the ocular, may be 

 detected in this way. 



§ 77. Dust or Cloudiness on the Objective.— Employ the same 

 ocular and objective as before (§ 76), and the fly's wing as object. 

 Focus and light well, and observe carefully the appearance. Rub gly- 

 cerin on one side of a slide near the end. Hold the clean side of this 

 end close against the objective. The image will be obscured, and can 

 not be made clear by focusing. Then use a clean slide, and the image 

 may be made clear by elevating the body slightly. The obscurity pro- 

 duced in this way is like that caused by clouding the front-lens of the 

 objective. Dust would make a dark patch on the image that would 

 remain stationary while the object or ocular was moved. 



If too small a diaphragm is employed, only the central part of the 

 field will be illuminated, and around the small light circle will be seen 

 a dark ring. 



§ 78. Relative Position of Objects or parts of the same object. — 

 The general rule is that objects highest up come into focus last in focus- 

 ing up, first in focusing down. 



§ 79. Objects Having Plane or Irregular Outlines. — As object 

 use three printed letters mounted in stairs in Canada balsam (PI. Ill, 

 Fig. 23, Ch. V). The first letter is placed directly upon the slide, and 

 covered with a small piece of glass about as thick as a slide. The 

 second letter is placed upon this and covered in like manner. The 

 third letter is placed upon the second thick cover and covered with an 

 ordinary cover-glass. The letters should be as near together as possi- 

 ble, but not overlapping. Employ the same ocular and objective as 

 above (§ 76). 



Dower the tube till the objective almost touches the top letter, then 

 look into the microscope, and slowly focus up. The lowest letter will 

 first appear, and then, as it disappears, the middle one will appear, 

 and so on. Focus down, and the top letter will first appear, then the 

 middle one, etc. The relative position of objects is determined exactly 

 in this way in practical work. 



§ 80. Transparent Objects Having Curved Outlines. — The suc- 

 cess of these experiments will depend entirely upon the care and skill 

 used in preparing the objects, in lighting, and in focusing. 



Employ a 5 mm. (i in.) or higher objective and a high ocular for all 



