82 INTERPRETATION OF APPEARANCES. {CH. III. 



For these experiments no condenser is to be nsed except where specifi- 

 cally indicated. 



§ 121. Dust or Cloudiness on the Ocular. — Employ the 16 mm. 

 (^3 in.) objective, low ocular, and fly's wing as object. 



Unscrew the field-lens and put some particles of lint from dark cloth 

 on its upper surface. Replace the field-lens and put the ocular in posi- 

 tion (§ 44). Eight 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 micro- 

 scope, and rotate it. The irregular object will rotate with the ocular. 

 Cloudiness or particles of dust on any part of the ocular may be detect- 

 ed in this way. 



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

 ocular and objective as before and the fly's wing as object. Focus 

 and light well, and observe carefully the appearance. Rub glycerin 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 cannot be made 

 clear by focusing. Then use a clean slide, and the image may be made 

 clear by elevating the tube slightly. The obscurity produced 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 station- 

 ary while the object or ocular is moved. 



If a small diaphragm is employed and it is close to the object, only 

 the central part of the field will be illuminated, and around the small 

 light circle will be seen a dark ring (Fig. 42). If the diaphragm is 

 lowered or a sufficiently large one employed the entire field will be 

 lighted. 



§ 123. Relative Position of Objects or parts of the same object. 

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

 focusing up, first in focusing down. 



Fig. 86. Letters mounted in stairs to show 

 the order of coming into focus. 



a, b, c, d. The various letters indicated 

 by the oblique row of black marks in the 

 sectional view. Slide. The glass slide on which the letters are mounted. 



§ 124. Objects having Plane or Irregular Outlines. — As object 

 use three printed letters in stairs mounted in Canada balsam (Fig. 86). 

 The first letter is placed directly upon the slide, and covered with a 



