92 INTERPRETATION OF APPEARANCES \CH, III 



own more extended and careful researches. The suspension of the judg- 

 ment whenever there seems room for doubt is a lesson inculcated by all 

 those philosophers who have gained the highest repute for practical 

 wisdom ; and it is one which the microscopist cannot too soon learn or 

 too constantly practice." 



For these experiments no condenser is to be used except where 

 specifically indicated. 



§ 130. Dust or Cloudiness on the Ocular. — Employ the 16 

 mm. ( 2 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 position (§ 48). Light the field well and focus sharply. The im- 

 age 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 

 microscope, and rotate it. The irregular objects will rotate with the 

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

 be detected in this way. 



§ 131. 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 ma}' 

 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 objec- 

 tive. Dust would make a dark patch on the image that would remain 

 stationary 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. 



S 132. 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. 



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

 use three printed letters in stairs mounted in Canada balsam (Fig. 89). 

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



