CH. Ill] INTERPRETATION OF APPEARANCES 101 



that they should not be misled, to the great waste of their time and 

 pains, by our errors. And it is due to ourselves that we should not 

 commit our reputation to the chance of impairment by the 'premature 

 formation and publication of conclusions which may be at once re- 

 versed by other observers better informed than ourselves, or may be 

 proved fallacious at some future time, perhaps even by our 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. 



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

 mm. (73 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 ocu- 

 lar in position (§ 55). Eight the field well and focus sharply. The 

 image will be clear, but part of the field will be obscured by the ir- 

 regular 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. 



§ 144. 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 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 



