Ch. IV] INTERPRETATION OF APPEARANCES 109 



The general law for the whole matter is to study the object in every 

 way possible (§ 21 8). 



For the experiments, § 188-201, no condenser is to be used, except 

 in a part of § 201. 



§ 187a. "The distinction between a dark element which is referable to 

 pigment and a dark element which is referable to the deflection of light can 

 generally be made by watching the effect produced by the alteration of the 

 focus. Where the dark element corresponds to a point from which light is 

 deflected a change of the focus will be associated with a change from dark to 

 bright. Where pigment is in question a change of focus will substitute only a 

 more diffuse for a less diffuse dark element." (Wright, p. 44.) 



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

 objective, 4.x or 5.x 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 (§ 45). Light 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 

 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. 



Unscrew the field-lens and remove the lint before proceeding. 



§ 189. A small bright field. — With low objectives (25-50 mm.), 

 if too small a diaphragm is used and put 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. 65). If the diaphragm is 

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

 lighted (see also § 90 for diaphragms with the condenser). 



§ 190. 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. 



§ 191. Objects having plane or irregular outlines. — As object 

 use three printed letters in stairs mounted in Canada balsam (fig. 66). 

 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 



