no 



INTERPRETATION OE APPEARANCES 



[Ch. IV 



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

 placed upon the second thick cover and covered with an ordinary 



Fig. 65. The Microscopic Field completely and only partly 



Illuminated. 



A The field completely illuminated; a net micrometer is used as object. 

 B The field is only partly illuminated; the same net micrometer is used as 

 object, but not all of it appears in the partially lighted field. 



cover-glass. The letters < should be as near together as possible, but 

 not overlapping. Employ the same ocular and objective as above 

 (§ 188). 



Lower the tube till the objective almost touches the top letter; 

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



1 



Fig. 66. Letters in Stairs to Determine Relative Position by Focus- 

 ing Up and Down. 



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. 



