102 INTERPRETATION OF APPEARANCES [CII.1I1 



small light circle will be seen a dark ring (Fig. 49). If the dia- 

 phragm is lowered or a sufficiently large one employed the entire 

 field will be lighted. 



145. 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. 



146. Objects having Plane or Irregular Outlines. As 

 object use three printed letters in stairs mounted in Canada balsam 

 (Fig. 96). 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 placed upon this and covered in like manner. The 

 third letter is placed upon the second thick cover and covered with 

 an ordinary cover-glass. The letters should be as near together as 

 possible, but not over-lapping. Employ the same ocular and objec- 

 tive as above (143). 



a FIG. 96. Letters mounted in 



b stairs to show the order of coming 



into focus. 



a, b, c, d. The various letters 

 indicated by the oblique roiv of black 



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

 mounted. 



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

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

 letter 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 deter- 

 mined exactly in this way in practical work. 



For example, if one has a micrometer ruled on a cover-glass 15- 

 25 hundredths mm. thick, it is not easy to determine with the naked 

 eye which is the ruled surface. But if one puts the micrometer 

 under a microscope and uses a 3 mm. (^ in.) objective, it is easily 

 determined. The cover should be laid on a slide and focused till 

 the lines are sharp. Now, without changing the focus in the least 

 turn the cover over. If it is necessary to focus up to get the lines 

 of the micrometer sharp, the lines are on the upper side. If one 

 must focus down, the lines are on the under surface. With a thin 

 cover and delicate lines this method of determining the position of 

 the rulings is of considerable practical importance. 



