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equal intensity, which with daylight is sometimes 

 difficult, as the sash of the window may be reflected 

 and show itself in the field as dark bands, or in the 

 case of lamplight the blue portion of the flame may 

 appear as a dark spot. These are only preliminary 

 directions but will suffice for the beginning. There 

 will be little difficulty in obtaining proper illumi- 

 nation at the outset, if one will bear in mind the 

 three necessary conditions when looking at the 

 back of the objective : 



Central illumination, 



Even illumination over the entire field, 



Mellow illumination. 



Defects in illumination which may not be ap- 

 parent will show when the eyepiece is again 

 attached, and defective lighting will be indicated, 



When dark points or shadows appear in the 



field, 

 When a shadow is thrown at one side of 



the object, 

 When the object appears to lie in a glare 



of light. 



In the first two cases the correction can be made 

 by suitably adjusting the position of the mirror. 

 In the latter by reducing the amount of light, 

 (which it may be said in passing, is seldom neces- 

 sary with medium or high power objectives), by 

 the use of a diaphragm. 



