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powers. The most frequently occurring fault is 

 the lack of perfect grinding and polishing of the 

 lens surfaces. If the former, it will show itself 

 as spots in the field and if the latter, as a series of 

 streaks and shadows, usually circular in form as if 

 the lens had been wiped with greasy fingers. 

 This may be the difficulty, and before passing 

 judgment the lenses should be carefully cleaned. 



Another defect may be in the glass itself, in the 

 so called striae, which will be indicated by dark 

 and light streaks across the field. 



Care must be taken not to confound small parti- 

 cles of dust which are apt to fall upon the field 

 lens and which at times are very prominent in the 

 field, with imperfections of the surface. These 

 can be distinguished from other defects only by 

 wiping, or using a camel's hair brush, and even 

 with the utmost care some particles are liable to 

 remain. 



The eyepiece often fits too closely in the tube 

 and when making observations with high powers, 

 a change of eyepiece is apt to disturb the object. 

 It should enter without any friction and still so 

 closely that it drops slowly into its place from the 

 compression of air in the tube when objective is 

 attached. 



Eyepieces are now generally made parfocal, 

 that is the equivalent foci are made to correspond, 



