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so that it is apparent that by the use of the draw-tube 

 the effect of the cover-glass may be partially neutral- 

 ized ; for instance, when by the use of a thin cover 

 the objective is spherically under-corrected, it may, to 

 a certain extent, be corrected by causing a correspon- 

 ding over-correction in the tube by increasing its 

 length. The use of the draw-tube for the purpose of 

 changing the amplification or for the matter of con- 

 venience can hardly be commended, except in cases 

 where adjustable objectives are used. 



Considerable also depends upon the perfection of 

 the eye-piece. I believe that, as a rule, too little care 

 is devoted to it ; at any rate, it is certain that while 

 any Huyghenian eye-piece for a telescope can be used 

 on a microscope, very few which have been made for 

 this can be used on a telescope ; and while it is true 

 that no such perfection may be required in the former, 

 it leaves such an indefinite range that it may become 

 difficult to place a limit for the perfect and imperfect. 

 In all work, and especially in testing, it should be seen 

 that the eye-lens, as well as the field-lens, are perfectly 

 clean. 



Among the absolutely necessary conditions in judg- 

 ing of the quality of an objective are perfect speci- 

 mens, especially if they are sections. A thick object 

 obstructs the light and generally makes it necessary 

 to go through so many layers or planes that it is diffi- 

 cult to get any one distinct ; the impression may thus 

 easily be given that the objective is at fault. The 

 difference between two objects of the same nature 



