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point and by knowing or determining the pitch of 

 screw ascertain the value of space traversed. 



It must however, be borne in mind that cover 

 glass of normal thickness be used, particularly in 

 oil immersion objectives, since if unusual thickness 

 is used, working distance may be entirely elimi- 

 nated. 



Magnifying Power.- -This is a question of vital 

 importance in a microscope, not so much as a 

 quality in itself, as in connection with the resolving 

 power. The inquiry should not be simply, how 

 many diameters an instrument will magnify, but 

 what the precision and extent of its definitions are 

 under a certain magnifying power. If a high 

 magnifying power is all that is desired, this may 

 be obtained to an almost unlimited extent by means 

 of simple lenses which may be procured at a small 

 pecuniary outlay ; but these do not give a distinct 

 image nor do they make structure visible which, 

 be it remembered, it is the purpose of the micro- 

 scope to do. 



The normal eye can distinguish from 200 to 250 

 lines to the inch and in a microscope such magni- 

 fying power should be used as will apparently 

 bring the structure which is sought after at least 

 up to this figure. To illustrate take a J- inch objec- 

 tive of 0.77 N. A. and a 2 inch eyepiece. An 

 objective of this kind properly corrected, resolves 

 the test-object Pteurvsigna aiigulatiun, in which the 



