be injured by the presence of chromatic or spherical 

 aberration or defective mechanical work, and it is a 

 matter of no uncommon occurrence that in objectives 

 of the same power and aperture there is a noticeable 

 difference in resolving power; or in objectives of 

 different numerical aperture, the one of less aperture 

 will have a greater resolving power than the other. 

 If we could accept the statements of makers as true 

 ones, a portion of this work and a vast amount of 

 literature would be unnecessary, but the writer has 

 occasion to know that this is not the case and that 

 there is a constantly increasing danger and tendency 

 to allow small defects to pass, in spite of the fact that 

 the general efficiency of the microscope has increased 

 in late years. 



To resolve a structure is to make it visible and the 

 resolving power is in direct ratio to the numerical 

 aperture and can be mathematically calculated. It can 

 be studied from the aperture tables already mentioned. 

 It will be seen that an objective with a numerical 

 aperture of 0.50 will make visible only one-half as 

 many lines in the same space as one with a numerical 

 aperture of i.o. This, it will be seen, refers only to 

 the resolving power of an objective and makes abso- 

 lutely no reference to its magnifying power. Now, as 

 we know that the purpose of the compound microscope 

 is to give magnifying power and that there is a certain 

 structure which is not visible to the eye, how can we 



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