92 



aperture may be, its effectiveness may be injured 

 by the presence of chromatic or spherical aberra- 

 tions or defective 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 aper- 

 ture 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 

 show one-half as many lines as one with a numer- 

 ical aperture of 1.0. This, it will be seen, refers 

 only to the resolving power of an objective and 

 makes absolutely no reference to its magnifying 

 power. Now, as we know that the purpose of the 

 compound microscope is to give magnifying power 



