36 



resolve easily or only glimpse them the latter when 

 they are hardly to be distinguished. The angular 

 aperture of an objective indicates the resolving 

 power, and the theoretical capacity of every degree 

 has been mathematically determined. However, this 

 standard is only reached approximately and to a vary- 

 ing extent. It is not by any means said that every 

 objective of a certain angular aperture will have a cor- 

 responding resolving power ; it is at this point that 

 the acute accuracy of work and superior judgment of 

 the optician in making proper corrections will invari- 

 ably give the best results. 



Minute structure such as bacter-ia can only be seen 

 by objectives having high numerical aperture, and 

 these are absolutely necessary in modern investiga- 

 tions. The many recent discoveries can only be 

 attributed to the increased resolving power. 



It is an error to suppose that the resolving power 

 may be improved by merely increasing the magnify- 

 ing power. It is an invariable quality of an objective 

 and has a fixed limit. The extent to which it may be 

 approached depends upon the nicety of manipulation, 

 but no amount of increase in magnifying power by 

 the eye-piece or any other means will carry it beyond 

 it ; on the contrary, it will lose in this respect if car- 

 ried beyond a certain point. 



Flatness of Field. Theyfc/;/in a microscope is 

 that portion which is observed in the eye-piece, and 

 its flatness may be observed when focused on a flat 

 object preferably a micrometer. It is said to be flat 



