fractions of millimeters or inches. After focusing upon 

 the lines, they will appear to become more curved and 

 less distinct as they near the edge, as shown in 

 Fig. 38. 



Flatness of field mainly depends upon the correction 

 of the spherical aberration and, as under the best 

 conditions this cannot be entirely 

 eliminated, it is impossible to 

 obtain absolute flatness. It may 

 also, however, be due to a faulty 

 eyepiece ; in this case it can 

 be determined by observing 

 whether it shows equally in 

 different objectives. With be- 

 ginners, especially, it is usually 

 most complained of, owing probably to the fact that 

 it is most easily discernable. While it is a desirable 

 quality and indicates to a considerable degree the 

 quality of objectives, it is impossible to obtain absolute 

 flatness of field in objectives of sufficient angular 

 aperture to meet the requirements of the present day 

 and especially in higher powers. 



Working Distance represents the free space 

 between the front lens in the objective and the upper 

 surface of the cover glass which protects the object, 

 when the objective is in focus and is corrected for that 

 cover glass. 



38. 



96 



