37 



when all portions of the object are seen over the 

 entire field at once without further focusing. When 

 not flat, it will be found that as 

 the image approaches the edge 

 of the field it becomes more and 

 more indistinct, and that the 

 objective must be correspond- 

 ingly adjusted ; in many cases 

 it remains indistinct or blurred, 

 and this may be considered the 

 Fig. ii. most serious fault. In the case 



of looking at straight parallel lines, such as in a 

 micrometer, they will appear to become more curved 

 as they near the edge, as shown in Fig. 11. 



Flatness of field mainly depends upon the correc- 

 tion of the spherical aberrations, and as under the 

 best conditions the latter cannot be entirely eliminated, 

 it is impossible to attain absolute flatness, except with 

 eye-pieces especially made for this purpose. It may, 

 however, also be due to a faulty eye-piece ; in this 

 case it can fairly be determined, by observing 

 whether it shows equally in different objectives. With 

 beginners, especially, it is usually most complained 

 of, owing probably to the fact that it is the most easily 

 noticable. It is a desirable quality and indicates to a 

 considerable degree the quality of objectives. While 

 it is impossible to obtain absolute flatness, the opti- 

 cian's effort is to obtain the nearest approach to it. As 

 a quality in itself, without regard to resolving power, it 

 is most easily obtainable, but in connection with this 

 quality it becomes difficult to acquire. 



