105 



a flat object preferably a micrometer. It is said 

 to be flat when all portions of the object are seen 

 over the entire field at once without further focus- 

 ing. 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 correspondingly adjusted ; in many cases 

 it remains indistinct or blurred, and this may be 

 considered the 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. 35, 



Fig. 35. 



Flatness of field mainly depends upon the cor- 

 rection of the spherical aberrations and as under 

 the best conditions the latter cannot be entirely 

 eliminated, it is impossible to attain 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 beginners, especially, it is usually 

 most complained of, owing probably to the fact 



