length. The fact that the periphery of the field is not 

 in focus at the same time as the center does not bespeak 

 a lack of spherical correction, but a lack of flatness of 

 field with which it is often confounded. 



Flatness of Field depends not only upon the ob- 

 jective itself, but upon the ocular and the cone of light 

 used, whereas the spherical aberration is inherent in the 

 objective itself. No field is absolutely flat. It is a 

 desirable quality in a lens but spherical and chromatic 

 corrections should never be sacrificed for it. Some 

 lenses appear to be "flatter" than they really are, because 

 their corrections are so poor that little contrast is 

 noticed between objects in the center of the field and at 

 the edge. Narrow cones of light give a flatter field than 

 wide ones. Thin objects are more critical tests for 

 flatness of field than thick ones. 



Working Distance is the free distance between the 

 cover glass and the objective when the latter is foccrsed. 

 It decreases generally with increasing power and numeri- 

 cal aperture of the objective. Of two lenses with the 

 same focal distance the one with the higher N. A. will 

 have the shorter working distance. The working dis- 

 tance also depends on the mounting of the front lens. 

 If the lens has a prominent mounting projecting beyond 

 its surface the working distance is lessened thereby. 

 THE OCULARS 



A certain magnification by the ocular will be neces- 

 sary, and sufficient, to bring out all the detail in the 

 image which can be secured from the numerical aper- 

 ture of the objective. If we use a higher ocular we lose 

 depth of sharpness and size of field, since they are both 

 inversely proportional to the magnification. We also 

 lose illumination, which varies inversely as the square of 

 the magnification. 



