97 



the surface of the objective front but are changed in 

 their course. If we make use of the same illustration 

 .and extend the refracted rays downward as shown 

 by the dotted lines e c, f d, f d, e c until they meet 

 at the axis, these points will be the apparant location 

 of the object and will appear to meet in the planes 

 a and d b instead of at o. To neutralize this con- 

 dition the objective will require to be spherically 



Fig. 33. 



under-corrected, by which is understood that the 

 marginal rays will eminate from a point near the 

 objective and the central rays at a greater dis- 

 tance from the objective, or as is shown in the 

 diagram, appear to come from exactly the same 

 points, which are the apparent positions of the 

 object or g c, h d, h d and g' c. 



If a thicker cover is used the objective will re- 

 quire to be more under-corrected, or if a thinner 



