this condition the objective will require to be spher- 

 ically under-corrected, by which is understood that the 

 marginal rays will eminate from a point near the objec- 

 tive and the central rays at a greater distance from the 



Fig. 37. 



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 e,h c,h' c, and g f e. 

 If a thicker cover is used the objective will require 

 to be more under-corrected, or if a thinner one, less 

 under-correction is needed, so that if an objective is 

 corrected for a definite thickness of cover, its correction 

 will be disturbed if greater or less thickness be used, 



