34 



extremely thin, has a marked influence on the 

 optical performance of an objective and this influ- 

 ence increases as the magnifying power of the 

 objective increases. 



For the purpose of illustration, we will imagine 

 a cover glass of considerable thickness (Fig. 31). 



O 



Fig. 31. 



o represents the source of light, or in this case the 

 object. While the rays from it are emitted in all 

 directions, we need only consider those coming 

 from the upper half, and for simplicity we will 

 select from them two pairs. As o a and o a strike 

 the lower surface of the cover glass they are 

 refracted toward the axis o i and on their emer- 

 gence from the upper surface of the cover glass 

 are again refracted away from the axis in the 

 direction of b c and b' c' t which is the original direc- 

 tion of the rays. The same action will take place 

 with the extreme rays o d and o d ', which will 

 emerge as shown at e f and e' f . 



