Cover Glass. At this point we must introduce 

 another feature which has not yet been considered but 

 which has an important bearing in the description of 

 angular aperture. As has been shown, in describing 

 the principles of refraction, rays which pass from air 

 into glass, are bent out of their course or refracted. 

 The cover glass is, as its name implies, a piece of glass 

 to cover the object ; but its purpose is really more to 

 protect and preserve the object, or with soft or limpid 

 objects to flatten them out, and to obtain by means of 

 the upper surface of the cover an optically plane sur- 

 face. Although extremely thin, it has a very pro- 

 nounced influence on the optical performance of an 

 objective and this influence increases as the magnifying 

 power of the objective increases. 



For the purpose of illustration, we will imagine a 

 cover glass of considerable thickness, Fig. 35, in which o 



represents the source 

 of light, or in this 

 case the object. As 

 o a and o a' enter at 



d\b 



i u 



b/ &/ the lower surface of 



a', 



/ ,/ ) the cover glass they 



XX / are refracted toward 



^ 



tne 



emerging at the up- 



per surface of the 



Fig. 35. cover g lass are a g ain 



74 



