81 



The various features which must be considered 

 as determining the quality of an objective are : 



Angular Aperture, 



Achromatism, 



Resolving Power, 



Flatness of Field, 



Penetration, 



Working Distance, 



Magnifying Power. 



Although these attributes may be considered 

 separately, some of them go hand in hand. The 

 presence or extent of one necessarily involves or 

 precludes another. 



M 



D 



Fig. 30, 



Angular Aperture.- -The angle which the 

 most extreme rays, which are transmitted through 

 the objective, make at the point of focus, is called 

 its angular aperture, or in short its angle, the 



