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 separ- 

 ately, some of them go hand in hand. The presence 

 or extent of one necessarily involves or precludes 

 another. 



Angular Aperture. The angle which the most 

 extreme rays transmitted through the objective make 

 at the point of focus is called the angular aperture, or, 



in short, the angle of the 

 objective, the extent of which 

 is expressed in degrees, and 

 of all the qualities in an ideal 

 objective, this is the most 

 important. Thus in Fig. 34, 

 d is considered the point of 

 focus, and c d e the angular 

 aperture. However, the above 

 definition has its limitations. 

 Fig 34 While in objectives of proper 



\; 

 V 



d 



70 



