the anterior, M the middle and P the posterior 

 systems ; thus also A is a single system, M a double 

 and P a triple one. 



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 and 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 pre- 

 cludes another. 



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, and of all the 

 qualities in an ideal objective, 

 this is the most important. Thus 

 p in Fig. 9, I) is considered the 

 point of focus, and C D E the 

 angular aperture. The above 

 definition has its limitations, 

 however. While in objectives 

 of proper construction it holds 

 true, there are many in which it 

 is not the case. For instance, 

 an objective may be so con- 

 structed that it may transmit a 

 considerable number of rays in 



