construction it holds true, there are many in which it is 

 not the case. For instance, an objective may be so 

 constructed that it may transmit a considerable number 

 of rays ir> excess of those which combine to form an 

 image and it is evident that as they do not aid in form- 

 ing an image, they serve no purpose and therefore 

 have no value in the consideration of angular aperture. 

 Objectives of the same power are made of varying 

 degrees of angular aperture, while others differing in 

 power may have the same angle. 



Diagrammatic representation of light transmitted by objectives of low (a) 

 and high (b) angular aperture. 



White light is radiated by objects equally in all 

 directions, but only those rays from one-half or up to 

 1 80 degrees of the object need be considered. It is 

 evident that the more rays which can be collected to 

 form an image, the more distinct will it become, thus 

 making it possible to see more detail. If of two objec- 



