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extent of which is expressed in degrees and of all 

 the qualities in an ideal objective, this is the most 

 important. Thus in Fig-. 30. D 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 constructed 

 that it may transmit a considerable number of 

 rays in excess of those which combine to form an 

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

 forming an image, they serve no purpose and 

 therefore have no value in consideration of ang- 

 ular aperture. 



As there are many objectives of the same power 

 but of different angular aperture, there are again 

 others of varying power but of the same angle. 



Light is radiated by an object equally in all 

 directions aiid the more of the rays which can be 

 collected to form an image, the more distinct will 

 it become and to a greater extent can we see detail. 

 If in two objectives one receives on its front sur- 

 face and transmits a larger number of rays than 

 another of equal power, we have a case where 

 power would indicate that we should see equally 

 well, but we will find that there is a difference, 

 due to the amount of angular aperture, in favor of 

 the wider angle. Or, in the case of two objec- 

 tives in which one has one-half the power of the 



