33 



While this is not a theoretically correct explanation 

 of angular aperture, it will serve the purpose of show- 

 ing its effect. As increase in aperture means increase 

 of resolving power and it will be seen that every degree 

 thus added increases the effectiveness of the objec- 

 tive. It is in this direction that modern advance has 

 been signalized. Whatever views rrray be held on the 

 advisability of narrow or wide angle objectives, the 

 optical standard of excellence depends particularly 

 on this quality. 



Objectives of the same angular aperture, but of dif- 

 ferent magnifying power (within ordinary limits), will 

 show the object equally well, provided they are other- 

 wise of the same quality, and it is also true that in 

 objectives of the same power but unequal angular 

 aperture, the one of wider angle will show an object 

 more brilliantly than the other, and, if the difference 

 be considerable, will show structure of which no trace 

 can be found with the narrow angle. These are facts 

 which are based upon natural laws, but there are 

 other conditions to be considered in connection with 

 them, which will be treated hereafter. 



It very often happens that objectives from different 

 makers, but of the same angle, show a considerable 

 variation ; this does not prove that the above princi- 

 ple is wrong, but is evidence that greater care or skill 

 has been bestowed on one than on the other. 



By arranging an objective with an immersion front, 

 its angular aperture may be considerably increased 

 over that of a dry front, and this explains why better 



