81 



excess of those which combine to form an image, and 

 it is evident that these should not be considered as 

 belonging to them. 



As there are many objectives of the same power, 

 but of different angular aperture, there are again oth- 

 ers of varying power, but of the same angle. Other 

 things being equal, it is the angular aperture of an 

 objective which determines the quality. It is expressed 

 in degrees, and is also spoken of as being wide, med- 

 ium or narrow, although this is indefinite and 

 depends considerably upon the power of the objec- 

 tive ; while the angle may be excessively wide for a 

 low power, it may be narrow for a higher one. 



For many years the extent to which angular aper- 

 ture could be carried was a matter of controversy, as 

 was also the use of objectives of wide and narrow 

 angles for different directions of work. It is, how- 

 ever, a matter of congratulation that the question is 

 at rest, although it has served a good purpose in pro- 

 mulgating a better knowledge of the subject. 



All objects emit rays, and it is evident that those 

 coming from one point and contained in a large angle 

 are more numerous than those in a small angle ; also 

 that as the angle more nearly approaches 180 degrees 

 the rays will be larger in number. 



It is assumed that two objects, B and B', are 

 equally bright, and therefore emit the same number 



