ANGULAR APERTURE. 



that point is the vertex, and the object-glass the base, and after 

 passing through the lens, they will converge to the corresponding 

 point of the object. 



Thus it appears that each point of the image is illuminated by 

 as many rays as are included within such a cone as we have here 

 described ; that is to say, one whose base is the object-glass, and 

 whose vertex is on the object. But it is evident that the number 

 of rays included in such a cone, depends exclusively upon the 

 magnitude of its angle, that is the angle L c L', formed by lines 

 drawn from a point, c, upon the object. 



14. This angle, which forms, therefore, an element of capital 



Fig. 6. 



importance in the estimation of the efficiency of the microscope, is 

 called the ANGULAR APERTURE of the object-glass. 



The effect produced by the variation of the angular aperture of 

 the object-glass, other things being the same, will be rendered 



11 



