ANGULAR APERTURE. 
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 
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 
