A D H 

 The whole aberration is therefore 



The aberration for a particular value of A varies as r, or as the square 

 of the radius of the aperture nearly. 



Let us examine what kinds of value the aberration in a lens assumes 

 in different cases. 



(1.) For the meniscus or concavo-convex lens, (r & ?' being both posi- 

 tive.) 



The aberration 



(2.) For the double concave lens r' is negative, 



(3.) For the double convex lens r is negative, 

 A=H 



To find the least circle of aberration into which all the homogeneal 

 rays of the same pencil, refracted by a lens, are collected. 



Let a = | aperture of the lens, 6 == distance of the point where the 

 extreme ray cuts the axis from the focus of refracted rays, c = distance 

 of the same point from the lens, x = rad. of the least circle of aberra- 

 tion, then 



_ x o 

 "TT 



Cor. If the focal length of the refractor, and the focus of incidence be 

 given, c is given and x varies as a b varies as 3 ; and on the same sup- 

 position the area of the least circle of aberration varies as a 6 . 



ACCELERATION of Falling Bodies, See Motion. 



ACCELERATION of Stars on Mean Solar Time. See Time. 



ACCELERATION of the Moon. See Moon. 



ADHESION, a term chiefly used to denote the force, with which the 

 surface of a solid remains attached to the surface of a liquid, after they 

 have been brought into contact. 



In the year 1773, Guyton-Morveau ascertained experimentally the 

 force of adhesion of eleven different metals to mercury. The surface of 

 oach metal was an inch (French) in diameter and polished. The follow. 

 t> 



