176 OPTICAL PRINCIPLES. 



Apian 1 atism (a, not, TrXavdw, to wander) . The ef- 

 fect of spherical aberration in rendering the image of 

 an object seen through a lens indistinct and misty may 

 now be intelligible. In order that such image may 

 be distinct, the rays emanating from each point of 

 the object must converge at one spot upon the retina. 

 But since, when spherical aberration exists, the mar- 

 ginal rays are more refracted than the central ones, 

 they will meet at foci before those formed by the lat- 

 ter ; and when the foci of one set are coincident with 

 the retina, so that the image would otherwise be di- 

 stinct, the latter is rendered confused and indistinct 

 by the rays of the other set. 



In this consideration, we imply that there are only 

 two sets of rays, the central and the marginal ; but 

 the central and marginal rays are not separate, for 

 the rays possess every intermediate degree of obliquity, 

 hence the foci and images are really innumerable. 



Now there are evidently two methods of destroying 

 or correcting spherical aberration, viz. by excluding 

 the marginal rays, or by altering their direction. 



The exclusion of the marginal rays is often adopted ; 

 and is effected by means of a diaphragm, or stop as 

 it is called. This consists of a plate of metal, with a 

 round aperture in the middle, and it is placed behind 

 the lens ; but it has the serious defect of diminishing 

 considerably the amount of light transmitted. 



The alteration of the direction of the marginal rays 

 is produced by refraction, a thin plano-concave lens 

 being placed in front of the convex one (PL XII. 

 fig. 25) . The doubly convex lens is composed of crown 

 glass, and the concave lens of flint glass, which has a 

 higher refractive and dispersive power than crown 

 glass. In this way we get a compound lens, which, 

 if the two lenses had the same refractive power, would 

 simply amount to a plano-concave lens with the mar* 

 ginal portions removed. But as the concave lens 

 consists of more highly refracting material than the 



