CHAP, ii.] SIGHT, 5 07 



is, as we have seen, an eye normally constituted in which the 

 power of accommodation has been lost of is failing through in- 

 creasing weakness of the ciliary muscle or a loss of elasticity in 

 the lens, or through the parts becoming rigid. 



Spherical Aberration. In a spherical lens the rays which 

 impinge on the circumference are brought to a focus sooner than 

 those which pass nearer the centre, and the rays proceeding from 

 a luminous point are no longer brought to a single focus at one 

 point but form a number of foci at different distances. Hence when 

 rays are allowed to fall on the whole of the lens, the image formed 

 on a screen placed in the -focus of the more central rays is blurred 

 by the diffusion-circles caused by the circumferential rays which 

 have been brought to a premature focus. In an ordinary optical 

 instrument spherical aberration is obviated by a diaphragm which 

 shuts off the more circumferential rays. In the eye the iris is an 

 adjustable diaphragm ; and when the pupil contracts in near vision 

 the more divergent rays proceeding from a near object, which tend 

 to fait on the circumferential parts of the lens, are cut off. As, 

 however, the refractive power of the lens does not increase regu- 

 larly and progressively from the centre to the circumference, but 

 varies most irregularly, the purpose of the narrowing of the pupil 

 cannot be simply to obviate spherical aberration ; and indeed the 

 other optical imperfections of the eye are so great, that such 

 spherical aberrations as are caused by the lens produce no obvious 

 effect on vision. 



Astigmatism. We have hitherto treated the eye as if its 

 dioptric surfaces were all parts of perfect spherical surfaces. In 

 reality this is rarely the case, either with the lens or with the 

 cornea. Slight deviations do not produce any marked effect, but 

 there is one deviation, known as regular astigmatism, which, present 

 to a certain extent in most eyes, very largely developed in some, 

 frequently leads to very imperfect vision. This defect is due to the 

 dioptric surface being not spherical but more convex along one 

 meridian than another, more convex, for instance, along the vertical 

 than along the horizontal meridian. When this is the case the rays 

 proceeding from a luminous point are not brought to a single focus 

 at a point, but possess two linear foci, one nearer than the normal 

 focus and corresponding to the more convex surface, the other 

 farther than the normal and corresponding to the less convex 

 surface. If the vertical meridians of the surface be more convex 

 than the horizontal, then the nearer linear focus will be horizontal 

 and the farther linear focus will be vertical, and vice versa. (This 

 can be shewn much more effectually on a model, than in a diagram 

 in which we are limited to two dimensions.) Now, in order to see 

 a vertical line distinctly, it is much more important that the rays 

 which diverge from the line in a series of horizontal planes should 

 be brought to a focus properly than those which diverge in the 



