292 



PHYSIOLOGY 



CHAP. 



The following table shows the different degrees of ametropia 

 that correspond to different lengths of the axis of the reduced 

 eye: 



The refractive power of the eye at rest, that is its static re- 

 fraction, alters with age. 



The eye of the new-born, according to Horstinann, is in 88 out 

 of 100 cases hyperme tropic. The curvature of the cornea in the 

 new-born is rather greater than in the adult ; the lens is almost 

 round, the anterior chamber much flattened, and the degree of 

 hypermetropia varies between 1 and 6 D. During infancy and 

 adolescence, up to the tenth year, the hypermetropia diminishes 

 slowly, until the sight becomes emmetropic ; but in the majority 

 of cases a slight degree of hypermetropia persists throughout life, 

 particularly in individuals who do not go to school. The investi- 

 gations of Falkenberg and Straub on recruits showed that young 

 people who appear to have normal sight are in many cases slightly 

 hypermetropic when the eye is under the influence of atropine, 

 which paralyses the activity of the ciliary muscle. 



Myopia is very rare in infancy ; but the percentage increases 

 with age, particularly among school children. 



In later years, at about fifty, emmetropic eyes become slightly 

 hypermetropic, and eyes that are somewhat hypermetropic become 

 more so, apart from the weakening of the mechanism of accom- 

 modation due to age. This fact depends on the changes brought 

 about by age in the structure and composition of the lens. 



In the different forms of ametropia the sight, owing to diffusion- 

 circles, is confused and indistinct in direct proportion to : 



(a) The degree of ametropia, that is, as the retina is farther 

 removed from the plane on which the image is formed. 



(V) The size of the cone of rays which penetrate the pupillary 

 aperture, that is, to the size of the pupil which limits the cone. 



In both these cases there is increased width of the diffusion - 

 circles, which produces a confusion of points and consequent 

 indistinctness of the outline of the images. If we look through a 

 small hole made by a pin in a card (stenopaic diaphragm) the 

 luminous cone that enters the eye is small, and the images become 

 sharper in proportion as the diameter of the diffusion -circles is 

 reduced. This is plain from Fig. 131, in which ab represents the 

 outline of a wide, cd of a narrow pupil ; is the object, / the 



