520 VISION 



Eyes of the third kind,, where the focal point falls behind the retina are 

 called hypermetropic, or long-sighted (Fig. 215, A). In order that incident 

 rays may be brought to a focus on the retina of such an eye, they must 

 already be convergent as they enter the eye. Since, however, converging rays 

 never occur in nature, it is evident that a hypermetropic eye, not provided 

 with artificial lenses, can focus parallel or divergent rays accurately only by 

 accommodation ; in short, the hypermetropic eye, if it is to see without glasses 

 must always be accommodated. 



Of the three kinds of eyes the emmetropic is without doubt the best adapted 

 to its purpose; for, as we have seen, rays from objects more than 5 m. distant 

 may be regarded as practically parallel for the eye so that the unaccommodated 

 emmetropic eye can form a distinct picture of all such objects. The hyper- 

 metropic eye can adjust itself for far distant and near objects by accommoda- 

 tion. But the myopic has no means at all of adjusting itself for distant objects 

 and from this point of view at least must be regarded as the least serviceable 

 of the three. 



The far point of the eye is that point from which proceed light rays with 

 the least divergence that can be focused by the eye. In the emmetropic eye 

 the far point lies of course at an infinite distance. The far point of the 

 myopic eye lies at a finite distance in front of the eye. The far point of 

 the hypermetropic eye lies behind the eye. It represents the point of con- 

 vergence of those rays which, after refraction in the unaccommodated eye, are 

 brought to a focus on the retina. 



We say therefore with reference to its structure that the refracting power 

 of the myopic eye is too strong, that of the hypermetropic eye too weak. 



By suitably chosen lenses both the myopic and the hypermetropic eye can 

 be made to focus parallel rays on the retina. If we place before a myopic eye 

 a double concave lens of such a strength as to give the parallel rays the direc- 

 tion they would have if they came from the far point of the eye, it is evident 

 that now the combination, lens -|- the eye, affects parallel rays, just the same 

 as does an emmetropic eye. 



If we place before a hypermetropic eye a double convex lens which con- 

 verges parallel rays to its far point, then the combination, lens -f- the eye, 

 must again be equal to the emmetropic eye. 



The degree of myopia or hypermetropia is measured by the refracting 

 power of the lens necessary to make the eye emmetropic. It is evident at 

 once that the focal point of this lens coincides with the far point of the eye 

 and the degree of myopia or hypermetropia is therefore expressed by the 

 reciprocal value of the distance of the far point from the eye. This correction 

 lens determines also the static refraction of the eye i. e., the amount of 

 refraction taking place without accommodation. 



4. OPTICAL DEFECTS OF THE EYE 



In our discussion thus far we have silently assumed that the e}^e is a 

 perfectly constructed optical instrument that its refracting media are per- 

 fectly transparent, their surfaces exactly spherical and the centers of curvature 



