REFRACTIVE ERRORS OF THE EYE 



27 



accommodation process, unaided by convex spectacles, is inadequate to 

 pull the image forward onto the retina and the sharp picture lies behind 

 the eye (hypermetropia or far-sightedness). In others, the eyeball is ab- 

 normally elongated (Fig, 12, bottom diagrams) and the image lies so 

 far forward in the vitreous (except when the object is very close to the 

 eye) that concave spectacles are required to move the focus of the lens 

 backward and place the image on the retina (myopia or near-sightedness) . 



Object At Great Distance; Object At Walking Distance: Object At Reading Distance 



15 



o o^ 



tr 

 H o 



tr — 



receptive (visual-cell) layer 

 rays focus behind eye 



some accommodation 



much accommodation 



rays focus in receptive layer 



rays focus in receptive layer 



no accommodation 



some accommodation 



2 — 



rays focus at inner surface 

 of receptive layer 



rays focus at outer surface 

 of receptive layer 



rays focus in receptive layer 



no accommodation 



little or no accommodation 



9r 



rays focus in front 

 receptive layer 



rays 

 of t 



rays still focus in front 

 of receptive layer 



rays focus in receptive layer 



Fig. 12 — Spherical refractive errors of the eye. 



Shows the extent of accommodation required, and the location of the images, in hyper- 

 metropic or far-sighted eyes (top row), normal eyes (middle row), and myopic or near- 

 sighted eyes (bottom row). 



A third refractive error to which the human eye is prone is 'astigmatism', 

 a condition in which the retinal image of a point is not a point but a 

 line, owing to one of the refracting surfaces (almost always the cornea) 

 being partly cylindrical as well as spherical in its curvature (Fig. 13). 

 This results in a blurring of objective lines running in certain directions. 

 The error is easily corrected, when it is regular as indeed it usually is, 

 by the appropriate counteracting cylindrical curvature formed on the 



