616 A TEXTBOOK OF PHYSIOLOGY 



Children possess a greater power of accommodation than adults. 

 The near point of vision gradually increases \vith advancing age, so 

 that at about the age of forty it is outside the normal reading distance 

 of 10 inches, and print has to be held farther away in order to be read. 

 This is the condition known as presbyopia, and is attributed to a 

 diminished elasticity of the lens and atonicity of the ciliary muscle. 

 It is remedied by biconvex glasses, which help to focus the rays 

 correctly on the retina. 



In the condition known as hypermetropia, or long sight, the 

 parallel rays are not brought to a focus, because the eyeball is shorter 

 than normal in its antero-posterior axis (Fig. 347, A). To see even 



FIG. 347. DIAGRAM SHOWING A, COURSE OF PARALLEL RAYS IN THE HYPER METROPIG* 

 EYE ; B, PATCH OF PARALLEL RAYS IN HYPERMETROPIC EYE AFTER CORRECTION 

 BY MEANS OF A CONVEX LENS. 



distant objects well, the subject has to accommodate slightly, and 

 this throws a strain upon the eye. The condition is corrected by a 

 convex lens (Fig. 347, B). 



The opposite condition is known as myopia. In this condition 

 a person is short-sighted, and cannot see distant objects at all. The 

 defect is due to an increased antero-posterior axis of the eye, the 

 eyeball being longer than normal (Fig. 348, A). This being the case, 

 parallel rays from a distance are brought to a focus in front of the 

 retina. The defect is often associated with some degree of astigma- 

 tism, and is corrected by the use of biconcave lenses of suitable 

 strengths (Fig. 348, B). 



The standard lens for ophthalmic purposes is one which has a 

 focal length of 1 metre ; its refractive power is then said to be 1 diopter, 

 or 1 D. A 2-diopter (2 D.) lens has therefore a focal length of L 



