ii4 HUMAN PHYSIOLOGY 



The power of positive accommodation varies at different 

 ages, being greatest in young children, since in early life the 

 lens is most convex. 



The distance of the near point in cms. is represented in 

 the accompanying figure. The "range of accommodation," 



FIG. 51. To show Variations in the power of Positive Accommodation 

 throughout life. 



i.e. the difference between the "near point" and the "far 

 point," steadily decreases as age advances. After about 

 sixty years of age, on account of the flattening of the lens, 

 not even parallel rays can be focussed except by using 



FIG. 52. To illustrate Presbyopia, Myopia, and Hypermetropia. A, emmetropic 

 eye; B, presbyopic eye; C, hypermetropic eye; D, myopic eye; N.P.o. 

 the near point, and F.P.'S., the far point of accommodation. 



positive accommodation. This is the fully developed con- 

 dition of Presbyopia old-sightedness (Fig. 52, B). 



Imperfections of the Dioptric Mechanism (1) Myopia. 

 In certain individuals the antero-posterior diameter of the eye 



