ACCOMMODATION 35 



focuses at one-half meter, a five diopter lens at one-fifth of a meter, and 

 so on. The emmetropic eye (Fig. 12, middle row of diagrams) focuses 

 parallel rays on its receptive layer when it is not accommodating. If now 

 a one-diopter lens is added, like a spectacle, in front of the relaxed eye, 

 an object one meter away will be imaged on the retina. A four-diopter 

 spectacle will enable the non-accommodating eye to image sharply an 

 object only a quarter of a meter distant. So, we may say that the amount 

 of accommodation being exerted by an emmetropic eye is four diopters 

 when, without a spectacle, it images an object at one-fourth of a meter. 



-near point at 2 



near point at reading distance 

 ^near point at arnn's length 

 ^^^^^^^j-near point at 13' 



50 60 70 



Age In Years 



Fig. 15- — Decrease of human accommodation with age, owing to the progressive hardening 

 of the body of the lens. Plotted from data of Donders on emmetropic subjects. 



By accommodating to a certain extent — four diopters' worth — the focus- 

 ing power of the crystalline lens has been increased by four diopters over 

 its strength when at rest; for, this amount of accommodation can take 

 the place of a four-diopter spectacle placed before the non-accommo- 

 dating eye. 



The range of accommodation — that is, the greatest increase in the 

 focusing power of the lens — which a person can produce is unfortunately 

 not a fixed quantity (Fig. 15). Almost as inevitable as death and taxes 

 is a decrease in that range, with age, to such an extent that the indi- 

 vidual (unless substantially myopic to begin with) becomes unable to 



