314 



THE SPECIAL SENSES. 



conveniently in the number of diopters which may be added to the 

 refractive power of the eye by the action of the ciliary muscle. 



Fig. 131a. — Curves showing the variation in the range of accommodation with age. Figures 

 along the abscissa represent years; figures along the ordinate represent diopters of accommo- 

 dation: C C, Represents the mean or average of over 1000 cases, while A A and B B gives the 

 extremes noted in these cases. It should be stated that the maximum accommodation or near 

 I)oint was reckoned from the anterior principal focus (13 mm. in front of the cornea) instead 

 of from the anterior surface of the cornea (Duane). 



The following table illustrates the usual range of accom- 

 modation for different ages. (Somewhat different results are ex- 

 pressed in the curve* represented in Fig. 131a): 



Years. 

 10 

 15 

 20 

 25 

 30 

 35 

 40 

 45 

 50 

 55 

 60 

 65 

 70 



Range of accommodation 

 in diopters. 

 . . 14 



12 

 10 



8.5 



7 



5.5 



4.5 



3.5 



2.5 



1.75 



1 



0.75 



0.25 



It will be noted that at sixty to seventy years of age the power 

 of accommodation has practically disappeared. The eye, if other- 

 wise normal, is set for parallel raj^s, that is, for objects at a distance 

 of 20 feet or more. Distinct vision of near objects can only be 

 obtained by using convex glasses of a suitable refractive power. 

 * Duane, "The Ophthalmoscope," Sept., 1912. 



