318 



THE SPECIAL SENSES. 



of the eye an amount which may be expressed as equivalent to 

 14 + D (^?p). It is as though the eye were left at rest and a 

 glass lens of 14 + D were placed against the cornea. The de- 

 creasing range of accommodation as age increases is expressed 

 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. Fit 

 along the abscissa represent years; figures along the ordinate represent diopters of accon 



Figures 

 accommo- 

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

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

 point 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): 



Range of accommodation 

 Years. in diopters. 



10 14 



15 12 



20 10 



25 8.5 



30 7 



35 5.5 



40 4.5 



45 3.5 



50 2.5 



55 1.75 



60 1 



65 0.75 



70 0.25 



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



