318 



THE SPECIAL SENSES. 



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



14 + D ( 



1000 



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. 



extremes noted in these 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. 13 la): 



Range of accommodation 



in diopters. 



14 



Years. 

 10 

 15 

 20 

 25 

 30 

 35 

 40 

 45 

 50 

 55 

 60 

 65 

 70 



12 



10 

 8.5 

 7 



5.5 

 4.5 

 3.5 

 2.5 

 1.75 

 1 



0.75 

 0.25 



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



