108 BIOPHYSICAL CHARACTERISTICS OF THE EYE 



Amplitude of Accommodation 



Objects at a great distance are seen distinctly, as far as their definition 

 permits, without accommodation. This condition is called the eye at 

 rest. Practically all objects beyond a distance of 20 to 30 ft (6 to 10 

 meters) focus on the retina without muscular effort; hence, this distance 

 is usually referred to as the far point. The near point is that point on 

 the axis which is seen distinctly when the crystalline lens has its greatest 

 refracting power. In youth this may be as little as 10 cm. The ampli- 

 tude of accommodation is defined as the distance of the near point from 

 the far point. 



TABLE III-3 



Loss op Accommodation with Advancing Years 



Age in years 10 15 20 25 30 35 40 45 50 55 60 65 70 



Power (F) of ac- 

 commodation in 

 diopters 14.0 12.6 11.2 9.9 8.5 7.1 5.7 3.7 1.9 1.2 1.0 1.0 1.0 



From A. Duane's curves [1912]. 



The faculty of accommodation is greatest in early life, and diminishes 

 rapidly with advancing years. In this process the near point gradually 

 recedes, but the far point remains practically stationary until the age of 

 50 years. At 10 years the amplitude or range is from infinity to 7 cm 

 when the maximum accommodation is used. At 20 and 40 years this 

 near point lies at 10 and 22.2 cm, respectively, from the principal point. 

 When the near point has retreated to a distance beyond 25 cm, so that 

 it is no longer possible to read or write conveniently without spectacles, 

 the condition of presbyopia, or old-age vision, has begun to set in. 

 After this it becomes necessary to add a convex lens to the eye so that 

 one may see distinctly at the usual working distance. The decreasing 

 power of accommodation as age increases is expressed conveniently in 

 the number of diopters which may be added to the refractive power* of 

 the eye. Table III— 3 shows the results obtained by A. Duane [1912], 

 from a comparative study of 1050 cases, for the mean power of accom- 

 modation for different ages. The near point is measured from the 

 anterior focus of the eye, i.e., from a point 15.2 mm in front of the cornea. 

 The gradual reduction in the power of accommodation is attributed to 

 the gradual decrease in elasticity of the lens. 



* Reciprocal of the focal length. Units diopters. 



