312 THE SPECIAL SENSES. 



of the object. The rays from it are so divergent that the refractive 

 surfaces are unable to bring them to a focus on the retina. Each 

 luminous point makes a diffusion circle on the retina, and the 

 whole image is indistinct. The distance at which the eye is just 

 able to accommodate and within which distinct vision is impos- 

 sible is called the near point. Observation shows that this near 

 point varies steadily with age and becomes rapidly greater in dis- 

 tance between the fortieth and the fiftieth year. In the case of the 

 normal eye the recession of the near point varies so regularly with 

 age that its determination may be used to estimate the age of the 

 individual. Figures of this kind are given (see also p. 314) : 



Age. Near Point. 



10 7 cm. or 2.76 in. 



20 10 " " 3.94 " 



30 14 " " 5.61 " 



40 22 " " 8.66 " 



50 40 " " 15.75 " 



60 100 " " 39.37 " 



This gradual lengthening of the near point is explained usually 

 by the supposition that the lens loses its elasticity, so that con- 

 traction of the ciliary muscle has less and less effect in causing an 

 increase in its curvature. The process starts very early in life, 

 and is one of the many facts which show that senescence begins 

 practically with birth. The change in near point in early life is so 

 slight as to escape notice, but after it reaches a distance of about 

 25 cm. (about 10 inches) the fact obtrudes itself upon us in the use 

 of our eyes for near objects, — reading, for example. The condition 

 is then designated as old-sightedness or presbyopia. Most normal 

 eyes become so distinctly presbyopic between the fortieth and the 

 fiftieth year as to require the use of glasses in reading. If no other 

 defect exists in the eye, this deficiency of the lens is readily over- 

 come by using suitable convex glasses to aid the eye in focusing 

 the rays. It is obvious that in such cases the glasses need not be 

 used except for near work. 



Far Point of Distinct Vision. — ^The normal eye is so adjusted 

 that parallel rays are brought to a focus on the retina. The far 

 point is therefore theoretically at infinity. Objects at a great 

 distance are .seen distinctly, as far as their size permits, without 

 accommodation, — that is, with the eye at rest. Practically it is 

 found that objects at a distance of 6 to 10 meters (20 to 30 feet) send 

 rays that are sufficiently parallel to focus on the retina without 

 muscular effort on the part of the eyes, and this distance, therefore, 

 measures the practical far point, punctum reniotum, of the normal 

 eye. The rays at this distance are, in reality, somewhat divergent, 

 and that they produce a distinct image without an act of accom- 

 modation may be due to the fact that the rods and cones, the really 

 sensitive, part of the retina, do not form a mathematical plane, but 



