296 THE SPECIAL SENSES. 



muscle effects a change in the curvature of the lens,* but none is 

 so simple and, on the whole, so satisfactory as the one suggested 

 bv Helmholtz. 



It is interesting to note that in fishes accommodation is effected in a 

 different way, namely, by movements of the lens forward and backward. 

 In these animals the eye when at rest is accommodated for near vision, and 

 to see objects at a distance the refractive power of the eye is diminished by 

 the contraction of a special muscle, retractor lentis, which pulls the lens toward 

 the retina, f 



Limit of the Power of Accommodation Near Point of 

 Distinct Vision. When an object is brought closer and closer to 

 the eye a point will be reached at which it is impossible by the 

 strongest contraction of the ciliary muscle to obtain a clear image 

 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 : 



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- 



* See Tscherning, " Optique physiologique," Paris, 1898 ; and Schoen, 

 " Archiv f. die gesammte Physiologic," 59, 427, 1895. 



fSee Beer, "Wiener klinische Wochenschrift, " 1898, No. 12. 



