760 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tion. This action of a convex lens in hypermetropia is indicated by the dotted 

 lines in Fig. 222, C, and the corresponding use of a concave lens in myopia is 

 shown in Fig. 222, B. 



The detection and quantitative determination of hypermetropia are best 

 made after the accommodation has been paralyzed by the use of atropia, by 

 ascertaining how strong a convex lens must be placed before the eye to pro- 

 duce distinct vision of distant objects. 



The range of accommodation varies very much from the normal in myopic 

 and hypermetropic eyes. In myopia the near-point is often 5 or 6 centimeters 

 from the cornea, while the far-point, instead of being infinitely far off, is at a 

 variable but no very great distance from the eye. The range of accommoda- 

 tion is therefore very limited. In hypermetropia the near- point is slightly 

 farther than normal from the eye, and the far-point cannot be said to exist, 

 for the eye at rest is adapted to bring converging rays to a focus on the retina, 

 and such pencils of rays do not exist in nature. Mathematically, the far-point 

 may be said to be at more than an infinite distance from the eye. The range 

 of effective accommodation is therefore reduced, for a portion of the accommo- 

 dative power is used up in adapting the eye to receive parallel rays. 



Presbyopia. The power of accommodation diminishes with age, owing 

 apparently to a loss of elasticity of the lens. The change is regularly pro- 

 gressive, and can be detected as early as the fifteenth year, though in normal 

 eyes it does not usually attract attention until the individual is between forty 

 and forty-five years of age. At this period of life a difficulty is commonly 

 experienced in reading ordinary type held at a convenient distance from the 

 eye, and the individual becomes old-sighted or presbyopic a condition which 

 can, of course, be remedied by the use of convex glasses. Cases are occasion- 

 ally reported of persons recovering their power of near vision in extreme old 

 age and discontinuing the use of the glasses previously employed for reading. 

 In these cases there is apparently not a restoration of the power of accommo- 

 dation, but an increase in the refractive power of the lens through local changes 

 in its tissue. A diminution in the size of the pupil, sometimes noticed in old 

 age, may also contribute to the distinctness of the retinal image, as will be 

 described in connection with spherical aberration. 



Defects of the Dioptric Apparatus. The above-described imperfections 

 of the eye viz. myopia and hypermetropia being generally (though not 

 invariably) due to an abnormal length of the longitudinal axis, are to be 

 regarded as defects of construction affecting only a comparatively small 

 number of eyes. There are, however, a number of imperfections of the diop- 

 tric apparatus, many of which affect all eyes alike. Of these imperfections 

 some affect the eye in common with all optical instruments, while others are 

 peculiar to the eye and are not found in instruments of human construction. 

 The former class will be first considered. 



Spherical Aberration. It has been stated that a pencil of rays falling 

 upon a spherical refracting surface will be refracted to a common focus. 

 Strictly speaking, however, the outer rays of the pencil i. e. those which fall 



