388 PHYSIOLOGICAL PHYSICS. [Chap. xxix. 



which, in the normal eye, = infinity. Therefore, nor- 

 mally p = -j ; 12 cm. is the focal distance of the con- 



vex lens, which represents the power of accommodation. 

 Presbyopia is the term applied when the range 

 of accommodation becomes diminished, usually as the 

 result of age. The punctual proximum is farther and 

 farther removed, probably because both a flattening 

 of the lens and a diminished elasticity prevent it 

 assuming the same degree of convexity as formerly, 

 The deficiency in accommodation may be rectified by 

 a convex lens, which, placed before the eye, would 

 give the rays the direction they would have if they 

 proceeded from the near point in a normal eye. Let 

 p = the normal near point, and let the near point of 

 the presbyopic eye be 30 cm., then 



10 30 " ~ / 



where / = the focal length of the desired lens. 

 For very fine work Donders makes P 8 Paris inches 

 (21 '66 cm.). Presbyopia, then, it is to be noted, is an 

 anomaly of accommodation, thus differing from short- 

 sightedness and long-sightedness, which are anomalies 

 of refraction ; these must now be considered. 



Anomalies of refraction. - - Hypermetropla 

 and myopia. We have seen that in the normal or 

 emmetropic eye, parallel rays of light are brought to a 

 focus on the retina. The eye may not be normal, 

 however, and the focus for parallel rays may be behind 

 the retina, or in front of the retina, in both cases 

 circles of diffusion being formed. The former condition 

 is termed hypermetropia or long sight, the latter 

 myopia or short sight. The reason of the terms long 

 or short sight is apparent. The hypermetropic eye 

 does not form images of objects at a long distance on 

 the retina with the accommodation in repose, else, in 



