THE SENSE OF SIGHT. 663 



oculi is followed by relaxation of the sphincter muscle and dilatation of the 

 pupil. Stimulation of the sympathetic is followed by a more pronounced 

 dilatation. The size of the pupil is the resultant of a balancing of these 

 two forces. 



OPTIC DEFECTS. 



Presbyopia. Presbyopia may be defined as a condition of the normal 

 eye in which the accommodation has become so reduced by age that reading has 

 become impossible at ordinary distances. As age advances the lens loses 

 its elasticity and the power to increase in refraction, and vision at the normal 

 reading distance becomes impossible. The near point, the punctum prox- 

 imum, therefore, advances toward the far point, or recedes from the indi- 

 vidual. The range of accommodation is also diminished. At forty years 

 the near point is about 22 cm.; at forty-five years it has receded to 28 cm. 

 This would indicate that the lens in these five years has lost i dioptry of refracting 

 power; at fifty years the near point recedes to 43 cm., and at sixty to 200 

 cm., indicating a loss in refracting power on the part of the lens of 2 and 4 

 dioptrics respectively. Convex lenses placed before the eyes having a 

 refracting power of i, 2, and 4 dioptrics would in the three instances return 

 the near point to its normal position. At the age of seventy the lens is 

 incapable of any increase during an accommodative effort. A lens of 4 

 dioptrics would therefore be required by such a man, for clear vision at 10 

 inches or 22 centimeters. 



Myopia. Myopia may be defined as a condition of the eye characterized 

 by an increase in the antero-posterior diameter or a hypernormal refracting 



FIG. 321. MYOPIA. Parallel rays FIG. 322. CORRECTION OF MYOPIA BY 

 focus at F, cross and form diffusion- A CONCAVE LENS. 



circles; divergent rays from A focus 

 on the retina. 



power of the lens. The former is the usual condition. Parallel rays of 

 light brought to a focus in front of the retina again diverge, giving rise to 

 diffusion-circles and indistinctness of the image. Divergent rays alone 

 are capable of being focalized on the retina in its new position. The distant 

 point, the punctum remotum is always at a finite distance, but approaches 

 the eye as the myopia increases. The near point is usually much nearer the 

 eye than 20 cm. For this reason the condition is termed near sight. 

 (Fig. 321). 



The increase in the length of the antero-posterior diameter may range 

 from a fraction of a millimeter up to 3.8 mm. With an increase of 0.16 mm. 

 the far point is but 200 cm. distant; and with an increase of 3.8 mm. it is but 

 10 cm. distant. Inasmuch as only divergent rays can be focalized by the 

 myopic eye normal vision can be restored by the use of a biconcave lens with 



