THE SENSE OF SIGHT. 



621 



vision at the normal near point becomes impossible. The latter, 

 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 refract- 

 ing power on the part of the lens of 2 and 4 dioptrics respectively. 

 Convex lenses placed before the eyes having a refracting power of 

 1,2, 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 4.5 diop- 

 tries would therefore be required for near vision. 



Myopia. This is a condition of the eye characterized by an 

 increase in the antero-posterior diameter or a hypernormal refracting 

 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 



FIG. 294. MYOPIA. Parallel rays 

 focus at F, cross and form diffu- 

 sion-circles; divergent rays from 

 A focus on the retina. (Hansett 

 and Sweet.} 



FIG. 295. CORRECTION OF MYOPIA 

 BY A CONCAVE LENS. (Hansell 

 and Sweet.) 



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 punctum remotum is always at a definite 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. 



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

 range from a fraction of a millimeter up to 10 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 a diverging power in the first 

 instance of 0.5 dioptry and the second of 10 dioptrics. 



Hypermetropia. This is a condition of the eye characterized by 

 decrease of the normal antero-posterior diameter or by a subnormal 

 refracting power of the lens. The former is the usual condition. 

 Parallel rays of light do not, therefore, come to a focus when the 



