;oo 



TEXT-BOOK OF PHYSIOLOGY 



in size. The near point, the point from which divergent rays can be focal- 

 ized, therefore advances toward the far point, or recedes from the individual. 

 The range of accommodation is, thereby, diminished. At forty years the near 

 point is about 22 cm., indicating an increase in refracting power on the part 

 of the lens of 4.5 dioptrics; at forty-five years the near point has receded to 

 28 cm. This indicates that the lens at this period has only a refracting 

 power of approximately 3.5 dioptrics, showing therefore a loss in the five 

 years of i dioptry; at fifty years the near point recedes to 40 cm., and at sixty 

 to 100 cm., indicating a loss in refracting power, ptf the part of the lens of 

 2 and 3.5 dioptrics respectively. Convex lenses placed before the eyes hav- 

 ing a refracting power of i, 2, and 3.5 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 

 dioptrics would therefore be required by such a man to return the near point 

 to its normal position, 22 cm. from the eye. 



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

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

 ting power of the lens. The former is the usual condition. In either case 

 parallel rays of light which enter the eye are brought to a focus in front of 



FIG. 304. MYOPIA. Parallel rays 

 focus at F, cross and form diffusion- 

 circles; divergent rays from A focus 

 on the retina. 



FIG. 305. CORRECTION OF MYOPIA BY 

 A CONCAVE LENS. 



the retina, after which they diverge and give rise to diffusion circles and indis- 

 tinctness of vision. Divergent rays, however, which enter the eye are focal- 

 ized as usual on the retina even 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. 304). 



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 established by placing before the eyes a bi- 

 concave lens with a diverging power in the first instance of 0.5 dioptry and 

 the second of 10 dioptrics (Fig. 305). 



Hypermetropia. Hypermetropia may be defined as 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 con- 

 dition. In either case parallel rays of light which enter the eye are, therefore, 

 not brought to a focus when the accommodation is suspended. Falling on 

 the retina previous to focalization, they give rise to diffusion-circles and indis- 

 tinctness of the image. As no object can be seen distinctly no matter how 



