DIOPTRIC MECHANISMS OF THE EYEBALL 



609 



rise finally to disturbances of vision which are known as presbyopia. 

 The ordinary reading distance is about ten inches. A smaller distance 

 than this is rarely made use of even in youth, when the point of near 

 vision is only three or four inches from the eye, on account of the 

 effort required to converge the axes of the two eyes to this extent. 

 The effect of the gradual diminution in the elasticity of the lens is 

 noticed as soon as the point of near vision recedes beyond ten or 

 twelve inches, i.e. 25 to 30 cm. This occurs as a rule between forty- 

 five and fifty, when we begin to experience difficulty in reading small 

 print, since the visual angle subtended by such print at a distance 

 over ten or twelve inches is too small to allow of distinct vision. 

 The condition of presbyopia, is remedied by employing reading 



FIG. 273. Accommodation in a bird's eye. (BEER.) 

 B, rest ; A, accommodation for near objects. 



glasses, i.e. by wearing spectacles which converge the rays of light 

 and so enable small objects to be brought nearer to the eye than its 

 near point. It is evident that these glasses must be strengthened 

 continually with advancing age. No trouble is experienced in seeing 

 distant objects, the refraction of the eye at rest remains as before. 

 The dimness of vision in extreme old age for distant as well as near 

 objects is due to changes, not in the refractive power of the eye, but 

 in the transparency of the refractive media, such as cataract, i.e. 

 opacity of the lens, opacity of the cornea, and so on. The former con- 

 dition can often be remedied by extracting the lens and replacing it 

 by glasses of ten dioptres. 



THE COMPARATIVE PHYSIOLOGY OF ACCOMMODATION 



The mechanism of accommodation which we have studied in man is found 

 with very little modification throughout the whole group of mammalia, though, 

 in the domestic animals at any rate, the range of accommodation is very 

 much less than in man. On examining other types of animals we meet, 

 as was shown by Beer, an amazing variety of methods by which the focusing 

 range of the eye may be altered. In order to bring distinct images of objects 



39 



