114 PRACTICAL PHYSIOLOGY 



sufficiently increased curvature of the crystalline lens to accommodate 

 for objects as near as eight inches. It may here be mentioned that in the 

 normal eye continuous exercise of the full power of accommodation 

 rapidly produces fatigue. It is impossible without fatigue to use more 

 than a half to two-thirds of the full accommodation for any protracted 

 period. The normal-sighted person instinctively avoids placing near 

 objects, e.g. a book, closer to the eye than about sixteen inches. 

 Similarly a presbyopic person whose near point is, say, ten inches, will 

 hold a book at about twenty inches distance. Unless the illumination 

 be extremely good the small size of the retinal image causes some diffi- 

 culty to occur in reading. This, however, may easily be corrected by 

 assisting the crystalline lens through using convex glasses, the degree of 

 convexity corresponding to the extent of failure of accommodating 

 power. These are, of course, only necessary when looking at objects 

 close to the eye. It may be mentioned here that the distance of the 

 near point gradually increases from infancy to old age. According to 

 Landolt it is about 3 inches at 10 years of age, 4 inches at 20 years, 5J 

 inches at 30 years, almost 9 inches at 40 years, 16 inches at 50 years, 

 40 inches at 60 years, at 70 years about 13 feet, and at 75 there is 

 practically no near point, in other words the power of accommodation 

 is generally lost. 



Ametropia. This is a term applied to all conditions of the eye in which 

 the retina does not lie at the focus for parallel rays. The retina may lie 

 in front of this focus when we have the condition of hypermetropia^ or 

 behind when myopia is the result, or the focus may be a linear one for 

 any distant luminous point when we have the condition of astigmatism. 



Hypermetropia. In this condition the antero-posterior axis of the eye 

 is generally too short. By some effort of accommodation, distant 

 objects may form a clear image on the retina, but the individual 

 suffering from this optical defect does not possess sufficient power of 

 accommodation to focus clearly near objects. Though the emmetropic 

 condition with much facility of accommodation is acquired at about the 

 age of eight years, before this stage is reached the eye is naturally 

 hypermetropic. A young child with marked hypermetropia and deficient 

 power of accommodation will in viewing near objects (e.g. reading), 

 make every effort with both eyes to accommodate for such objects. In- 

 cluded in this effort would be an exaggerated action of the muscles pro- 

 ducing convergence of the optic axes of the eyes, leading to squint, but 

 such squint will frequently be removed on correcting the optical defect. 



EXPERIMENT. Using Kiihne's artificial eye, place the retinal screen 

 in the position necessary to obtain a clear image of the external 

 luminous arrow. In this position of the retina the condition of the eye 



