544 THE NERVOUS SYSTEM. 



from an indefinite distance, and therefore sensibly parallel, are brought 

 to a focus at the retina (Fig. 142). Vision is accordingly distinct, even 

 for the heavenly bodies, provided their light be neither too dim nor too 

 brilliant. For objects situated nearer the eye, the convexity of the lens 

 increases with the diminution of distance, and vision remains perfect. 

 But there is a limit to the change in shape, of which the lens is capa- 

 ble ; and when this limit is reached, a closer approximation of the 

 object destroys the accuracy of its image. For ordinary normal 

 eyes, in the early or middle periods of life, accommodation fails and 

 vision becomes indistinct, when the object is placed at less than 15 

 centimetres (6 inches) from the eye. 



Between these two limits, of 15 centimetres and infinity, the accom- 

 modation required is by no means in simple proportion to the distance. 

 The accommodation necessary for objects situated respectively at 15 

 and 30 centimetres from the eye (6 inches and 12 inches), is much 

 greater than for the distances of one yard and two yards. The 

 farther the object recedes from the eye, the less difference is pro- 

 duced, in the divergence of the rays, by an additional distance ; and 

 consequently less change is required in the refractive condition of 

 the eye. It is generally found that no sensible effort of accommoda- 

 tion is needed for objects situated beyond fifty feet from the observer ; 

 while within this limit the accommodation necessary for distinct vision 

 increases rapidly with the diminution of distance. 



An eye which is capable of distinct vision, throughout the whole 

 range between 15 centimetres and an indefinite distance, is, in this 

 respect, a normal eye, and is said to be emmetropic ; that is, its 

 powers of accommodation are within the natural limits or measure- 

 ments of this function. 



Presbyopic Eye. The power of accommodation naturally diminishes 

 with the advance of age; and observation shows that this diminution 

 dates from the earliest period of life. Infants often examine minute 

 objects at very short distances, in a manner which would be imprac- 

 ticable for the healthy adult eye ; and the minimum distance of dis- 

 tinct vision at twenty years of age is placed by some writers' at ten 

 centimetres instead of fifteen. The power of increasing the convexity 

 of the lens to this extent is soon lost ; and, as it continues to diminish, 

 a time arrives, usually between the ages of 40 and 50 years, when the 

 incapacity of accommodation for near objects begins to interfere with 

 the ordinary occupations of life. When this condition is reached, the 

 eye is said to be presbyopia. Its vision is still perfect for distant 

 objects, but it can no longer adapt itself to those in close proximity. 

 To remedy this defect the patient employs a convex eye-glass, which 

 gives him an increased refraction for the examination of near objects ; 

 and he is thus enabled to read or write at ordinary distances and in 

 characters of the ordinary size. 



The use of a convex eye-glass does not restore the perfection of 

 sight as it existed beforehand. In the normal eye, the degree of 



