SENSE OF SIGHT. 635 



time, its details are by no means clearly understood ; and explanations, 

 varying more or less from that given above, have been proposed by 

 observers of very high authority. The direction and degree in which 

 pressure would be exerted, by muscular fibres attached like those in the 

 interior of the eye, are too imperfectly known to warrant a positive 

 statement in this respect. 



Limits of Accommodation for the Normal Eye. The normal eye is 

 so constructed that rays emanating from a single point, though coming 

 from an indefinite distance, and therefore sensibly parallel to each other, 

 are brought to a focus at the retina (Fig. 203). Vision is accordingly 

 distinct, even for the heavenly bodies, provided their light be neither too 

 dim nor too excessive in brillianc3 T . For bodies situated nearer to the 

 eye, the convexity of the lens increases with the diminution of the dis- 

 tance, and vision still remains perfect. But there is a limit to the change 

 of shape which the lens is capable of assuming ; and when this limit is 

 reached, a closer approximation of the object necessarily 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 amount 

 of accommodation required is by no means in simple proportion to the 

 variation of the distance. The change of accommodation necessary for 

 objects situated respectively at 15 and 30 centimetres from the eye (6 

 inches and 12 inches), is much greater than that corresponding to the 

 distances of one yard and two yards. The farther the object recedes 

 from the eye, the less diiference is produced, in the sensible divergence 

 of the rays, by any additional increase of distance ; and consequently 

 less variation is required in the refractive condition of the eye to pre- 

 serve the accuracy of its image. It is generally found that no sensible 

 effort of accommodation is required for objects situated at any distance 

 beyond fifty feet from the observer ; while within this limit the amount 

 of accommodation necessary for distinct vision increases rapidly with 

 the diminution of the distance. 



An eye which is capable of accommodating for distinct vision, through- 

 out the whole range included 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 placed within the natural limits 

 or measurements of this function. 



Presbyopia Eye. The power of accommodation diminishes naturally 

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

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

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

 ticable for the healthy adult eye ; and the minimum distance of distinct 

 vision at twenty years of age is placed by some writers at ten centi- 

 metres instead of fifteen. The power of increasing the convex^ 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 



