REASON FOR ACCOMMODATION 253 



retically, it lies at infinite distance, for such an eye must accommodate 

 to some extent to bring even parallel rays (theoretically hailing from in- 

 finity), let alone rays which diverge ever so slightly, to a focus on the 

 retina (Fig. 101). The near point of a far-sighted eye is likewise farther 

 away than that of an emmetropic eye. In myopia the near point is very 

 close, and the far and commencement points coincide. 



But hypermetropia and myopia are abnormalities in human eyes only 

 because human visual requirements are best met by what we call the 'nor- 

 mal' condition of emmetropia. We walk and run fast enough to require 

 sharp vision, without the expense of intraocular muscular exertion, for dis- 

 tances greater than twenty feet ahead of us. If we habitually travelled 

 faster — as we are coming to do in this motor age — we should not need 

 a more distant commencement point, since by relaxing our accommo- 

 dation we can see sharply as far ahead as the clarity of the atmosphere 

 allows. But if we habitually crawled on our bellies, we would be much 

 better off with a closer commencement point, else we should constantly 

 be exerting accommodation for the distances we most needed to be able 

 to s^e ahead. Civilized men are so dependent upon clear images of things 

 which they manipulate with their hands that they need a fairly close near 

 point. If we were all engravers, we would be better off with a still closer 

 one — and the 'normal' human eye would be a myopic one. 



That the human eye, ideally, is emmetropic is thus a mere coincidence, 

 and not a sine qua non for all animal eyes. Naturally, there are many 

 animals with diverse habits which make them need farther or closer com- 

 mencement points and near points than ours. It is absurd to call their 

 eyes inferior or disharmonious simply because they do not happen to be 

 emmetropic. 



Devices Which Make Accommodation Unnecessary — We have 

 seen in earlier chapters that most of the advantages in vision seem to be 

 on the side of large eyes as against smaller ones. Here, with the matter 

 of accommodation, the shoe is on the other foot. In a small animal with 

 small eyes which, ceteris paribus, looks customarily at small objects, the 

 retinal image not only shifts less laterally when the object moves sidewise 

 in the opposite direction, but recedes less within the retinal thickness 

 when the object approaches. Consequently such an eye has a much closer 

 commencement-point than a large eye — it need not begin to accommo- 

 date until the object is much nearer. Not only that, but the visual cells 

 are no smaller in small eyes — just as they are no larger in large eyes, 



