EYESTRAIN. 739 



right movement and proper ' senseing ' of the world thus depends upon 

 the exactness of this image-making power of the eye. It seems prob- 

 able, indeed, that one of the greatest factors in the survival of the 

 fittest or in the elimination of the unfit, in biologic and social evolution, 

 has been this overlooked optical inaccuracy of the eye. In the chase, 

 in battle, in games, in all tribal and industrial competitions and tasks, 

 the imperfectly-seeing must have gone down before those whose eyes 

 saw more perfectly and whose answering hand and foot executed the 

 precedent ocular command more speedily and precisely. 



The malcurvature, or astigmatism, of the eye, may obviously be of 

 widely varying degrees, and may be placed in any possible one of the 

 180 degrees of its half-circle. So infinite are the permutations of these 

 amounts and placings, and so complicated may astigmatism be with 

 any kinds of short-sightedness or long-sightedness that, as emphasized, 

 in all probability no one eye in the world has identically the same 

 optical measurements and powers as another. No pair is with- 

 out optical imperfections and none has exactly the same ones as those 

 of any other. The chief and continuous cause of astigmatism is the 

 pressure of the upper lid upon the cornea just at the upper edge of the 

 pupil. The astigmatism in the vast majority of all eyes shows that the 

 cornea is curved more in its approximately vertical than in its horizontal 

 meridian. The few cases in which this kind of curvature is not present 

 are really ' exceptions which test the rule/ The habitual placing of 

 the upper lid at this precise line of the cornea is caused by the necessity 

 of shading the cornea or protecting it from the light which would enter 

 from above and dazzle or harm the delicate retina. Every one when 

 facing a bright light has found how bad vision is for at least some 

 minutes afterwards. This semi-paralysis of retinal function would 

 make activity slow and inconsequent, if not often jeopardize life. It 

 has been a helpful factor during the evolutionary struggle in preserving 

 the organism. 



A study of the position of the upper eyelid and coordinated pupil- 

 lary movements in animals would show each type meeting or avoiding 

 the difficulty by many devices. In such animals as fishes, birds, owls 

 and many forest-roving animals there is danger from above as well as 

 below, so the upper lid is kept well retracted and the pupil wide open. 

 In the owl the pupil is large even in daylight, and hence this bird is then 

 in a dazed condition of mind, and flight is dangerous. The mechan- 

 isms designed to give definition to the retinal image, to shade the retina 

 so that it may recuperate its sensibility for the next instant's instan- 

 taneous image, accentuate the fact that the formation of the retina 

 was perhaps the most difficult task encountered in the development of 

 higher forms of life on the globe. All organic success depended upon 

 that special success. 



