Page:Popular Science Monthly Volume 67.djvu/745

Rh right movement and proper 'senseing' of the world thus depends upon the exactness of this image-making power of the eye. It seems probable, 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 without 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 pupillary 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 mechanisms designed to give definition to the retinal image, to shade the retina so that it may recuperate its sensibility for the next instant's instantaneous 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.