Page:Popular Science Monthly Volume 67.djvu/746

740 And few have the most dim notion of the complexity of the organ of vision in man, or of the amazing difficulties of 'Biologos' in fashioning and perfecting it. Millions of finger tips are bunched together in the one-inch cup of the eyeball, from whence run about 425,000 nerve fibrils to a topographic mechanism of sensation in the occipital lobe. The eye can see an object 1/1,000 of an inch in diameter. The cones and rods are only 1/10,000 or 1/14,000 of an inch in diameter, and a million cones at the macula occupy a space of only 1/10 of an inch square. These crowded finger tips perceive the shape of the picture and the intensities of the light stimuli of all illuminated objects, of a millionth of a millionth of the kinetic power of any other physiologic force, and of so short a duration as the 0.00144 of a second. And out of these infinitesimal waves the sensations called light and color are created. The mechanism which creates them must be in intimate and instant connection with the centers initiating and controlling every other sensation, of every motion, of every muscle of the body. Imagine for an instant what takes place in every animal and human being every day of its existence: e. g., a traveler tells of a monkey pursued by another, and running over and through the tops of the trees of an African forest faster than a deer could run on open ground. The flashing repetitive momentary glances of the eyes, before, back and all about a hundred objects must be coordinated with a mathematical precision to accurate unity and brilliant action of every muscle of the body. Similar perfection of eye and motion has been evolved in every higher animal of the world, and in every savage, and in every child. In man there is no danger from above, the eyes are rarely raised to the sky; the pupil consequently has a wide range of movement, and to shade the pupil the lid drops to its upper border. Doubtless we possess a more appropriate color sense, retinal discrimination and visual judgment because of the device.

The need of a shading of the retina to produce clear and quick imaging, 'resensitizing of the visual purple' it has been called, is so great that there are distinguishable a surprising number of separately acting mechanisms, all working to this end:

 1. The shadows cast by the retinal arteries, veins and capillaries. 2. Reflections and shadings from the individual corpuscles of the blood in these vessels. 3. The shadings of vitreous cells called muscæ volitantes. 4. The pigmentary layer of the front part of the retina continuous with that of the iris. 5. The iris-pigmentation, lack of which constitutes the tragedy of albinism. 6. The continuous narrowing and widening of the pupil. 7. The pigmentation of the skin of the lid, and the comparative opacity of the lid-structure itself. 8. The eye-lashes have an additional screening function, while allowing vision or suggestion of an object above.