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The sense of vision is a question of length of vibration, and not of multitude of impacts; but there's a kind of shock to the retina, a nasty giddy confusion just at the time if the eyes are open. Keep 'em shut." "Shut," I said. "Good!" "And the next thing is, keep still. Don't begin to whack about. You may fetch something a nasty rap if you do.
I was not, of course, at that time aware that this apparent paradox was occasioned by the center of the visual area being less susceptible of feeble impressions of light than the exterior portions of the retina.
Another of the physical effects of their fear was that in the congested condition of the blood vessels of the retina they beheld thousands upon thousands of small black specks flitting past them, as if it had been possible to distinguish the flying bullets. "Confound the luck!" Jean stammered.
In the same manner, when a fiery meteor shoots across the night, it appears to leave a long lucid train behind it, part of which, and perhaps sometimes the whole, is owing to the continuance of the action of the retina after having been thus vividly excited.
The mechanism of the eye may be perfect, the retina and the optic nerve may faithfully perform their duties, but if the brain behind the eye be defective, the comprehension of the object or some of its properties is lost to the intelligence of the individual. Some people are "color blind."
The retina, which is the third and innermost membrane of the eye, is the expansion of the optic nerve, and constitutes the immediate seat of vision.
The optic nerve, whose expansion forms the retina, receives this image and transmits it to the mind. Arnott has well remarked, that "a whole printed sheet of a newspaper may be represented on the retina on less surface than that of a finger nail; and yet not only shall every word and letter be separately perceivable, but even any imperfection of a single letter.
Numerous other instruments of precision are now in constant use, among the most wonderful of which perhaps is the ophthalmoscope, whereby we are enabled to subject the retina and the intervening media of the eye to minute visual examination.
These circumstances, though they very much perplexed the experiments till they were investigated, admit of a satisfactory explanation; for while the rays from the bright internal object in exp. a. fall with their full force on the center of the retina, and, by fatiguing that part of it, induce the reverse spectrum, many scattered rays, from the same internal pink paper, fall on the more external parts of the retina, but not in such quantity as to occasion much fatigue, and hence induce the direct spectrum of the pink colour in those parts of the eye.
Now as the images, that are painted on the retina of the eye, are no other than signs, which recall to our imaginations the objects we had before examined by the organ of touch, as is fully demonstrated by Dr. Berkley in his treatise on vision; it follows that the human creature has greatly more accurate and distinct sense of vision than that of any other animal.
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