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First Law of Vision.—Law of Outward Projection of Retinal Impressions. - An image is formed on the retinal screen.
We have seen that the whole object of the complex arrangement of lenses placed in front of the retina is the formation of images. But we do not see the retinal images. We do not see anything in the eye, but something outside in space. It would seem, then, that the retinal image impresses the retina in a definite way; this impression is then conveyed by the optic nerve to the brain, and determines changes there, definite in proportion to the distinctness of the retinal image ; and then the brain or the mind refers or projects this impression outward in a definite direction into space as an external image, the sign and facsimile of an object which produces it. We shall see hereafter how important it is that we regard what we see as external images, the signs of objects which produce them, and these external images themselves as projections outward of retinal images.
This law of outward projection is so important that we will stop a moment to show that it is not a new law specially made for the sense of sight, but only a modification of a general law of sensation. After doing so, we will proceed to illustrate by many phenomena, so as to fix it well in the mind.
Comparison with Other Senses.—The general law of sensation is, that irritation or stimulation in any portion of the course of a sensory fiber is referred to its peripheral extremity. Thus, if the sciatic nerve be laid bare in the upper thigh and then pinched, the pain is felt not at the part injured but at the termination of the nerve in the feet and toes. If the ulnar nerve be pinched in the hollow on the inner side of the point of the elbow, pain is felt in the little and ring fingers, where this nerve is distributed. In amputated legs, as is well known, the sense of the presence of a foot remains, and often severe neuralgic pains are felt in the feet and toes. The pain, which in this case is caused by a diseased condition of the nerves at the point of amputation, is referred to the place where the diseased fibers were originally distributed. In nerves of common sensation, therefore, injury or disease, or stimulation of any kind in any part, is referred to the peripheral extremity of the nerve-fibers. Now the peculiarity of the optic nerve is, that it refers impressions not to its peripheral extremity only, but beyond
But when we find great differences in the functions of tissues, such as occur in this case, we can generally find the steps which fill up the gap. A thoughtful comparison of the phenomena of the different senses will, we believe, reveal these steps. We repeat here what has already been said in a general way on page 5. Commencing with the lowest of the specialized senses, the gustative, an impression on the nerves of taste is referred, as in the case of common sensory fibers, to their peripheral extremity: the sensation is on the tongue. In the case of the olfactive, we have a sensation still at the peripheral extremity, i. e., in the nose, but also a reference to an external body at a distance as its cause. Here the objective cause and the subjective sensation are separated, and both distinct in the mind. In the case of the auditive nerve, the sensation is no longer perceived, or at least is very imperfectly perceived, in the ear, but is nearly wholly objective, i. e., referred to the distant sounding body. Finally, in the case of the optic nerve, the impression is so wholly projected outward that the very reminis
cence of its subjectivity is entirely lost. We are perfectly unconscious of any sensation in the eye at all.
Illustrations of this Property.—We will now try to make this property clear by many illustrative experiments.
Experiment 1.-If the retina or the optic nerve in any portion of its course were irritated in any way, by pinching, by scratching, or by electricity, we should certainly not feel any pain at all, but see a flash of light. But where? Not at the place irritated, nor at the peripheral extremity only, not in the eye, but beyond in the field of view, and at a particular place in that field, depending on the part of the retina irritated. Of course, this experiment can not be easily made. It has been made, however, by passing a spark of electricity through the head or through the eye in such wise as to penetrate the retina or traverse the optic nerve. nomenon has also been observed in cases of extirpation of the eye at the moment of section of the optic nerve. (Helmholtz.)
Experiment 2. Phosphenes.-Press the finger into the internal corner of the eye: you perceive a brilliant colored spectrum in the field of view on the opposite or external side. The spectrum thus produced has a deep steel-blue center, with a brilliant yellow border, and reminds one of the beauty spots on a peacock's feather or a butterfly's wing. Remove the pressure to any other part, and the spectrum moves also, but retains its opposite position in the field of view. In this familiar experiment the pressure indents the sclerotic and causes a change or irritation on the forward portion of the retina; and any change whatever on the retina is always referred directly outward at a right angle to the point impressed, and therefore to the opposite side of the field of view.
These colored spectra have been called phosphenes. Observe, again, the projection is in a perfectly definite direction depending on the part of the retina impressed.
Experiment 3. Muscce Volitantes.—If we gaze on a white wall or ceiling, or, still better, on a bright sky, we see indistinct motes floating about in the field of view on the wall or sky, and slowly gravitating downward. Sometimes they are undulating, transparent tubes, with nucleated cells within ; sometimes they are like inextricably tangled threads, or like matted masses of spider's web; sometimes they are slightly darker spots, like faint clouds. They are called muscoe volitantes, or flying gnats. What are they? They are specks or imperfections in the transparency of the vitreous humor. As fishes or other objects floating in midwater of a clear lake on a sunny day cast their shadows on the bottom ooze, even so these motes in the clear medium of the vitreous humor, in the strong light of the sky,
cast their shadows on the retiFig. 26.
nał bottom. Now, as already said, all changes in the retina, of whatever kind, whether produced by images, or shadows, or mechanical irritations, are projected outward into the field of view, and appear there as something visible.
Experiment 4. Purkinje's INTERNAL VIEW OF THE Retina, Figures.—Stand in a dark room
showing the retinal vessels ramifying over the surface, with a lighted candle in hand. but avoiding the central spot. Shutting the left, hold the can(After Cleland.)
dle very near the right eye, within three or four inches, obliquely outward and forward, so that the light shall strongly illuminate the retina. Now move the light about gently, upward, downward, back and forth, while you gaze intently on the wall opposite. Presently the field of view becomes dark from the intense impression of the light, and then, as you move the light about, there appears projected on the wall and covering its whole surface a shadowy, ghost-like image, like a branching, leafless tree, or like a great bodiless spider with many branching legs. What is it? It is an exact but enlarged image of the bloodvessels of the retina (Fig. 26). These come in at the entrance of the optic nerve, ramify in the middle layer, and therefore in the strong light cast their shadows on the bacillary or receptive layer, of the retina. The impression of these shadows is projected outward into the field of view, and seen there as an enlarged shadowy image. These have been called Purkinje's figures, from the discoverer.
Experiment 5. Ocular Spectra.- Look a moment steadily at the setting sun, and then, turning away the eye, look elsewhere—at the sky, the ground, the wall : a vivid colored spectrum of the sun (or many of them, if the eye has not been steady while regarding the sun) is projected into the field of view, and follows all the motions of the eye. This spectrum, on a bright ground, like the sky, to my eye is first green, then blue, then purple, then rose, and so gradually fades away. The spectrum is equally seen when the eye is shut; but then being projected on a dark ground, the color is apt to be complementary to that of the same spectrum seen against the bright ground of the sky. It is first blue, then yellow, then green, and so fades. The explanation is obvious. The strong impression of the image of the sun on the retina induces a change which lasts some time; the sun brands its image on the ret