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Representative in the Visual Field of the Blind Spot.-Since every condition of the retina has its visible representative in the field of view, it may be asked, “If there be a blind spot, why do we not see it, when we look at a white wall or bright sky, as a black spot, or a dusky or dim spot, or a peculiar spot of some kind ?” I answer: 1. With both eyes open there are, of course, two fields of view partly overlapping each other. Now the invisible spots in these two fields do not correspond, and therefore objects in the invisible spot of one eye are seen perfectly by the other eye, and hence there is no invisible area for the binocular observer. But it will be objected that even with one eye we see no peculiar spot on a white wall. I therefore add: 2. That we see distinctly only a very small area about the point of sight, and distinctness decreases rapidly in going from this point in any direction. Therefore the correspondent or representative in the field of view may well be overlooked, unless it be conspicuous, i. e., strongly differentiated from the rest of the general field. 3. But if this were all, close observation would certainly detect it. The true reason is very different, and the explanation is to be sought in an entirely different direction. Writers on this subject have expected to find a visible representative, and have sought diligently but in vain for it. But the fact is, they ought not to have expected to find it. The expectation is an evidence of confusion of thought --of confounding blackness or darkness with absence of visual activity. Blackness or darkness is itself but the outward projection of the unimpressed state of the bacillary layer; but there is no bacillary layer here. We might as well expect to see a dark spot with our fingers as in the representative of the blind spot. A black spot, or a dark spot, or a visible spot of any kind, is
not the representative in space of a blind or insensitive retinal spot. The true representative of a blind spot is simply an invisible spot, or, in other words, a spot in which objects are not seen. If we could differentiate it in
any way, it would be visible, which it is not. As it can not be differentiated in any way, the mind seems to extend the general ground color of the neighboring field of view over it. This is, however, a psychological rather than a visual phenomenon. It is for a similar reason that it is impossible to see any limit to the field of view, except where it is limited by the parts of the face, as nose, brows, etc. There is a certain limit horizontally outward where vision ceases, but it is impossible to detect any line of demarkation between the visible and the invisible.
3. Erect Vision.— Retinal images are all inverted. External images or signs of objects are outward projections of retinal images. How, then, with inverted retinal images, do we see objects in their right position, i. e., erect? This question has puzzled metaphysicians, and many answers characteristic of this class of philosophers have been given. The true scientific answer is found in what is called the “law of visible direction.” This law may be thus stated : When the rays from any radiant strike the retina, the impression is referred back along the ray-line (central ray of the pencil) into space, and therefore to its proper place. For example: The rays from a star (which is a mere radiant point) on the extreme verge of the field of view to the right enter the eye and strike the retina on its extreme anterior left margin; the impression is referred straight back along the ray-line, and therefore seen in its proper place on the right. A star on the left sends its rays into the
eye and strikes the right side of the retina, and the
impression is referred back along the ray-line to its appropriate place on the left. So also points or stars above the horizon in front impress the lower portion of the retina, and the impression is referred back at right angles, or nearly at right angles, to the impressed surface, and therefore upward; and radiants below the horizon, on the ground, impress the upper half of the retina and are referred downward.
Comparison with Other Senses.—There is nothing absolutely peculiar in this; but only a general property of sense refined to the last degree in the case of sight, owing to the peculiar and exquisite structure of the bacillary layer of the retina. For example: Suppose, standing with our eyes bandaged, any one should with a rod push against our body. We immediately infer the direction of the external rod by the direction of the push. Or another example : Suppose we stood naked in a pond of placid water, with eyes bandaged, and some one on shore agitated the water; the advancing waves would after a while reach us and tap gently upon the sensitive skin. Could we not infer the direction of the distant cause from the direction of the blows? Is it any wonder, then, that when the rays
of light crossing one another in the nodal point punch against the interior hollow of the retina, we should infer the direction of the cause by the direction of the punch; i. e., that we should refer each radiant back to its proper place in space?
Thus it is seen that it is in no wise contrary to the general law of the senses, that we should refer single radiants, like stars, back to their proper place in space and see then there. But objects are nothing else than millions of radiants, each with its own correspondent focal point in the retinal image. Each focal impression
is referred back to its correspondent radiant, and thus the external image is reconstructed in space in its true position, or is reinverted in the act of projection.
Law of Visible Direction. After these illustrations and explanations we return to the law, and restate it thus: Every impression on the retina reaching it by a ray-line passing through the nodal point is referred back along the same ray-line to its true place in space. Thus, for every radiant point in the object there is a correspondent focal point in the retinal image; and every focal point is referred back along its ray-line to its own radiant, and thus the external image (object) is reconstructed in its proper position. Or it may be otherwise expressed thus : Space in front of us is under all circumstances the outward projection of retinal states. With the eyes open, the field of view is the outward projection of the active or stimulated state of the retina; with the eyes shut, the field of darkness is the outward projection of the unstimulated or passive state of the retina. Thus the internal retinal concave with all its states is projected outward, and becomes the external spatial concave, and the two correspond, point for point. Now the lines connecting the correspording points, external and internal, cross each other at the nodal point, and impressions reach the retina and are referred back into space along these lines; or, in other words, these corresponding points, spatial and retinal, exchange with each other by impression and external projection. This would give the true position of all objects and of all radiants, and therefore completely explains erect vision with inverted retinal image.
We see, then, that the sense of sight is not exceptional in this property of direction - reference. But what is exceptional is the marvelous perfection of this
property—the mathematical accuracy of its perception of direction. This is the result partly of the remarkable structure of the bacillary layer. Every rod and cone has its own correspondent in space, and the extreme minuteness and therefore number of separably discernible points in space are measured by the minuteness and therefore number of the rods and cones of the bacillary layer. Also the perpendicular direction of the rods and cones to the retinal concave is probably related to the direction of projection of impressions into space, and therefore to the accuracy of the perception of direction.
Illustrations of the Law of Direction. There are many interesting phenomena explained by this law, which thus become illustrations of the law.
Since inverted images on the retina are reinverted in projection and seen erect, it is evident that shadows of objects thrown on the retina, not being inverted, ought to become inverted in outward projection, and therefore seen in this position in space. This is beautifully shown in the following experiment.
Experiment 1.-Make a pin-hole in a card, and, holding the card at four or five inches distance against
the sky before the right eye with the left eye shut, bring the pin-head very near to the open eye, so that it touches the lashes, and in the line of sight: a perfect inverted image of the pin-head will be seen in the pinhole. If, instead of one, we make several pin-holes, an inverted image
of the pin-head will be seen in each pin-hole, as shown in Fig. 26. The explanation is as follows: If the pin were farther away, say six inches or