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reverse of the objects in space—that the upper part of the field of view corresponds to the lower part of the retina, and the lower part of the former to the upper part of the latter. Similarly the right and left sides of the field correspond to the left and right sides respectively of the retina.
These two laws—the law of external projection and the law of direction-are the two most fundamental laws of vision. The one shows why objects are seen externally in space; the other gives the exact place where they are seen-i. e., the relative position of objects and parts of objects. Together they explain all the phenomena of monocular vision except color. The whole science of monocular vision is but a logical explication of these two laws. It is necessary, however, to take up some points and explain them more fully by this law.
1. 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 thinkers for many centuries and many and various answers have been given.
Theories of Erect Vision.-1. First, there have been metaphysical theories characteristic of this class of thinkers. According to these, erect and inverted are purely relative terms. If all things are inverted, then nothing is inverted? There is no up and down to the soul, etc. 2. Nativistic Theory.—It is a native or inherited endowment, for which no reason can be given. 3. Empiristic Theory. It is learned by experience by each individual for himself.
The first we put aside as being non-scientific. The
second and third are each true to some extent, and may and must be combined and reconciled. It is acquired by experience; yes, but not by individual experience, but by ancestral experience, acquired and accumulated through the whole line of evolution of the eye from the lowest animals to man—from the earliest times to
To the individual, however, it is native-inherited.
But leaving aside the question of origin, a strictly scientific explanation is an analysis of the phenomena and their reduction to a general law. This law is the “law of visible direction” already explained. 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, pass through the nodal point, 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 along the ray-line 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. Suppose we were standing captive and blindfolded on the plains of Arizona surrounded by Apaches shooting arrows at us from every side. Would we not be able, by the part struck and by the direction of the push, to refer each arrow back along its line of flight to the place whence it came? 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 nowise 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 them 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. If we decompose objects into their component radiants it is at once seen that the question of erect vision is nothing more than a question of seeing things in their right places.* 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 reinverted and reconstructed in its proper position. Or it may be otherwise expressed thus : Space in front of us is under all circumstances the cutward 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 corresponding 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. This is easily understood by referring to Fig. 27, page 68.
* Some may say, some have said (“Science,” vol. ii, p. 268, 1895), that we are not warranted in explaining by one law things so disparate as sensation of light and sensation of touch. The answer is plain. Direction is not a sensation, but an idea underlying all the senses. It is a matter of space perception, and therefore in this regard it is right to reduce all the senses to a common law.
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 is 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. That this accurate perception of direction and therefore of erectness of objects is not a matter of judgment acquired by individual experience, but is inherited and therefore immediate, is proved by the fact that an infant, as soon as it takes notice at all, turns its eyes toward the light, and therefore must see the light in its true position. As already said, erect vision is a mere question of seeing things in their right places. A child six years old, operated on for congenital cataract and blindness, saw things in their proper position and right side up from the first, but could not judge of distance. This had to be learned by experience.*
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,
*“ Revue scientifique," October 29, 1892.