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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 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.
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 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.
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
more, then light from the pin would be brought to focal points and produce an image on the retina; and this. image, being inverted, would by projection be reinverted, and the pin would be seen in its real position. In the above experiment, however, the pin is much too near the retina to form an image. But nearness to the retinal screen, though unfavorable for producing an image, is most favorable for casting a sharp shadow; and while retinal images are inverted, retinal shadows are erect. The light streaming through the pin-hole into the eye casts an erect shadow of the pin-head on the retina. This shadow is projected outward into space, and by the law of direction is inverted in the act of projection, and therefore seen in this position in the pin-hole. It is further proved to be the outward. projection of a retinal shadow by the fact that, by multiplying the pin-holes or sources of light, we multiply the shadows, precisely as shadows of an object in a room are multiplied by multiplying the lights in the room.* Experiment 2.—If we look
L at a strong light, such as the flame of a candle or lamp, or a gas-flame, at some distance and at night, and thus bring the lids somewhat near together, we observe long rays streaming from the light in many directions, but chiefly upward and downward. Fig. 27 gives the phenomenon as I see it. The explanation is as follows: In bringing the lids near
* This phenomenon was first explained by the author in 1871. See “Philosophical Magazine," vol. Ixi, p. 266.
together, the moisture which suffuses the eye forms a concave lens, as in Fig. 28 (hence the phenomenon is much more conspicuous if there be considerable moisture in the eyes). This watery lens will be saddle-shapedi. e., concave vertically and convex horizontally. Now the rays from the light (L, Fig. 27) which penetrate the center of the pupil will pass directly on without refraction except what is normal, and make its image (Fig.
28, I') on the central spot. But the rays which strike the curved surface of the watery lens will be bent upward to b and downward to a. Thus the light, instead of being brought to a focal point, is brought to a long focal line, 6 a, on the retina, with the image of the light in the middle at L'. The upper portion of this line b L' will be projected outward and downward, and form the downward streamers of Fig. 27; while the lower portion of the retinal impression a L' will be projected outward and upward, and form the upward streamers of Fig. 27. To prove this, while the streamers are conspicuous, with the finger lift up the upper lid: immediately the lower streamers disappear; now press down the lower lid: immediately the upper streamers