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Thus, then, I conclude that the mind perceives relief instantly, but not immediately; for it does so by means of double images, as just explained. This is all that is absolutely necessary for the perception of relief; but it is probable—nay, it is certain—that the relief is made clearer by a ranging of the point of sight back and forth, and a successive combination of the different parts of the object or scene or pictures, as maintained by Brücke.

Return to the Comparison of the Eye and the Camera. -It is time now to return to, and to continue, our comparison of the eye and the photographic camera. We have seen that both the camera and the eye are equally optical instruments contrived for the purpose of making an image; but we have also seen that in both this image is only a means by which to attain a higher end, viz., to make a photographic picture in the one case, and to accomplish distinct vision in the other. In both also, in order to accomplish its higher purpose, there must be a sensitive receiving plate, viz., the iodized silver plate in the one, and the living retina in the other. In both, finally, there are wonderful changes, chemical or molecular, or both, in the sensitive plate. Let us then continue the comparison.

1. In the photographic camera when accomplishing its work there are three images which may be mentally separated and described. First, the light-image. This is what we see on the ground-glass plate. It comes and goes with the object in front. It is the facsimile in form and color of the object, but diminished in size and inverted in position. Second, the invisible image. When the ground-glass plate is withdrawn and the sensitive plate substituted, the light-image falling on this plate determines in it wonderful molecular changes,

which are graduated in intensity exactly according to the intensity and kind of light in the light-image: the aggregate effect is therefore rightly called an image, though it is invisible. Third, the visible image, or picture. The operator then takes the plate with the invisible image to a dark room, and applies certain chemicals which develop the image—i. e., which determine certain permanent chemical changes, which in intensity and kind are exactly proportioned to the antecedent molecular changes, and therefore graduated over the surface exactly as the molecular changes of the invisible image were graduated, and hence also exactly as the light of the light-image was graduated. This is the permanent photographic picture—the facsimile in form of the object which produced it.

So also in the work of the eye, vision, we may mentally separate and may describe three corresponding images. First, there is the light-image, which is formed in the dead as well as the living eye. Second, the invisible image. The light-image, falling on the sensitive living retina, determines in its substance molecular changes which are graduated in intensity and kind exactly as the light of the light-image is graduated in intensity and color, and may therefore be rightly called an image, even though it be invisible, and the nature of the molecular changes be inscrutable. Third, the external visible image. The invisible image, or the molecular changes which constitute it, is transmitted to the brain, and by the brain or the mind is projected outward into space, and hangs there as a visible external image, the sign and facsimile in form and color of the object which produced it.

2. Again, as there are certain effects which can not be produced by one camera—as two cameras from two positions take two slightly different pictures of the same object or the same scene, which when combined in the stereoscope produce the clear perception of depth of space—even so the two eyes act as a double camera in taking and a stereoscope in combining two slightly different images of every object or scene, so as to give a clear perception of binocular perspective.

We have thus carried the comparison as far as comparison is possible. But there is this essential difference between the two-essential because found everywhere between human and natural mechanism: In the one case we trace mechanism and physics and chemistry throughout. In the other we also trace mechanism, exquisite mechanism, but only to a certain point, beyond which we discover something higher than mere mechanism. We trace physics and chemistry to a certain point, but as we pursue the investigation we find something superphysical and superchemical, or else a physics and a chemistry far higher than any we yet know. At a certain point molecular and chemical change is replaced by sensation, perception, judgment, thought, emotion. We pass suddenly into another and wholly different world, where reigns an entirely different order of phenomena. The connection between these two orders of phenomena, the material and the mental, although it is right here in the phenomena of the senses, and although we bring to bear upon it the microscopic eye of science, is absolutely incomprehensible, and must in the very nature of things always remain so. Certain vibrations of the molecules of the brain, certain oxidations, with the formation of carbonic acid, water, and urea, on the one side, and there appear on the other sensations, consciousness, thoughts, desires, volitions. There are, as it were, two sheets of blotting

paper pasted together; the one is the brain, the other is the mind. Certain ink-scratches and ink blotchings, utterly meaningless, on the one, soak through and appear on the other as intelligible writing. But how or why we know not, and can never hope even to guess.

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CHAPTER V.

JUDGMENT OF DISTANCE, SIZE, AND FORM.

We are now prepared to understand the modes of estimating distance, size, and form; for these modes are founded partly on monocular and partly on binocular vision.

As already stated, the direct and simple sense-impressions given by the optic nerve are light, its intensity, its color, and its direction. These can not be analyzed into simpler elements, but distance, size, and form are judgments based upon these.

Distance. We judge of distance by means of the different forms of perspective already described on page 142: 1. By focal adjustment, or monocular perspective. The eye adjusts itself for distinct vision for all distances from infinite distance to five inches. By experience we know distance from the amount of effort necessary to adjust for perfect image, and therefore distinct vision. Judgments based on this are tolerably accurate from 5 inches to several yards. Beyond 20 feet it is too small to be appreciable. 2. By axial adjustment, or binocular perspective. The greater or less amount of optic convergence necessary to produce single vision is a far more accurate mode of judging of distance than the last. It is reliable from near the root

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