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Now the painter can imitate the aërial perspective. He skillfully diminishes the brightness, dulls the sharpness of outline, and blues the tinge of all objects, in proportion to their supposed distance, so as to produce the effect of depth of air. He can also and still more perfectly imitate the mathematical perspective, by diminishing the size of objects and the distance between them as he passes from his foreground to his background. But he can not imitate the focal perspective, and still less can he imitate the binocular perspective. This is artificially given only in the stereoscope, and is the glory of this little instrument. Focal perspective is unimportant to the painter, because imperceptible at the distance at which pictures are usually viewed ; but the want of binocular perspective in painting interferes seriously with the completeness of the illusion. Therefore the illusion is more complete and the perspective comes out more distinctly when we look with only one eye. In a natural scene it is exactly the opposite : the perspective is far more perfect with both eyes open, because then all the forms coöperate.
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 When the ground-glass plate is withdrawn and the sensitive plate substituted, the light-image falling on this plate determines in it wonderful molecular changes, which are graduated in intensity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, and which comes and goes with the object. 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, there are certain effects which can not be produced by one camera or by one eye. 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—but only phantom 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 a real space.
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. Certain physical phenomena-molecular vibrations, decompositions, and recompositions occur, and there emerge, how we know not, psychical phenomenathoughts, emotions, etc. Aladdin's lamp is rubbed— physical phenomenon-and the genie appears-psychical phenomenon.
THEORIES OF BINOCULAR PERSPECTIVE.
Wheatstone's Theory.—To Wheatstone is due the , credit of having discovered the fact that two slightly dissimilar pictures—dissimilar in the same way as the two retinal images of a solid object or of a scene—when united, produce a visual effect similar to that produced by an actual solid object or an actual scene. He also invented the stereoscope to facilitate the combination of such pictures. His theory of these effects was as follows : In viewing a solid object or a scene, two slightly dissimilar images are formed in the two eyes, as already explained; but the mind completely unites or fuses them into one. Whenever there occurs such complete mental fusion of images really dissimilar in this particular way, and therefore incapable of mathematical coincidence, the result is a perception of depth of space, or solidity, or relief. In the stereoscope, therefore, he supposes that the two slightly dissimilar pictures are mentally fused into one, and hence the appearance of depth of space follows as the necessary result of this mental fusion.
This theory is still widely held by physiologists ; but it is evidently the result of imperfect analysis of visual impressions. In stereoscopic diagrams it is always possible to detect the doubling on which the per