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VISUAL (OPTICAL) ILLUSIONS
Illusions in the movement of an object. Encyclopedia of visual illusions
At leisure / Visual (optical) illusions << Back: portrait illusions >> Forward: Illusions of color vision The great Russian physiologist I. M. Sechenov, on the issue of visual perception of movements, stood on a materialistic point of view. He wrote: "... in relation to movements that the eye can follow, the imagined and the actual coincide with each other." It turns out that when the object of observation moves, a number of visual illusions are also encountered, which are due to some properties of our visual apparatus. Even Claudius Ptolemy (II century AD) in his "Optics" says that if a circle with a colored sector is put into rotation, then the whole circle seems to be colored to us. Obviously, even the ancients knew that a fire moving at a certain speed in a circle turns for us into a continuous ring of fire. Our eye has the ability to retain a visual impression for a fraction of a second, although the visible object has already disappeared from the field of vision. The visual sensation of light takes some time to occur. If a brightly lit surface suddenly appears in front of the dark-adapted eye, then the visual sensation from it arises after about 0,1 sec. With a smaller difference in the brightness of the adaptation field and the resulting light surface, this time increases to 0,2-0,3 seconds, with a larger one it is reduced. At the same time, the strength of the emerging visual sensation at first sharply increases - the "flash" seems brighter than in reality, but then the normal sensation of brightness "comes" relatively quickly. To this inertia of vision is added the inertia of the nervous system, in which the signal from the organs of vision and the response signal from the motor organ propagate, albeit with great, but not with infinite speed. An average of 0,19 seconds elapses from the moment a medium-strength signal is given to the moment a person responds. For individuals, this time ranges from 0,15 to 0,225 seconds. When a person perceives a signal with one eye, he reacts to this signal more slowly: the "lag" is approximately 0,015 seconds. Only in the first half of the 1825th century did they begin to use this feature of the visual perception of moving objects. So, in 125, a device was built in France, the so-called "taumatrope" *, which is a piece of cardboard, on one side of which, for example, a cage is drawn, and on the other - a bird (Fig. XNUMX). * (Greek: "tauma" - focus, "trope" - wheel.)
With a quick rotation and simultaneous observation of both sides of the cardboard, the bird will appear to be sitting in a cage. You can attach a piece of cardboard with drawings on both sides to the axis of the top. The same experiment can be done with a card that has a galloping horse on one side and a jockey on the other (Fig. 126). A number of the most diverse variants of this toy are possible: a hunter without game and with game, two separate parts of the same word, a ballerina separately from a partner, etc.
By the way, the illusion of a bird being in a cage can be obtained in another way. You should take half of the postcard and place it vertically between the bird and the cage so that the shadow of the postcard does not fall on the fig. 125, then lean the postcard along with the drawing to the nose and look with one eye at the cage, and with the other at the bird. In this case, it turns out that the bird has moved and entered the cage. This illusion is explained by the merging of the images of the object in the right and left eyes in our minds into a single visual image (stereo effect). In 1829, the Belgian physicist J. Plateau built an instrument, which he called a "phenakistiscope" *, consisting (Fig. 127) of a cardboard circle divided into several sectors with the same number of windows; the sectors contain images of a wood splitter in successive positions when splitting a log with an axe. If you stand in front of a mirror and look through the window while the circle is rapidly rotating, you will get the impression of a wood splitter. * (Phenakistiscope is a deceptive vision.)
The Plateau spiral is also known, on which one can observe a consistent pattern of movement. If the disk with the spiral (Fig. 128) is rotated clockwise, then after a long fixation of it with the eye, we get the impression of contracting all the branches of the spiral to the center; when the spiral rotates in the opposite direction, we see the divergence of the spirals from the center to the periphery. If, after a long examination of a moving spiral, we look at stationary objects, we will see their movement in the opposite direction. So, for example, if, after a long observation of the area from the window of a moving train or of water from the window of a moving steamer, we shift our gaze to stationary objects inside the carriage or steamer, then it will seem to us that they are also moving, but in the opposite direction. These illusions are associated with successive moving images.
Everyone is familiar with the illusion of vision, when from the window of a standing train you see how a neighboring train is moving off. You feel like your train is slowly leaving the station. You are already accustomed to linking moving images in your mind with your movement. You are looking out the window from a courier train traveling at a speed of 60 kilometers per hour. Red flowers grow on the slopes of the embankment, and you want to know them: what are they, roses, poppies or dahlias? However, the flowers flicker and it is not possible to recognize them, although the train is moving only 16 meters per second. It is known that the swallow flies at a speed of about 90 m / s and grabs tiny insects on the fly, flies like an arrow through holes slightly larger than itself. Consequently, she sees all the objects around her, and her visual impressions do not merge. A person cannot follow the details of more or less rapid movements. Therefore, we sometimes find it strange to take snapshots of a walking person, etc. It would be correct to say that the reality of things, as they are perceived by our vision, is more accurately conveyed by fine art than by instant photography. Following "toys" like those shown in Fig. 125-127, followed by a number of inventions that allow you to see moving figures when the disks rotate. All these devices were the forerunners of modern cinema, and, in essence, the action of all of them is based on the ability of the eye to retain the light effect produced on it for some time. The eye still "sees" what has already disappeared for about 0,1 second. So, in modern cinema, when changing 24 frames per second and when the projector window is blocked at the moment of changing the frame with a special screen (obturator), our eye does not notice this change and perceives not the movement of the tape, but the slower movement of the figures projected onto the screen. The simultaneous brightness contrast of achromatic surfaces can be conveniently observed, in addition to the method shown in Fig. 107, using the disc fig. 129.
If this disk is rapidly rotated around its axis, then six rings are obtained, the brightness of which changes from white at the extreme to black in the center of the disk. Objectively, these rings will have the same brightness over their entire radial width; subjectively, where any ring is in contact with a lighter one, it seems noticeably darker; where it touches the nearest darker one, it appears lighter. Helmholtz explains this as a deception of our judgment, he says: "A person of average height next to a very tall one seems small, because at this moment we clearly see that there are higher people, but we do not see that there are also lower ones. The same a person of average height, placed next to a short one, will appear tall." It is clear that the experience of shading the dark spot over the entire surface of the disk during its rotation is associated with the phenomenon of preservation of the visual impression. The same experiment is made with a color disc to observe the phenomenon of color mixing. The stroboscopic* methods currently used in technology for measuring the duration of periods of rapidly occurring processes are based on the principle of preserving a visual impression for tenths of a second. For example, an observer armed with a high-speed shutter examines a rotating disk through it, and the shutter is activated just at such a moment in time when the disk occupies a strictly defined position. With a shutter frequency of more than 10 times per second, some sector of the disk or a radius drawn on it will appear to the observer to be stationary. * (From the Greek "strobos" - a whirlwind, whirling.) Another way to obtain a stroboscopic effect is to illuminate the rotating part under study with short-term light flashes. If the repetition rate of flashes coincides with the number of revolutions of the part per second, and the interval between flashes is less than 0,1 second, then in this case the rotating part will appear to the observer as stationary. Television also uses the law of conservation of visual impression. In this case, on the luminescent screen of the cathode-ray tube of the receiver, the electron beam, at a very high speed, "draws" the image of the picture we see, moving along horizontal lines and shifting vertically from line to line. In fact, it exactly repeats the movements of another electron beam moving in the same way over the image received in the television studio transmitter. Due to the high speed of the electron beam moving from the top of the screen in lines to its lower border, we do not notice this movement, but perceive the entire image as a whole. The electron-beam method for decomposing an image transmitted over a long distance was first proposed in 1907 by the Russian scientist B.L. Rosing. A very interesting illusion associated with the appearance of color on a black and white rotating disk (Fig. 130) was observed in the last century by Benham and is now used in psychophysiological experiments. By spinning the disk at 6-10 revolutions per second clockwise in a sufficiently bright light, we will notice colored rings on the disk. The ring more distant from the center acquires a blue-violet hue, followed by greenish, yellowish and reddish rings. When the disk is rotated counterclockwise, the order of the colored rings is reversed. On the peripheral ring of another disk shown in Fig. 131, a reddish coating appears, and on the inside it is bluish, of course, if this disk is put into rotation. As the rotation speed increases, the bluish coating disappears, and the entire disc will appear reddish.
The appearance of color when changing the speed of alternation of black and white stripes is now attracting the attention of researchers working on the problems of color television. However, the existing explanations of this illusion cannot be considered complete and exhaustive. Many illusory movements are explained both by the phenomenon of the preservation of a visual impression, and by some physiological phenomena that have not yet been sufficiently elucidated that take place in the process of visual perception (Fig. 132-135).
A number of phenomena of illusory movement are known when moving objects are observed through a slit or a small hole in the screen. So, for example, if a round disk is moved in front of a slit in the screen from the side opposite to the observer, then it seems to us an ellipse, with a fast movement of the disk it will seem that the major axis of the ellipse lies vertically, and with a slow movement it seems horizontal. Examples of illusory movements are very common to us in ordinary conditions; we present here some more of them. So, from the window of a fast moving train, we see that all the objects of the landscape surrounding the train are moving. Observing the moon on a cloudy night, we see that it moves quickly relative to the still clouds. "Over the fields, but over the clean moon flies like a bird ...", - is sung in a Russian folk song. The Chinese saying is perfectly true: "Look over the railing of the bridge and you will see how the bridge floats on still water." The spokes of a fast-moving bicycle appear to us to be merged; the oscillating string seems to us blurred between fixed knots, etc. In some old physics textbooks, the ability of the eye to retain a visual image for some time was considered as one of the shortcomings of our organ of vision. However, with this "flaw" in mind, man has created art forms as powerful and accessible as cinema and television. Author: Artamonov I.D. << Back: portrait illusions >> Forward: Illusions of color vision
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