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VISUAL (OPTICAL) ILLUSIONS
Disadvantages and defects of vision. Encyclopedia of Visual Illusions
At leisure / Visual (optical) illusions << Back: The device of the eye and visual sensations >> Forward: Illusions associated with the structural features of the eye Let us try to understand in more detail the previously given individual indications of shortcomings in the structure of the eye, the causes of inaccurate perception of certain visual images. Disadvantages and defects of vision can be classified in some way. Firstly, there are irregularities of the normal human eye, inherent in everyone without exception, these are aberrations * of the optical system of the eye (spherical, astigmatism and chromatic), the presence of a blind spot, irradiation and entoptic phenomena. * (From Latin - deviation.) Secondly, there are individual, sometimes congenital, sometimes acquired with age, visual defects - these are myopia and hyperopia, strabismus, night and color blindness. Thirdly, we can name the general psychological patterns of visual sensations, such as the absolute threshold, thresholds of discrimination, the relationship between the stimulus and the strength of sensation, adaptation, simultaneous contrast, sequential images and the relationship of visual sensations with other psychological processes. Each of these patterns can affect the accuracy and reliability of the visual perception of objective reality. Finally, fourthly, brightness contrast, spectral sensitivity, degree of relief, as well as the inertia of vision, also limited by the corresponding limits of perception, can either interfere with the disclosure of the essence of the phenomenon, or be used to the benefit of the process of cognition. Thus, the limitations and features of the normal eye and individual visual defects greatly limit the role of visual sensations in the cognition of the surrounding world. If we also take into account the subjectivity of the threshold values of visual functions, the wide variety of properties of the visual organs in different people, it becomes clear how little accurate information about the world around us we receive using only our own sensations. It is widely believed that it is difficult to meet two people with exactly the same properties of the visual apparatus. Here, for example, is the reasoning of one young man on this score. "My friend and I sit in the summer among fragrant greenery on the banks of a forest river and observe a wonderful variety of phenomena. My friend is blond with bluish-gray eyes. He wears glasses that correct astigmatism since childhood. I am a brunette, I have dark brown eyes. I think I can see fine, although I'm told that I sometimes roll my left eye hard so that the pupil is near the bridge of my nose. I don't know if my friend sees the same way I do. How can I know what he sees? However, it is possible and even certain that I would see a strange picture if I suddenly looked at everything around me through the eyes of my friend. Are these arguments of a young man fair? With regard to the formal difference in the properties and defects of the optical structures of the eyes of a young man and his friend, these arguments are valid. However, the visual process is not just the fixation of optical images of objects on the retina, but also the simultaneous work of brain centers, the activity of the central nervous system, and the use of accumulated experience. Therefore, the reasoning of the young man about the visual perceptions of him and his friend is unfair. On the one hand, the eyes of each person have their own defects, to which their owner has already adapted and completely imperceptibly eliminates them at some stages of visual perception. On the other hand, both comrades, being members of the same social and social environment, use the same results of the evolutionary development of the visual apparatus, the same social experience accumulated by mankind, and their common modern views on nature. Therefore, from a conversation between them, it can be seen that they perceive with their different eyes the phenomena of the external world in the same way, with very few exceptions. This explains the objectivity of visual sensations, as well as their relativity, i.e., the presence in them of a share of fallacy, unreliability. The next section will be devoted to the influence of structural features of the normal human eye on the erroneous perception of reality. Here we will dwell especially on individual, congenital or acquired defects of vision, so that later, when describing some illusions, we will refer to this brief information. If you check the accommodation of a normal eye, it turns out that the most distant point that the eye can distinguish in a calm state is theoretically at infinity, but practically at a distance exceeding 15 m. This point is considered farthest. The point that can be seen clearly at the closest distance from the eyes is called the near point. For a normal eye, this point is at a distance of 10-15 cm. The distance separating the far point from the near one is called the accommodation distance. If a clear image of a point on the retina of the eye is obtained when it is removed to a distance of no more than 35 cm - the eye suffers from mild myopia, from 35 to 10 cm - an average degree, and if the greatest distance of clear vision does not exceed 10 cm - a strong degree of myopia. According to fig. 4, the degree of myopia is determined by the angle that is formed between the ray aN coming from infinity and the ray coming from the far point A, i.e., the angle aNA or, which is the same, NAM. The measure of accommodation is determined by the difference between the angles NBM and NAM at the far and near points. For example, suppose that for some short-sighted eye, the far point is 180 mm from the eye, and the near one is 100 mm. In this case, the degree of myopia is expressed by an angle of 1/180=0,0056, i.e. 5,6 D (diopter)*. The measure of accommodation is expressed by the angle difference 1/100-1/180=4/900=0,0044, i.e. 4,4 D. * (Angles expressed in thousandths are usually called diopters in optics. One diopter is the refractive power of a lens whose focal length is 1 m.)
The myopic eye has the main focus of the refractive system in front of the retina. If the object under consideration approaches the eye, then its image approaches the retina. In the case of myopia, either the axis of the eye is too long, or the curvature of the lens is large, or the refractive power of other media of the eye is large. Myopia is corrected with concave glasses. A far-sighted eye is either too short, or its lens has a small curvature. Images of objects in this case will be obtained behind the retina, and such an eye in an unstressed state cannot see any objects clearly. In fact, as objects approach from a distance, the point of convergence of their rays in the eye goes even further beyond the retina. Only by resorting to the effort of accommodation can this eye see at all, and it sees distant objects better than near ones. Farsightedness is corrected by glasses with convex (positive) lenses that increase the refractive power of the eye. The greatest degree of farsightedness occurs with aphakia, i.e., in the absence of the lens and the inability of the eye to accommodate. Far-sighted eyes should be distinguished from senile eyes, sometimes almost devoid of the ability to accommodate due to a decrease in the elasticity of the lens over the years. In this case, the nearest point is more and more removed from the eye. By the age of 45, this point is already beyond the distance at which the normal eye distinguishes objects well. Senile farsightedness is corrected with the help of convex lenses that bring both near and far points closer. In people suffering from strabismus, the visual line (look) of one eye is directed to an object that attracts attention, and the line of the other eye is deviated towards the nose (converging or internal strabismus) or temple (divergent or external strabismus) up or down. The degree of strabismus is determined by the angle formed by the line of sight of the squinting eye and the normal direction. There are two types of strabismus: friendly and paralytic. In the first case, the motor muscles of the eyes are normal and their movements are coordinated, but the position of the eyes relative to each other always remains incorrect. Perhaps a constant deviation of the same eye, and sometimes alternately; squints first one eye, then the other. In those cases where one eye sees better than the other, the fixing eye always turns out to be the best eye, and the second, worse eye deviates. But one has only to close the best eye and the worse seeing eye begins to fix, and the closed second eye becomes deviated. This type of strabismus is caused by a disorder of the motor part of the apparatus of deep vision, high degrees of farsightedness or myopia, poor vision in one of the eyes. Coordinated vision with both eyes, which gives us the opportunity to receive plastic deep images, is lost. Concomitant strabismus often develops in early childhood and can be corrected with prismatic spectacles. In case of severe strabismus, one of the prisms corrects the existing deviation of the visual axis of the squinting eye, and the other partially deflects the axis of the other eye, ensuring the restoration of binocular vision. Paralytic strabismus occurs as a result of paralysis of one of the several motor muscles of the eye due to a disease of the central nervous system. In this case, the movement of the affected eye lags behind, and its axis is deviated to the side. Sometimes at the same time there is a doubling of visible objects. With the help of glasses, this type of strabismus cannot be eliminated: surgery helps here. There are cases of half-blindness, i.e., loss of half (right or left) of the field of vision, also due to a disease of the central nervous system. Abnormalities of the eye in relation to the basic patterns of light and color perception occur in the form of defects called "night blindness" and color blindness. "Night blindness" (hemeralopia) is a disorder of light perception, manifested in the form of a sharp decrease in visibility in dim lighting at dusk or at night. At the onset of darkness, when things lose their chromatic hues for us, a normally sighted person is still quite easily oriented through his peripheral vision. A subject suffering from hemeralopia feels completely helpless, does not distinguish anything, bumps into objects, etc. Dark adaptation in this case is either noticeably weakened or completely absent. The causes of this visual defect are often poor nutrition (lack of fat, vitamin A) or prolonged work in excessively bright light. Color blindness can be complete or partial. Persons suffering from complete color blindness do not distinguish any color tones; they do not distinguish a multi-color pattern from a single-color one. For them, shown in Fig. II poppy and cornflower differ from each other in the contours and brightness of the image.
All color tones for a normal seeing eye can be reproduced by mixing in appropriate proportions at least three colors taken as primary (red, green and blue). Therefore, normally sighted people are called trichromats. The phenomenon of abnormal trichromatism was discovered in 1880 by D. Rayleigh. Persons suffering from this visual anomaly could reproduce all colors by mixing the same three colors as persons with normal vision, but they added too much green. Thus, a mixture that seems white to them is actually green, a mixture that seems white to us they consider pink. Persons with complete color blindness are monochromatic, since they receive all the shades of objects only due to variations in the intensity of the same stimulus. Complete color blindness is very rare. Partial color vision disorders are more common, for example, when the subject, perceiving all the colors available to him, mixes only the two main ones - green and blue (red-blind or color blind - this is color blindness of the first kind - protonopia) or red and blue (green-blind - color blindness of the second kind - deuteronopia). Finally, the third type of partial color blindness (tritonopia) is "purple color blindness". Protonopia suffered from the famous English chemist D. Dalton, who for the first time in 1794 described this deficiency in his vision. Dalton drew attention to the fact that the geranium flower, which seemed pink to everyone, seemed to him blue during the day, and in the evening, by candlelight, red. Everyone assured him that they did not see any striking difference in the color of geranium petals day and night. This observation prompted Dalton to study the peculiarity of his vision, and he found that red, orange, yellow and green seemed to him almost the same: he called them all yellow. But he could distinguish between blue and purple colors well. Dalton said that the blood seemed bottle green to him, and the grass almost red. It is hard to imagine how Dalton, who suffered from such a pronounced color blindness, did not discover it before the age of 26. Perhaps this was a consequence of our ability to ignore what is familiar. A person suffering from color blindness can often think that he is right and others are wrong. In life, cases of acquired color blindness are known, but in most cases this defect of vision is congenital and is transmitted through the female line, mainly to male offspring. About 4% of all men suffer from color blindness, while among women all types of color blindness are much less common - no more than 0,5% of all women. For the second group of colorblinds (deuteronopes), a characteristic feature is the inability to distinguish light green from dark red and violet from blue, while they do not mix purple with blue, but mix with gray. The third type of partial color blindness is observed much less frequently. For newts, the entire spectrum contains only shades of red and green. For many professions, color blindness is not a major disadvantage. But there are professions where the ability to distinguish colors confidently and strictly is essential - for example, a color-blind person working as a machinist, helmsman, driver, etc., can always cause a disaster by taking one signal color after another. A color-blind person who cannot determine the colors of solutions and dyes cannot successfully work in some operations in the chemical, printing, textile and other industries. The professions of the artist, botanist, tailor, physician and some others also require normal color vision. At present, tables are used to test for color blindness, where spots of another are placed among spots of one color, which together make up some number, letter or figure for any normally sighted person. Color-blind people cannot distinguish the color of these spots from the color of the spots that serve as a background, and therefore they cannot "read" the corresponding numbers, letters or figures. On fig. I, placed on a colored insert, shows the test table of prof. E. B. Rabkina, on which the red-blind - color-blind does not see the circle, and the green-blind does not see the triangle. Color vision in modern conditions will give a person more and more pleasure not only from the perception of beauty in nature and in painting, but also in new forms of art - color photography and cinema. Color is becoming more and more important in industry, as it turns out that the productivity of labor depends to a large extent on the right color of production facilities and equipment. The time is not far off when color television, and then color music, will become widespread.
Further study and more and more effective consolidation of the beneficial properties of the human visual apparatus makes it possible to neutralize, and partially and completely eliminate the harmful effects of shortcomings and defects in our vision. Author: Artamonov I.D. << Back: The device of the eye and visual sensations >> Forward: Illusions associated with the structural features of the eye
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