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Illusions associated with the structural features of the eye. Encyclopedia of visual illusions

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The optical system of the eye is not free from spherical and chromatic aberrations.

The essence of spherical aberration is that the focus for rays entering the eye parallel to its axis and at a small distance from it is farther from the pupil than the focus for rays that are more distant from the axis. The edges of the pupillary space refract light more than its middle. Partly for this reason, as stated earlier, we see small light sources in the form of radiant stars. It is easy to verify the presence of spherical aberration of the eye by doing such an experiment.

If printed text is placed in front of the eye closer than the distance of the best vision, when it is no longer possible to clearly see the letters, and then a piece of paper with a small hole is taken and placed in front of the very eye, then the letters will again become clearly visible. If you hold a black thread in front of a bright flame, then it seems broken to us - circles of light scattering on the retina cover the thread on both sides and make it invisible.

In an effort to better see the object, we "squint", bringing the eyelids together, and thereby reduce the hole through which light rays pass into the eye. As a result, the edges of the pupil and the lens are "turned off" from work, spherical aberration is reduced, and we see the object more clearly and sharply. In bright light, when the pupil constricts, spherical aberration decreases and we see better.

The eye is not an achromatic system: the focus of violet rays is located 0,43 mm closer to the lens than the focus of red rays if the eye is accommodated to infinity. Therefore, objects, especially white ones, illuminated with white light, give an image on the retina surrounded by a colored border. Usually we do not notice her, as she is very weak. However, it is easy to detect it with the help of simple experiments, for example, by examining Fig. 5.

Fig.5. If you bring this drawing very close to the eye, then near the black circles we will see colored borders. This indicates that the eye is not free from chromatic aberration.

We will observe the same effect if we look through a small hole in a piece of paper at the edge of the roof against the background of a bright sky. Raising the leaf so that the rays fall on the periphery of the pupil, we note that the sky near the roof will seem reddish. The foregoing is easy to explain if we remember that the reverse image is obtained on the retina and that when rays fall on the edge of the lens, blue rays are refracted more than red ones. Chromatic aberration of the eye creates difficulties when viewing scales or interference fringes, as well as when observing celestial bodies using astronomical instruments.

There are known cases of myopia in people only at dusk, when the outlines of visible objects become less sharp. If at the same time the clear visibility of objects is limited to a distance of 2 m, then the resulting myopia corresponds to 0,5 diopters. During the day, the eye has a maximum sensitivity in the yellow-green region of the spectrum, and at dusk the maximum sensitivity shifts to the blue-green region. The eye, like a lens, refracts blue-green rays more strongly than yellow ones. Therefore, nocturnal myopia occurs in humans due to chromatic aberration of the eye. In addition, in low light, the pupil of the eye expands and the edges of the lens begin to play a large role in the formation of the image on the retina. Consequently, nocturnal myopia is to some extent due to the spherical aberration of the eye.

Astigmatism* eyes. Astigmatism of the eye is called its defect, usually due to the non-spherical (toric) shape of the cornea and sometimes the non-spherical shape of the lens surfaces.

* (Greek "stigma" - dot.)

Astigmatism of the human eye was first discovered in 1801 by the English physicist T. Jung. In the presence of this defect (by the way, not in all people it manifests itself in a sharp form), there is no point focusing of rays falling parallel to the eye, due to different refraction of light by the cornea in different sections.

With strong astigmatism, a person sees clearly, for example, only vertical lines, and sees horizontal lines blurry, or vice versa (Fig. 6). Pronounced astigmatism is corrected by glasses with cylindrical glasses, which refract light rays only in the direction perpendicular to the axis of the cylinder.

Rice. 6. The astigmatic eye sees figure A as shown on the right.

Eyes completely free from this deficiency are rare in humans, as can be easily seen by considering the figs. 7, 8 and 9.

Rice. 7. Concentric, close to each other circles appear interrupted, not equally spaced from each other

Rice. 8. The same (monotone) shading of the quadrants of this circle does not seem equally bright

Rice. 9. Certain letters appear darker depending on the position of the pattern relative to the eye.

To test the eyes for astigmatism, ophthalmologists often use a special table (Fig. 10), where twelve circles have shading of equal thickness at regular intervals. An eye with astigmatism will see the lines of one or more circles as blacker. The direction of these more black lines allows us to conclude the nature of the astigmatism of the eye.

Rice. 10. Figure for determining the degree and measure of astigmatism of the eye

If astigmatism is due to the non-spherical shape of the lens surface, then when moving from a clear vision of horizontal objects to viewing vertical objects, a person must change the accommodation of the eyes.

Most often, the distance of clear vision of vertical objects is less than that of horizontal ones. This is partly due to the visual defect "overestimation of vertical lines", which will be discussed later (see paragraph 5).

Blind spot. The presence of a blind spot on the retina of the eye was first discovered in 1668 by the famous French physicist E. Mariotte. Mariotte describes his experience, which makes it possible to verify the presence of a blind spot, as follows: “I attached a small circle of white paper on a dark background, approximately at eye level, and at the same time asked the other circle to be held to the side of the first, to the right at a distance of about two feet but a little lower so that the image of it falls on the optic nerve of my right eye, while I close my left. I stood opposite the first circle and gradually moved away, keeping my right eye on it. , which measured about 9 inches, completely disappeared from view. I could not attribute this to its side position, for I distinguished other objects that were even more sideways than he was; I would have thought that he had been removed if I had not found him again with slightest movement of the eyes. It is known that Marriott amused the English king Charles II and his courtiers by teaching them to see each other without a head.

* (1 foot equals 0,3048 m, 1 inch equals 25,4 mm.)

The retina of the eye in the place where the optic nerve enters the eye does not have light-sensitive endings of nerve fibers (rods and cones). Consequently, the images of objects falling on this place of the retina are not transmitted to the brain.

You can verify the presence of a blind spot by looking at any of Fig. 11, 12 and 13. In these figures, the blind spot for the right eye is found to the right of the central beam, and for the left - to the left. Under these conditions, in the first case, the right side of the figure disappears, and in the second, the left side. Therefore, for the right eye, it is necessary to set the drawing so that the left part of the drawing is directly opposite the eye (for example, the central circle in Fig. 11 and 12 or the cross in Fig. 13), and for the left - the right part of the drawing. Then, if necessary, remove or zoom in on the drawing, or move it a little to the side until a clear effect is achieved.

Fig. 11

Fig. 12

Rice. 13. Close the left eye and look with the right at the figure depicted on the left, holding the drawing at a distance of 15-20 cm from the eye. At a certain position of the drawing relative to the eye, the image of the right figure will cease to be visible. Rice. 11 and 13 allow you to detect the blind spot of both the right and left eyes

Academician S. I. Vavilov wrote about the structure of the eye: “How simple the optical part of the eye is, how complex its perceiving mechanism is. Not only do we not know the physiological meaning of the individual elements of the retina, but we are not able to say how appropriate the spatial distribution of light-sensitive cells, to what you need a blind spot, etc.

Before us is not an artificial physical device, but a living organ in which advantages are mixed with disadvantages, but everything is inextricably linked into a living whole.

A blind spot, it would seem, should prevent us from seeing the whole object, but under normal conditions we do not notice this. Firstly, because the images of objects falling on the blind spot in one eye are not projected onto the blind spot in the other; secondly, because the falling parts of objects are involuntarily filled with images of neighboring parts that are in the field of view.

If, for example, when looking at black horizontal lines, some areas of the image of these lines on the retina of one eye fall on a blind spot, then we will not see a break in these lines, since our other eye will make up for the shortcomings of the first. Sections of "straight lines" passing through the blind spot of any eye will be continued by our consciousness along the shortest path even if in reality the lines have a break or a bend in this place. So, for example, if the blind spot is against the "middle of the cross", we will "see" the cross even if in reality its four branches do not connect in the middle.

Here is another interesting experience. If we hold a sheet of white paper with a red spot in front of us so that this red spot is not visible, for example, with the right eye, we will still see the spot with the left eye, that is, we will see a sheet of paper with a red spot, which is true. If, however, we take completely white paper, and hold red glass in front of the left eye, then the whole paper will appear reddish-white, and the place corresponding to the blind spot of the right eye does not differ from the rest of the background. Even when observing with one eye, our reason compensates for the lack of a retina and the disappearance of some details of objects from the field of view does not reach our consciousness. The blind spot is quite large (at a distance of two meters from the observer, even a person's face can disappear from the field of view), but under normal conditions of vision, the mobility of our eyes eliminates this "lack" of the retina.

Irradiation*. The phenomenon of irradiation consists in the fact that light objects against a dark background seem to be enlarged against their real sizes and, as it were, capture part of the dark background. This phenomenon has been known since very ancient times. Even Vitruvius (I century BC), the architect and engineer of Ancient Rome, pointed out in his writings that when dark and light are combined, "light devours darkness." On our retina, the light partly captures the place occupied by the shadow.

* (In Latin - incorrect radiation.)

The initial explanation of the phenomenon of irradiation was given by R. Descartes, who argued that an increase in the size of light objects occurs due to the spread of physiological excitation to places adjacent to a directly irritated place in the retina.

However, this explanation is currently being replaced by a new, more rigorous one, formulated by Helmholtz, according to which the following circumstances are the root cause of irradiation.

Each luminous point is depicted on the retina of the eye in the form of a small circle of scattering due to the imperfection of the lens, inaccurate accommodation, etc. When we examine a light surface against a dark background, due to aberrational scattering, the boundaries of this surface seem to move apart, and the surface seems to us larger than its true geometric dimensions; it seems to extend over the edges of the dark background surrounding it. The effect of irradiation is the sharper, the worse the eye is accommodated.

Due to the presence of circles of light scattering on the retina, under certain conditions (for example, very thin black threads), dark objects on a light background can also be subjected to illusory exaggeration - this is the so-called negative irradiation.

There are a lot of examples when we can observe the phenomenon of irradiation, it is not possible to give them here in full. The presence of irradiation is clearly confirmed by Fig. 14-19.

Rice. 14. A white square on a black background on the right seems larger than a black square on a light background on the left

Rice. 15. White figures on a black background appear larger than black figures on white.

Rice. 16. Black stripe on a white background (left) seems narrower than a white stripe on a black background

Rice. 17. The square is crossed out by two wide black stripes that appear to be pinched where they touch the sides of the square (as shown on the left)

Rice. 18. The crosshairs of the black stripes do not appear black, but gray. This effect can only partly be explained by irradiation

Rice. 19. Wires that cross at sharp angles appear to be broken, as shown at the bottom of the figure. If the wind swings the wires, then the white gap oscillates in time with these swings.

The great Italian artist, scientist and engineer Leonardo da Vinci, in his notes, says the following about the phenomenon of irradiation:

"When the Sun is visible behind leafless trees, all their branches that are opposite the solar body are so reduced that they become invisible, the same will happen with the shaft placed between the eye and the solar body. I saw a woman dressed in black, with a white band on the head, and the latter seemed twice as wide as the width of the shoulders of the women who were dressed in black.If from a great distance we look at the battlements of the fortresses, separated from each other by intervals equal to the width of these teeth, then the intervals seem to be much larger than the teeth ... ".

The great German poet Goethe points out a number of cases of observations of the phenomenon of irradiation in nature in his treatise "The Teaching of Flowers". He writes about this phenomenon as follows:

"A dark object seems to be smaller than a light object of the same size. If we consider simultaneously a white circle on a black background and a black circle of the same diameter on a white background, then the latter seems to us about 1/5 smaller than the first. If the black circle is made correspondingly larger, they will seem equal The young crescent of the moon seems to belong to a circle of a larger diameter than the rest of the dark part of the moon, which is sometimes distinguishable in this case.

The phenomenon of irradiation in astronomical observations makes it difficult to observe thin black lines on objects of observation; in such cases it is necessary to stop the lens of the telescope. Physicists, due to the phenomenon of irradiation, do not see thin peripheral rings of the diffraction pattern.

In a dark dress, people seem thinner than in a light one. Light sources visible from behind the edge produce an apparent notch in it. The ruler, from which the flame of the candle appears, is represented with a notch in this place. The rising and setting sun makes a notch in the horizon.

A few more examples. The black thread, if "held in front of a bright flame, seems to be interrupted in this place; the incandescent filament of an incandescent lamp seems thicker than it really is; light wire on a dark background seems thicker than on a light one. Bindings in window frames seem smaller, than they really are.A statue cast in bronze looks smaller than one made of plaster or white marble.

The architects of ancient Greece made the corner columns of their buildings thicker than others, given that these columns from many points of view will be visible against the background of a bright sky and, due to the phenomenon of irradiation, will appear thinner.

We are subjected to a peculiar illusion in relation to the apparent magnitude of the Sun. Artists tend to draw the Sun too large compared to other depicted subjects. On the other hand, in photographic landscape shots, which also show the Sun, it seems unnaturally small to us, although the lens gives a correct image of it.

Note that the phenomenon of negative irradiation can be observed in such cases when a black thread or slightly shiny metal wire appears thicker on a white background than on black or gray. If, for example, a lace maker wants to show off her art, then it is better for her to make lace from black thread and spread it on a white lining. If we observe the wires against a background of parallel dark lines, such as a tiled roof or brickwork, then the wires appear thickened and broken where they cross each of the dark lines. These effects are also observed when the wires are superimposed in the field of view on a clear outline of the building.

Probably, the phenomenon of irradiation is associated not only with the aberration properties of the lens, but also with the scattering and refraction of light in the media of the eye (a layer of liquid between the eyelid and the cornea, media filling the anterior chamber and the entire interior of the eye). Therefore, the irradiative properties of the eye are obviously related to its resolving power and radiant perception of "point" light sources (Fig. 20). The ability of the eye to overestimate acute angles is connected with aberrational properties, and therefore, partly with the phenomenon of irradiation.

Rice. 20. Black lines are sharply distinguishable only in some radial directions; on others they merge into a gray background. To perceive the whole figure, the eye must make circular movements.

Author: Artamonov I.D.

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