Persons with imperfect sight always have illusions of vision; so do persons with normal sight. But while the illusions of normal sight are an evidence of relaxation, the illusions of imperfect sight are an evidence of strain. Some persons with errors of refraction have few illusions, others have many; because the strain which causes the error of refraction is not the same strain that is responsible for the illusions.
The illusions of imperfect sight may relate to the color, size, location and form of the objects regarded. They may include appearances of things that have no existence at all, and various other curious and interesting manifestations.
When a patient regards a black letter and believes it to be grey, yellow, brown, blue, or green, he is suffering from an illusion of color. This phenomenon differs from colorblindness. The color-blind person is unable to differentiate between different colors, usually blue and green, and his inability to do so is constant. The person suffering from an illusion of color does not see the false colors constantly or uniformly. When he looks at the Snellen test card the black letters may appear to him at one time to be grey; but at another moment they may appear to be a shade of yellow, blue, or brown. Some patients always see the black letters red; to others they appear red only occasionally. Although the letters are all of the same color, some may see the large letters black and the small ones yellow or blue. Usually the large letters are seen darker than the small ones, whatever color they appear to be. Often different colors appear in the same letter, part of it seeming to be black, perhaps, and the rest grey or some other color. Spots of black, or of color, may appear on the white; and spots of white, or of color, on the black.
Large letters may appear small, or small letters large. One letter may appear to be of normal size, while another of the same size and at the same distance may appear larger or smaller than normal. Or a letter may appear to be of normal size at the near-point and at the distance, and only half that size at the middle distance. When a person can judge the size of a letter correctly at all distances up to twenty feet his vision is normal. If the size appears different to him at different distances, he is suffering from an illusion of size. At great distances the judgment of size is always imperfect, because the sight at such distances is imperfect, even though perfect at ordinary distances. The stars appear to be dots, because the eye does not possess perfect vision for objects at such distances. A candle seen half a mile away appears smaller than at the near-point; but seen through a telescope giving perfect vision at that distance it will be the same as at the near-point. With improved vision the ability to judge size improves.
The correction of an error of refraction by glasses seldom enables the patient to judge size as correctly as the normal eye does, and the ability to do this may differ very greatly in persons having the same error of refraction. A person with ten diopters of myopia corrected by glasses may (rarely) be able to judge the sizes of objects correctly. Another person, with the same degree of myopia and the same glasses, may see them only one-half or one-third their normal size. This indicates that errors of refraction have very little to do with incorrect perceptions of size.
Round letters may appear square or triangular; straight letters may appear curved; letters of regular form may appear very irregular; a round letter may appear to have a checkerboard or a cross in the center. In short, an infinite variety of changing forms may be seen. Illumination, distance and environment are all factors in this form of imperfect sight. Many persons can see the form of a letter correctly when other letters are covered, but when the other letters are visible they cannot see it. The indication of the position of a letter by a pointer helps some people to see it. Others are so disturbed by the pointer that they cannot see the letter so well.
Multiple images are frequently seen by persons with imperfect sight, either with both eyes together, with each eye separately, or with only one eye. The manner in which these multiple images make their appearance is sometimes very curious. For instance, a patient with presbyopia read the word HAS normally with both eyes. The word PHONES he read correctly with the left eye; but when he read it with the right eye he saw the letter P double, the imaginary image being a little distance to the left of the real one. The left eye, while it had normal vision for the word PHONES, multiplied the-shaft of a pin when this object was in a vertical position (the head remaining single), and multiplied the head when the position was changed to the horizontal (the shaft then remaining single). When the point of the pin was placed below a very small letter, the point was sometimes doubled while the letter remained single. No error of refraction can account for these phenomena. They are tricks of the mind only. The ways in which multiple images are arranged are endless. They are sometimes placed vertically, sometimes horizontally or obliquely, and sometimes in circles, triangles and other geometrical forms. Their number, too, may vary from two to three, four, or more. They may be stationary, or may change their position more or less rapidly. They also show an infinite variety of color, including a white even whiter than that of the background.
A period following a letter on the same horizontal level as the bottom of the letter may appear to change its position in a great variety of curious ways. Its distance from the letter may vary. It may even appear on the other side of the letter. It may also appear above or below the line. Some persons see letters arranged in irregular order. In the case of the word AND, for instance, the D may occupy the place of the N. or the first letter may change places with the last. All these things are mental illusions. The letters sometimes appear to be farther off than they really are. The small letters, twenty feet distant, may appear to be a mile away. Patients troubled by illusions of distance sometimes ask if the position of the card has not been changed.
When the eye has imperfect sight the mind not only distorts what the eye sees, but it imagines that it sees things that do not exist. Among illusions of this sort are the floating specks which so often appear before the eyes when the sight is imperfect, and even when it is ordinarily very good. These specks are known scientifically as “muscae volitantes,” or “flying flies,” and although they are of no real importance, being symptoms of nothing except mental strain, they have attracted so much attention, and usually cause so much alarm to the patient, that they will be discussed at length in another chapter.
When the sight is imperfect the subject, on looking away from a black, white, or brightly colored object, and closing the eyes, often imagines for a few seconds that he sees the object in a complementary, or approximately complementary, color. If the object is black upon a white background, a white object upon a black background will be seen. If the object is red, it may be seen as blue; and if it is blue, it may appear to be red. These illusions, which are known as “after-images,” may also be seen, though less commonly, with the eyes open, upon any background at which the subject happens to look, and are often so vivid that they appear to be real.
Persons with normal sight see the sun white, the whitest white there is; but when the sight is imperfect it may appear to be any color in the spectrum – red, blue, green, purple, yellow, etc. In fact, it has even been described by persons with imperfect vision as totally black. The setting sun commonly appears to be red, because of atmospheric conditions; but in many cases these conditions are not such as to change the color, and while this still appears to be red to persons with imperfect vision, to persons with normal vision it appears to be white. When the redness of a red sun is an illusion, and not due to atmospheric conditions, its image on the ground glass of a camera will be white, not red, and the rays focussed with a burning glass will also be white. The same is true of a red moon.
After looking at the sun most people see black or colored spots which may last from a few minutes to a year or longer, but are never permanent. These spots are also illusions, and are not due, as is commonly supposed, to any organic change in the eye. Even the total blindness which sometimes results, temporarily, from looking at the sun, is only an illusion.
The idea that the stars should twinkle has been embodied in song and story, and is generally accepted as part of the natural order of things; but it can be demonstrated that this appearance is simply an illusion of the mind.
All the illusions of imperfect sight are the result of a strain of the mind, and when the mind is disturbed for any reason illusions of all kinds are very likely to occur. This strain is not only different from the strain that produces the error of refraction, but it can be demonstrated that for each and every one of these illusions there is a different kind of strain. Alterations of color do not necessarily affect the size or form of objects, or produce any other illusion, and it is possible to see the color of a letter, or of a part of a letter, perfectly, without recognizing the letter. To change black letters into blue, or yellow, or another color, requires a subconscious strain to remember or imagine the colors concerned, while to alter the form requires a subconscious strain to see the form in question. With a little practice anyone can learn to produce illusions of form and color by straining consciously in the same way that one strains unconsciously; and whenever illusions are produced in this way it will be found that eccentric fixation and an error of refraction have also been produced.
The strain which produces polyopia is different again from the strain which produces illusions of color, size and form. After a few attempts most patients easily learn to produce polyopia at will. Staring or squinting, if the strain is great enough, will usually make one see double. By looking above a light, or a letter, and then trying to see it as well as when directly regarded, one can produce an illusion of several lights, or letters, arranged vertically. If the strain is great enough, there may be as many as a dozen of them. By looking to the side of the light or letter, or looking away obliquely at any angle, the images can be made to arrange themselves horizontally, or obliquely at any angle.
To see objects in the wrong location, as when the first letter of a word occupies the place of the last, requires an ingenuity of eccentric fixation and an education of the imagination which is unusual.
The black or colored spots seen after looking at the sun, and the strange colors which the sun sometimes seems to assume, are also the result of the mental strain. When one becomes able to look at the orb of day without strain, these phenomena immediately disappear.
After-images have been attributed to fatigue of the retina, which is supposed to have been so overstimulated by a certain color that it can no longer perceive it, and therefore seeks relief in the hue which is complementary to this color. If it gets tired looking at the black C on the Snellen test card, for instance, it is supposed to seek relief by seeing the C white. This explanation of the phenomenon is very ingenious but scarcely plausible. The eyes cannot see when they are closed;; and if they appear to see under these conditions, it is obvious that the subject is suffering from a mental illusion with which the retina has nothing to da. Neither can they see what does not exist; and if they appear to see a white C on a green wall where there is no such object, it is obvious again that the subject is suffering from a mental illusion. The after-image indicates, in fact, simply a loss of mental control, and occurs when there is an error of refraction, because this condition also is due to a loss of mental control. Anyone can produce an afterimage at will by trying to see the big C all alike – that is, under a strain; but one can look at it indefinitely by central fixation without any such result.
While persons with imperfect sight usually see the stars twinkle, they do not necessarily do so. Therefore it is evident that the strain which causes the twinkling is different from that which causes the error of refraction. If one can look at a star without trying to see it, it does not twinkle; and when the illusion of twinkling has been produced, one can usually stop it by “swinging” the star. On the other hand, one can start the planets, or even the moon, to twinkling, if one strains sufficiently to see them.
The illusions of normal sight include all the phenomena of central fixation. When the eye with normal sight looks at a letter on the Snellen test card, it sees the point fixed best,- and everything else in the field of vision appears less distinct. As a matter of fact, the whole letter and all the letters may be perfectly black and distinct, and the impression that one letter is blacker than the others, or that one part of a letter is blacker than the rest, is an illusion. The normal eye, however, may shift so rapidly that it appears to see a whole line of small letters all alike simultaneously. As a matter of fact there is, of course, no such picture on the retina. Each letter has not only been seen separately, but it has been demonstrated in the chapter on “Shifting and Swinging” that if the letters are seen at a distance of fifteen or twenty feet, they could not be recognized unless about four shifts were made on each letter. To produce the impression of a simultaneous picture of fourteen letters, therefore, some sixty or seventy pictures, each with some one point more distinct than the rest, must have been produced upon the retina. The idea that the letters are seen all alike simultaneously is, therefore, an illusion. Here we have two different kinds of illusions. In the first case the impression made upon the brain is in accordance with the picture on the retina, but not in accordance with the fact. In the second the mental impression is in accordance with the fact, but not with the pictures upon the retina.
The normal eye usually sees the background of a letter whiter than it really is. In looking at the letters on the Snellen test card it sees white streaks at the margins of the letters, and in reading fine print it sees between the lines and the letters, and in the openings of the letters, a white more intense than the reality. Persons who cannot read fine print may see this illusion, but less clearly. The more clearly it is seen, the better the vision; and if it can be imagined consciously – it is imagined unconsciously when the sight is normal – the vision improves. If the lines of fine type are covered, the streaks between them disappear. When the letters are regarded through a magnifying glass by the eye with normal sight, the illusion is not destroyed, but the intensity of the white and black are lessened. With imperfect sight it may be increased to some extent by this means, but will remain less intense than the white and black seen by the normal eye. The facts demonstrate that perfect sight cannot be obtained with glasses.
The illusions of movement produced by the shifting of the eye and described in detail in the chapter on “Shifting and Swinging” must also be numbered among the illusions of normal sight, and so must the perception of objects in an upright position. This last is the most curious illusion of all. No matter what the position of the head, and regardless of the fact that the image on the retina is inverted, we always see things right side up.