William Horatio Bates

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”