178 Illusions of Imperfect and Normal Sight
strated that this appearance is simply an illusion of the mind.
CAUSE OF THE ILLUSIONS OF IMPERFECT SIGHT
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 demon strated 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 strain ing consciously in the same way that one strains un consciously; 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, ar ranged vertically. If the strain is great enough, there