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More about illusions from the great “Bates”
ILLUSIONS OF FORM
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.
Illusions of Number
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 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 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.