174 Illusions of Imperfect and Normal Sight
the normal eye does, and the ability to do this may differ very greatly in persons having the same error of refrac tion. 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.
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 checker-board or a cross in the center. In short, an in finite variety of changing forms may be seen. Illumi nation, 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 normally with both eyes. The word PHONES he read correctly with the left eye;