176 Illusions of Imperfect and Normal Sight
pear 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 some times ask if the position of the card has not been changed.
ILLUSIONS OF NON-EXISTENT OBJECTS
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 scientifi cally as "muscae volitantes," or "flying flies," and al though 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.
ILLUSIONS OF COMPLEMENTARY COLORS
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 approxi mately complementary, color. If the object is black upon a white background, a white object upon a black back ground 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.