180 Illusions of Imperfect and Normal Sight
but one can look at it indefinitely by central fixation without any such result.
While persons with imperfect sight usually see the stars twinkle, they do not necessarily do so. Therefore it is evident that the strain which causes the twinkling is different from that which causes the error of refrac tion. If one can look at a star without trying to see it, it does not twinkle; and when the illusion of twinkling has been produced, one can usually stop it by "swing ing" the star. On the other hand, one can start the planets, or even the moon, to twinkling, if one strains sufficiently to see them.
ILLUSIONS OF NORMAL SIGHT
The illusions of normal sight include all the phe nomena of central fixation. When the eye with normal sight looks at a letter on the Snellen test card, it sees the point fixed best, and everything else in the field of vision appears less distinct. As a matter of fact, the whole letter and all the letters may be perfectly black and distinct, and the impression that one letter is blacker than the others, or that one part of a letter is blacker than the rest, is an illusion. The normal eye, however, may shift so rapidly that it appears to see a whole line of small letters all alike simultaneously. As a matter of fact there is, of course, no such picture on the retina. Each letter has not only been seen separately, but it has been demonstrated in the chapter on "Shifting and Swing ing" that if the letters are seen at a distance of fifteen or twenty feet, they could not be recognized unless about four shifts were made on each letter. To produce the impression of a simultaneous picture of fourteen letters,