CHAPTER XIV NORMAL ILLUSIONS For One. THE problem is to measure certain normal illusions in visual perception of space. The observer must bear the following principles clearly in mind: a. These illusions are normal; if they do not appear in the record, that is proof of either incompetence or abnormality in space estimation.* b. These experiments involve a clear distinction between what looks right and what is right. We shall here devote attention to the way things look, not what they really are. The observer is asked to make adjustments so that they look right according to his very best ability, but to make any allowance on the basis of a guess or knowledge of the possible direction and magnitude of the illusion is forbidden. The value of these experiments depends upon the power of self-possession in observing this distinction, and its chief training value also lies in this. The assignment of this experiment is an expression of confidence in the ability and integrity of the observer. c. The observer must perform all the experiments * Barring rare cases of special training in certain types of illusion. before checking any or verifying the accuracy of his judgment by any means whatever.* d. The book should lie upon the table in a normal position for reading, except when otherwise specified, and the observer should maintain the same position with reference to it throughout the series of experiments. This will prevent sighting. 1. The Arc Terminal Illusion. a. One Horizontal Distance. Place three silver dollars or other coins,† edge to edge, in a straight row on the table. Remove the middle coin and place it to the right, in the same line, in such a position that the distance between the adjacent edges of the moved coin and the coin now in the middle shall be equal to the original distance across the three coins. Measure and record the distance between the middle and right-end coins, but do not measure the standard or verify by it. b. Two Vertical Distances.-Place the three coins edge to edge as before; push the middle one straight out forward into such a position that the distance from this coin to each of the other coins shall look equal to the original distance across the three coins. Record the two variable distances only. In the following experiments geometrical figures are made variable by combining some figure in Pl. I on the *If, by any adventure or slip in self-command, the observer should gain objective knowledge of the validity of his judgment or aid in the estimation, this must be recorded, as it is sure to influence the following experiments. In the absence of coins, use three disks cut from paper. tissue-paper with some figure in Pl. II or Pl. III. Tear the tissue-paper off at the perforation and use it with the printed face down. As printed numbers or letters in the plate would be disturbing elements, we may simply agree upon the following designation of the figures in the plates: Pl. II: A, the feathered figure at the top; B, the second figure from the top; C, the slanting bar with the three dots; D, the forked figure at the bottom. Pl. III: A, the large double figure in the upper left-hand corner; B, the slanting, crossed bar with its dot above; C, the cylinder top; D, the parallel lines; and E, the horizontal line. 2. The Angle-line Terminal Illusion: Lengthening and Shortening Effects. Place the tissue-paper over Pl. II so that the apex of the angle of the figure on the tissue-paper lies over the base-line of Fig. A and points toward the left. Bisect the base-line by placing the apex of the angle so that it looks to be at the middle of the line. Measure and record the left section only. 3. The Angle-line Terminal Illusion: Shifting Effect. Bisect the base-line of Fig. A, Pl. II, as above, but use one of the dots on the tissue-paper instead of the apex of the angle figure to mark the middle point. Measure and record the left section only. 4. The Secondary-figure Terminal Illusion: Attraction of Regard in Length. Place the long line of the tissue-paper to the right of Fig. B, Pl. II, by the side of the book; * cover one end with a piece of thick paper *To measure in the book, mark the distance with some sharp point on the edge of a sheet of paper and then measure that on the millimeter scale. |