CHAPTER XIV NOKMAL ILLUSIONS

For One, The problem is to measure certain normal illusions in visnal perception of space. The observer must bear the f oUowing principles clearly in mind :

a. Thèse illusions are normal ; if they do not appear in the record, that is proof of either incompétence or abnormality in space estimation.*

b. Thèse experiments involve a clear distinction between what looks right and what is right. We shall hère dévote 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 thèse experiments dépends 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 ail the experiments

* Barrîng rare cases of spécial training in certain types of lusion.

Plate I.

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before checking any or verifying the acciiracy of his judgment hy 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 référence to it throughout the séries of experiments. This will prevent sighting.

1. The Arc Terminal Illusion, a. One Horizontal Distance. — Place three sîlver 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 Une, 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.

6. 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 f rom 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 PI. 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.

f In the absence of coins, use three disks eut from paper.

174 NOKMAL ILLUSIONS

tîssue-paper with some figure in PI. 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 éléments, we may simply agrée upon the following désignation of the figures in the plates :

PI. 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.

PI. 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 PI. 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, PI. 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, PI. II, by the side of the book ; * cover one end with a pièce 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.

Plate II.

NOKMAL ILLUSIONS 177

and adjust this paper until the exposed part of fhe tissue-paper line looks equal to the middle line of Fig. B. Measure and record the varied line only.

5. The Attraction of Regard in Direction. — Place the tissue-paper over Fig. C, PL II, so that the short line on the tissue-paper coïncides with the line of the three dots, then slide the tissue-paper upward and toward the left, within the same line marked by the dots, until the upper end of the moved line looks to be where the lower line would strike if continued upward in its présent direction as a straight line. Measure and record the distance between the right dot and the right end of the moved line only.

6. The Oppel Illusion: The Bending Effect. a. Upper Line, — Place one of the two dots on the tissuepaper on the joint of Fig. D, PI. II, and turn the tissuepaper so that the other dot shall indicate what direction the lower left line would take if continued upward to the right as a straight line. Sighting is, of course, forbidden. Measure the distance between the movable dot and the end of the line above it only.

6. Lower Line. — Make the same kind of a measurement for the upper left line.

7. The Poggendorff Illusion: The Breaking Effect.— Place one of the dots on the tissue-paper over the right vertical line of Fig. A, PI. III ; indicate with this dot the point at which the short slanting line at the left would strike if continued as a straight line across the

space between the two verticals. Measure and record the distance from the top of the line to the dot only.