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 estimatioin.*
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____________________