Abstract
A variation of the method of paired comparisons was used to investigate the Delbouf illusion. Significant variation in the magnitude of the illusion including positive and negative effects was noted. This variation did not differ significantly from a formula proposed by Piaget for describing this type of illusion. The method achieved an accuracy of the order of 1\160 in. within the range of variation of the illusion using 69 cases. The obtainable accuracy depends largely on the number of Ss available. The reliability of the method by the Gulliksen‐Tukey formula was 87.