MEASUREMENTS OF ILLUSIONS AND HALLUCINATIONS IN NORMAL LIFE, BY C. E. Seashore, Ph.D. PART FIRST. Illusions op weight : influence of knowledge of size on JUDGMENT OF WEIGHT. When an object lifted is found heavier than was expected, it is overestimated, and when it is found lighter than was expected, it is underestimated. The strongest and most frequent illusion of this kind in normal life is perhaps that which is caused by our accustomed associations between the properties of size and weight of objects. The aim in Part First of this research is to investigate the nature and extent of the illusions of weight as caused by knowledge of the size of the body lifted. Illusions of weight from other sources are incidentally considered. The experiments were made between October, 1893, and May, 1895. The problem is a development from a test made by Gilbert1 on the suggestive force of size on judgment of weight. Dresslar2 has made a statistical study of the same illusion, and it had previously been noticed and subjected to experiment by others.3 5 Since the present experiments were completed, a monograph by Griffing* has appeared, in which he touches upon the same illusion in so far as it is connected with the sense of impact and pressure ; Flournoy' also reports a very popular experiment on the same illusion. 1 Gilbert, Researches on the menial and physical development oj school children. Stud. Tale Psych. Lab., 1894 II 43-45, 59-63. ! Dresslar, Studies in the psychology of touch, Am. Jour. Psych., 1894 VI 313. 8 Charpentier, Analyse de quelques éléments de la sensation de poids, Archives de Physiol., 1891 (5) III 126. Müller and Schumann, Ueber die psychologischen Grundlagen der Vergleichung gehobener Gewichte, Archiv, f. d. ges. Physiol. (Pflüg.), 1889 XLV 37. 4 Griffing, On the sensations of pressure and impact, Psych. Rev., 1895 II Suppl. I. 5 Flournoy, De l'influence de la perception visuelle des corps sur leur poids apparent L’Année Psych., 1894 I 198.