Joseph Rémi Léopold Delbœuf

Joseph Rémi Léopold Delbœuf (30 September 1831, Liège, Belgium – 14 August 1896, Bonn, Germany) was a Belgian experimental psychologist who studied visual illusions including his work on the Delboeuf illusion. His father, a pewter craftsman, died early in Delboeuf’s life. He received a scholarship from the University of Bonn and continued his postdoctoral research under philosopher and mathematician Friedrich Ueberweg.

Delboeuf is known for his description of the Delboeuf illusion in 1893. At the First International Congress on scientific and experimental hypnotism (1889), a motion was put forward to ban non-medical practitioners from using hypnosis. However, Delboeuf argued that a medical degree was not required to practice hypnotism; rather, it should be used freely, yet with caution. To test his hypotheses, he suggested that he would need two identical patients to be treated with different approaches. Instead, he used the symmetry of the opposite sides of the body. They argued that subjects drifted into a dream-like state in which they became aware of the suggestion and time remaining in this state. Their idea was very similar to a double consciousness theory suggested by Pierre Janet. They highlighted that subjects are unable to remember the suggestion when they wake because the memory is only available in the dream-like or hypnotic state. He argued that, in order to use geometry to find the fundamental qualities of “determinations of space,” we must first understand the concepts of both “determination” and “space.” In this paper, he independently discovered Euclidean postulate 5. Postulate 5 states that, if a line intersects two straight lines that together form two interior angles on the same side that sum to less than 180 degrees, then the two straight lines must meet on that side.

Psychophysics Delboeuf first conducted psychophysics experiments at the University of Gand (1855-1866). He primarily worked on Weber's Law (proposed by Fechner):

Where S stands for the strength of a sensation, I stands for the intensity of an external stimulus, and K is a constant. Delboeuf proposed that a sensation consists of two aspects: (1) formation of the sensation and (2) how sense organs function. Delboeuf is well known for his work on this illusion. Later in his career, he published an explanation of the Muller-Lyer Illusion in terms of eye movements instead of the mis-estimation of angles theory Brentano had published. Many studies have been conducted on the Delboeuf illusion (1865) since the publication of his original study, and many explanations for this illusion have been put forth.

Works

De la moralité en littérature (1861) De la psychologie comme science naturelle, son présent et son avenir (1875) Le sommeil et les rêves (1885) La mémoire chez les hypnotisés (1886) De l'influence de l'éducation et de l'imitation (1886) Une Visite à la Salpêtrière (1886) De la prétendue veille somnambulique (1887) Magnétisme: À propos d'une Visite À L'Écold de Nancy (1889) comme Quoi il n'y a pas d'hypnoitisme (1891) Quelques Considérations sur la Psychologie de l'Hypnotisme (1892) Note sur certaines illusions d'optique; essai d'une theorie psychophysique de la maniere dont l'oeil apprecie les distances et les angles (1865) Seconde note sur de nouvelles illusions d'optique: Essai d'une theorie psychophysique de la maniere dont l'oeil apprecie les grandeurs (1865) Etude psychophysique. Recherches theoriques et experimentales sur la mesure des sensations et specialement des sensations de lumiere et de fatigue Memoires couronnes et autres memoires publies par l'Academie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique (1873) La mesure des sensations (I): Reponses a propos du logarithme des sensations (1875) Analyse du memoire: Theorie generale de la sensibilite (1875) Theorie generale de la sensibilite Memoires couronnes et autres memoires publies par l'Academie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique (1876) La psychologie comme science naturelle: Son present et son avenir (1876) La loi psycho-physique: Hering contre Fechner (1877) La loi psychophysique et le nouveau livre de Fechner (1878) Elements de psychophysique generale et speciale (1883) Examen critique de la loi psychophysique: Sa base et sa signification (1883)

Articles in Popular Science Monthly