A Graeco-Egyptian scholar in Alexandria, who wrote astronomical and other texts, including the Optics (c. 170), which contained his views on perception.
His account of an earth-centred universe with eight concentric crystal spheres, each of the first seven carrying an independently moving heavenly body, the outermost the 'fixed' stars — with an elaborate system of epycycles to explain the various motions, including reversals of motion of Mars — dominated scientific thinking until the 16th century. It was described in his Algamist ('the greatest'). His ideas stem from Hypparchus (fl. 160–125bc) who discovered the precession of the equinoxes, estimated the distances of the sun and moon from measurements, and is credited with inventing trigonometry. Ptolomy is far better remembered because the Algamist and other writings remained in print and were accepted 'bibles of belief' for many centuries. They affected how music was conceived, from the Ptolomaic 'music of the spheres', modified much later by Kepler, when the sun was seen as dominating the planetary system with earth a minor player. His Optics influenced the more famous work by Alhazen. Ptolemy combined the mathematical, philosophical, and physiological traditions. He held an extramission–intromission visual theory: the rays from the eye formed a cone, the vertex being within the eye, and the base defining the visual field. The rays knew their own length, and thus conveyed information about the distance and orientation of surfaces. Size and shape were determined by the visual angle combined with perceived distance and orientation (this being one of the earliest statements of size and shape constancy). Ptolemy used these principles to explain many objective and subjective phenomena connected with illumination and colour, size, shape, movement, and binocular vision. He also divided illusions into those caused by physical, optical, or judgemental factors. He gave an obscure explanation of the moon illusion based on the difficulty of looking upwards.
(Published 2004)
— Helen E. Ross/Richard L. Gregory
- Bibliography
- Smith, A. M. (1996). Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary.
