The French physiologist Augustin Charpentier (1852-1916) published the first accounts of the size-weight illusion-the observation that if two objects differ in size but have equal mass, the smaller will feel heavier when lifted. In the current paper, translations are presented of Charpentier's much-cited 1891 paper on weight perception and the size-weight illusion, and his little-known brief 1886 paper which contains the earliest experimental data on the illusion. Charpentier explained weight illusions in terms of the sense of effort involved in lifting the object and the contrast with the expected effort. Modern research shows that people quickly adapt and use the appropriate force to pick up objects, but the illusion persists even when appropriate force is used; expectations therefore affect the perceptual system more strongly than the motor system.