The beginnings of the movement designated under the name of Neo-Impressionism can be traced back to about 1880. The movement is a direct offshoot of the first Impressionism, originated by a group of young painters who admired it and thought of pushing further still its chromatic principles. The flourishing of Impressionism coincided, as a matter of fact, with certain scientific labours concerning optics. Helmholtz had just published his works on the perception of colours and sounds by means of waves. Chevreul had continued on this path by establishing his beautiful theories on the analysis of the solar spectrum. M. Charles Henry, an original and remarkable spirit, occupied himself in his turn with these delicate problems by applying them directly to aesthetics, which Helmholtz and Chevreul had not thought of doing. M. Charles Henry had the idea of creating relations between this branch of science and the laws of painting. As a friend of several young painters he had a real influence over them, showing them that the new vision due to the instinct of Monet and of Manet might perhaps be scientifically verified, and might establish fixed principles in a sphere where hitherto the laws of colouring had been the effects of individual conception. At that moment the criticism which resulted from Taine's theories tried to effect a _rapprochement_ of the artistic and scientific domains in criticism and in the psychologic novel. The painters, too, gave way to this longing for precision which seems to have been the great preoccupation of intellects from 1880 to about 1889.