"Pisarro explained the Neo-Impressionist theories to his dealer Durand-Ruel in a letter written towards the end of 1886. He stressed the importance of Seurat's role as inventor of the theory, and described the new function of colour, which replaced the mechanical mixtures of pigments with optical mixtures, where colours partially fused in the spectator's eye. The component parts of each optical colour mixture were to be painted in separate touches so that they retained their colour purity. When colours were mixed on the palette, they could only be combined with close neighbors on the colour circle, so as to avoid excessive dulling of the hues. Pissaro noted that the great colour theorists who had influenced Seurat's thinking were Chevreul, the Scott Maxwell, and the American Ogden Rood. Optical colour mixtures, they argued, were more luminous than mixed pigments."