In his statement of theory of 1890. Seurat said: 'the means for expression is the optic mixture of tones and colours.' Optical mixture meant placing the components required to mix any given colour separately on the canvas so that at a certain distance they would mix in the spectator's eye. Coloured light mixes up towards white (white light is the mixture of all the colours of the spectrum) whereas the mixture of pigments on the palette goes down to grey. Optical mixture gives Divisionist paintings their characteristic intense blond luminosity. At the head of his statement of theory Seurat wrote, 'Aesthetic: Art is Harmony' and, further on. 'Harmony is the analogy of contraries'. In colour. Seurat said, these contraries were; 'complementaries. that is a certain red opposed to its complementary, etc. red-green, orange-blue, yellow-violet ...' This aspect of the theory accounts for one of the most notable features of Seurat's paintings, the way in which colours are accompanied by their complementaries. This can be seen particularly clearly in 'Le Bec du Hoc' in the birds in the sky, which are basically blue but covered in dots of orange. Equally striking is the border painted by Seurat all round the edge of the canvas (the colours on the frame are not by him). In the foreground, where there is green grass, the border is seeded with a predominance of red but where it edges the blue of the sky it becomes predominantly orange. These complementaries harmonise - they complete each other - but they also react with each other to give the painting vibrancy and life.
Seurat's method was immensely laborious and his paintings are elaborately wrought objects. But he always began by making pure Impressionist studies on the spot, and as is the case with C?zanne. Seurat's paintings powerfully convey the feeling of human intelligence shaping the immediate responses of the artist to nature into the enduring forms of art. This effect is particularly heightened in Seurat by the strongly sculptural quality of his forms.
Simon Wilson, Tate Gallery: An Illustrated Companion, Tate Gallery, London, revised edition 1991, p.109