Esthetic:
Art is Harmony.
Harmony is the analogy between opposites and the analogy between elements similar in tonal value, color, and line, considered in terms of the dominant, and under the influence of lighting, in gay, calm or sad combinations.
The opposites are:
For tonal value, a more luminous (lighter) tone against a darker one.
For color, the complementaries. i.e., a certain red contrasted with its complementary, etc. [red—green, orange—blue, yellow—violet]
For line, those forming a right angle.
Gaiety in terms of tonal value is a luminous dominant tonality, in terms of color, a warm dominant color; in terms of line, lines above the horizontal.
Calmness in terms of tonal value is an equal amount of dark and light; in terms of color, an equal amount of warm and cool; and in terms of line, the horizontal.
Sadness in terms of tonal value is a dominant dark tonality; in terms of color, a cold dominant color; and in terms of line, downward directions.
Technique:
Given the phenomena of the duration of a light impression on the retina, synthesis is the unavoidable result. The means of expression is the optical mixture of tonal values and colors (both local color and the color of the light source, be it sun, oil lamp, gas, etc.), that is to say, the optical mixture of lights and of their reactions (shadows) in accordance with the laws of contrast, gradation, and irradiation.
The harmony of the frame contrasts with that of the tonal values, colors, and lines of the picture.
From Pages 17-19 of Seurat in Perspective edited by Norma Broude, Englewood Cliffs: Prentice-Hall, 1978.