AbstractThe question of a fundamental aesthetics and, derivable from it, a harmony based on rational principles, is an old one. A remarkable attempt at an answer is that provided some 100 years ago by the French “psychobiophysicist” Charles Henry (1859–1926). In 1888 he published a color circle and an aesthetic protractor with which a systematic evaluation of colors and forms for their aesthetic qualities was possible. Both the circle and the protractor, were based on Henry's psychophysiological principles of dynamogeny and inhibition. A theory of rhythm was added by Henry related to prime numbers or their reciprocal fractions. Thus, an aesthetics founded upon what Henry considered to be sound scientific principles resulted. Henry's ideas and theories had a considerable impact on contemporary neo‐impressionist artists, particularly George Seurat and Paul Signac.