In 1723, after reviewing the French edition (1722) of Isaac Newton’s Opticks (1704), where he encountered Newton’s concept of analogy formation, the French mathematician Louis-Bertrand Castel (1688–1757) began occupying himself with color-sound analogies. In addition, Castel studied, among other things, the light-sound comparisons in the writings of Athanasius Kircher, especially the color-sound-interval tables in his Musurgia universalis (1650).

The purposes of Castel’s Clavecin oculaire were manifold: 1. A deaf person could visually enjoy the beauty of music, a blind person acoustically judge color, and those who could both see and hear could savor both all the more intensely. 2. Painters, who had previously defined the harmony and disharmony of colors on the basis of personal taste and feeling, could now learn the secret of color combinations on a rational basis by means of the transfer of music theory to color theory. 3. Color would obtain a mobility that it could never achieve on a fixed canvas. 4. Music could be fixed to a canvas in such a way that one could wallpaper a room with music and calmly gaze at what during a concert passes by too quickly.

Postulating that seeing and hearing were structured according to identical principles, Castel concluded that this also applied for the other senses, and he even set his sights on a Clavecin pour tous les sens.

Castel’s ideas are likewise to be regarded in the context of a general fascination with optical illusions and visual spectacles, which is reflected in the popularity at the time of the camera obscura, the magic lantern, anamorphoses, and fireworks.

Castel attached a belt to each key of a harpsichord so that when a key was depressed, a window in a box placed on top of the harpsichord would open. Candles burning behind stained glass were used as light sources; in this way, the color assigned to a particular note became visible.

There exist a number of deviating representations of color organs by Castel’s contemporaries, as they apparently describe the various early forms of the Clavecin. Many of the structural details of the instrument remain vague, as neither an illustration nor a construction sketch survive. There are also no descriptions of what the color play looked like; it is only certain that Castel was unable to present moving and changing forms.

Castel struggled with the technical problems of his Clavecin in Paris for nearly thirty years, and, by his own account, he presented it to an audience of 50 on December 21, 1754, in a half-hour concert with four encores, and again on January 1, 1755, to 200 persons. However, there are no further existing sources for these two premiere performances in the history of the color organ. There is also no information about the music that was played: Castel presumably performed melodies or at most two-part compositions, each at a slow tempo. The richly adorned harpsichord music of that period, by composers such as François Couperin or Jean-Philippe Rameau, would most likely have overstrained the cumbersome mechanics of the Clavecin. Castel probably presented silent music as a pure play of colors on the one hand, and a music visualization on the other that was synchronized to the sounds.

Castel’s writings triggered a controversial discussion in France on the relationships between colors and sounds, one in which intellectual giants of the period, such as Denis Diderot, Jean-Jacques d’Ortous de Mairan, Jean-Jacques Rousseau, and Voltaire participated. The standard and detailedness of these debates are unparalleled in the history of the color organ.