(Johann Gottlob Krüger, 1743)

Anyone observing the beautiful colors that arise in the prism is easily led to the insight that it might be possible to delight the eye by the alternation and blending of the seven colors just as much as the ear by the seven tones. I have sketched such a machine…which is not unworthy of the name of ocular harpsichord.

“The ocular harpsichord was a kind of ‘thought experiment,’ a realization of an idea in an imagined instrument.” Thus Thomas L. Hankins has aptly characterized the ocular harpsichord proposed by French mathematician Louis Bertrand Castel in 1725. Based on an analogy between the seven tones of the diatonic scale and the seven colors of the rainbow (which Isaac Newton had advanced in his Opticks, 1704), Castel argued that it should be possible to create music for the eyes by replacing the pitches of a harpsichord with colors. Considering himself a “philosopher” not an “artisan,” Castel had little interest in constructing an instrument to demonstrate the color music he derived theoretically. Yet criticism and curiosity about the idea drove him to build an ocular harpsichord to prove its viability. Castel never published technical details or illustrations, however, and all that survives of his efforts are rather vague descriptions by Castel’s visitors, some laudatory some dismissive, of test instruments that revealed colored silks or glasses, illuminated from behind by candles.
In an essay entitled “On a new kind of music, enjoyed by the eyes” (1743), the German professor Johann Gottlieb Krüger proposed a design for an ocular harpsichord in direct response to Castel’s. As Krüger understood it, Castel’s instrument captured the sequential, melodic aspect of music without its simultaneous, harmonic aspect. Krüger’s remedy was to have pitches appear not as colored materials next to one another but rather as colored lights superimposed upon one another, thereby producing color chords. Each key of the instrument would activate both a harpsichord mechanism and a lever that allowed the light from a candle shine to through a colored glass window onto a screen. The size of the window corresponded to the pitch, such that the lowest sounding pitch produced the largest circle of light on the screen, and higher pitches appeared as progressively smaller circles layered towards its center. Though Krüger’s ocular harpsichord design was quite detailed and captured Goethe’s notice, there is no mention of it being tested in practice. Rather, the imagined instrument served another “thought experiment” regarding the analogy between pitch and color and the possibility of music for the eyes, based on a different idea of what music in essence is.
Text: Johann Gottlob Krüger, Naturlehre, 3rd edition, vol. 1
Images: Johann Gottlob Krüger, “De novo musices, quo oculi delectantur, genere” in Miscellanea Berolinensia, ad incrementum scientiarum ex scriptis Societati Regiae Scientiarum exhibits edita 7 (1743)