Louis-Bertrand Castel, a French natural philosopher, died Jan. 9, 1757, at age 58. Castel was a Jesuit, one of many of his order to engage in scientific pursuits. In 1725, he wrote a letter in which he proposed the idea of an “ocular harpsichord”, an instrument that would play colors instead of sounds. This was not an entirely new proposal, as Athanasius Kircher (a Jesuit of the previous generation) had wondered about making “visible music,” and Isaac Newton (most definitely not a Jesuit) had noticed the parallel between the 7 colors of the visible spectrum and the 7 notes of the musical scale. But neither Kircher nor Newton actually contemplated building an instrument that would play thirds and fifths of red, blue and yellow.
Castel followed up his initial letter of 1725 with a longer treatise, Optique des couleurs (1740), in which he further developed his idea. The ocular harpsichord, as proposed here, would have small pieces of colored glass covered by tiny curtains; when the proper key was pressed, the curtain would be lifted and light would shine through the glass. The great challenge, of course, was to figure out which colors corresponded to which musical notes, so that a deaf person, viewing the play of colors, would experience melodies and harmonies, just like a listener exposed to conventional music. Castel devised what he thought was an appropriate scale of colors, but it is not clear whether he ever built an actual ocular harpsichord, or instead just talked about it. The latter was more the Jesuit way–Kircher and Gaspar Schott and many of the other great Jesuit inventors rarely built the magic lanterns, speaking trumpets, and perpetual motion machines that they discussed so extensively in their treatises; invention was more of a mental exercise for the Jesuits than a contribution to technology. All that survives is a caricature of Castel, made by a contemporary, that shows him playing what is almost certainly an imaginary color keyboard (first image).
We do not have the Optique des couleurs in our Collections, but we do have two other treatises by Castel, the title pages of which we display above. It is evident just from the title pages that Castel spoke his own mind quite freely and was not shy about taking on the likes of Isaac Newton, Gottfried Leibniz, and Voltaire. Castel’s quarrels with his contemporaries seems to have arisen from his belief that they relied too much on singular experiments and were missing the big picture, which can be revealed only by analogy. Demonstration by analogy was definitely out of favor in Enlightenment France, which is probably why Castel and his ocular harpsichord rapidly disappeared from view.
Dr. William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to firstname.lastname@example.org.