Isaac Newton may well have been the most casual purveyor of trippy ideas in history. His work forms the backbone of modern physics, sure, but most of his corpus is equally filled with esoteric ruminations that would be right at home on the pages of the r/whoadude subreddit.
One of the most infamous examples is his theory, presented in the 1704 treatise Opticks, that the prismatic spectrum of seven colors was somehow related to the seven musical tones of a diatonic scale. He even drew up a diagrammatic wheel in which he assigned certain notes to what he thought might be their corresponding colors.
Though this theory has since been discredited, it was widely believed by musicians, scientists, and other thinkers who lived in Newton's wake. Among them was the French mathematician Louis Bertrand Castel who invented a modified harpsichord inspired by Newton's theories in 1725.
Castel had a DIY approach, and personally outfitted his instrument with sixty multi-hued lanterns rigged to be exposed when certain notes were played. The idea, wrote Castel, was that "the pressing of the keys would bring out the colours with their combinations and their chords; in one word, with all their harmony, which would correspond exactly to that of any kind of music."
Castel called the instrument the "clavecin pour les yeux"—which translates to "harpsichord for the eyes"—but it has become popularly known as the ocular harpsichord or color organ. He played it for visiting scholars over the subsequent decades, many of whom produced their own riffs on his design. In addition, several illustrious figures weighed in on its value—Voltaire critiqued the underlying principles of the concept, while the baroque composer Georg Philip Telemann produced several musical pieces for it, further cementing the harpsichord's reputation as a new wave instrument.
The ocular harpsichord's growing fame hastened the development of visual music as an artform, and some historians also see it as "the direct stimulus of the use of synaesthetic imagery in literature," according to physics historian Maarten Franssen.
As Franssen notes, it's hard to prove that such a nebulous assertion is true, through reading a little Keats might win you over. "Taste the music of that vision pale"? That's a line that ties together taste, hearing, and vision in one neat, triple synaesthetic bow. It's unclear if Keats ever saw an ocular harpsichord, but he is one of many Romantic poets who fits into the artistic tradition that the invention helped spark.
Even today, musicians are still developing instruments based on the same concepts that fascinated Castel. Electronic devices like the light organ or the AudioCube are recent examples of instruments that convert sound into light, while the laser harp turns light into sound. It goes to show that the impulse to "see music" or "hear colors" has deep roots in the scientific revolution of the 1700s, when audiences gathered to marvel over instruments that played light.