"that little instrument which shows musicians the beat..."

Man Ray, Objet Indestructible (1923-1975) - (Readymade wooden metronome with photograph of an eye)
Tate Magazine Issue 3: Eye of the Beholder
"The metronome, that little instrument which shows musicians the beat, is really the earliest music, a primitive music. I owned one myself and, when I painted, I let in run, you know, and, like a pianist, I did so and so many strokes while painting, isn’t that so? And the metronome registered everything. I did miss the visual aspect, though. That’s why I attached an eye to the pendulum and, when the eye moved from right to left and from left to right, I had the feeling someone was watching me paint or looking at the picture. Sometimes it might stop and then I knew that the picture was very bad and destroyed it, do you understand? But one day I got my revenge; the metronome began to tick so hard and got on my nerves so much that I took a hammer and smashed it into a thousand pieces. It was, after all, called Objet à détruir [Object to be Destroyed], and it was right for I then destroyed it. But after the great historic retrospectives on Dada I was asked to exhibit the metronome so I put an eye on it and made it run again."

György Ligeti, (1923-2006), "Poema sinfónico para 100 Metrónomos", 1962