isaac newton was twenty-three, plague was shutting down cambridge, and he was bored enough to press a convex lens against a flat piece of glass and stare at it.
what he saw: concentric rings of color, sharp near the center where the glass almost touched, getting fainter and tighter as they spread outward. dark rings alternating with vivid ones — red, blue, green — like a target painted by someone who understood wavelengths before wavelengths were a thing.
the setup
he called them "coloured rings." we call them newton's rings. same principle: a thin film of air trapped between two glass surfaces. light bounces off the bottom of the lens and off the top of the plate. the two reflected waves interfere. where the air gap is just right, they cancel (dark). where it's half a wavelength off, they reinforce (bright).
because the lens is curved, the air gap increases smoothly with radius. so the interference pattern becomes rings instead of fringes.
"the parts of the rings appeared at all distances from the center, and the light which made them was sometimes reflected from the upper surface of the air, and sometimes from the lower."
that's newton in opticks, 1704. he'd been sitting on the observation for nearly forty years.
the history
he noticed the rings around 1666, during his so-called annus mirabilis — the same stretch where he was inventing calculus and getting hit by apples, or so the myth goes. he didn't publish immediately. he was busy, and also he was isaac newton, which meant he was paranoid and secretive.
he finally described the rings in a letter to the royal society in 1675. robert hooke had already been pushing a wave theory of light. newton used the rings to argue the opposite: light was corpuscles, tiny particles. he proposed "fits of easy reflection and easy transmission" — periodic intervals where particles were alternately more likely to bounce or pass through. he was essentially describing wave interference without admitting waves existed.
"each ray of light is a body, and the fits of easy reflection and easy transmission are the alternate dispositions of the ray to be reflected or refracted."
he was wrong about the mechanism. right about the math. the periodicity he measured matched what you'd expect from wave interference. he even estimated the thickness of the air film to extreme precision — down to the millionth part of an inch, he claimed. the man could measure.
the full wave explanation didn't arrive until thomas young's double-slit experiment in 1801, then fresnel mathematically buried the corpuscle theory for good. newton's rings became the textbook demo for thin-film interference, which is a little ironic given why he studied them.
the context
the experiment sits at a weird historical crossroads. newton was trying to kill the wave theory. instead he produced the most beautiful evidence for it. the rings don't care what you believe about light. they just appear, regular and indifferent, whenever you press curved glass flat.
he published the complete treatment in opticks in 1704, when he was sixty-two and president of the royal society. the book is structured as a series of queries and observations, almost conversational. he walks through the ring experiments in book two, part four — the thickness measurements, the color sequences, the way the pattern shifts when you press harder and squeeze the air gap thinner.
it's strange to read now. he is so careful, so precise, so obviously dancing around an explanation he refuses to name.
"light is neither this nor that, but something which we do not understand."
he never wrote that, by the way. but he might as well have.
the thought is mine. the words are written by janis, my hermes agent.