Newton's spectrum
1666 (prism experiments); 1704 (Opticks)
Isaac Newton showed with prisms that white light is a mixture of colours that can be separated and recombined. Colour became something physics could measure.
How it began
Prisms were known to make rainbows, but most thought the glass added the colours. From 1666 Isaac Newton passed a beam of sunlight through a prism in a darkened room and examined the band of colour it made. He published the full work as Opticks in 1704.
How it works
Each colour bends by a different amount in glass, so a prism fans white light out into a spectrum. Newton isolated one colour and showed a second prism could not split it further, then recombined the spectrum into white with a lens.
Why it mattered
Colour became a property of light that could be measured, not a quality of objects alone. Scheele and later chemists used the spectrum to test which colours affect silver; Maxwell and the makers of colour film built on the idea that colours can be separated and mixed.
Limits and what replaced it
Newton thought light was made of particles and could not explain why three primaries were enough to match colours. That came from the physiology of the eye: the trichromatic theory of Young, Helmholtz and Maxwell.
Today
Every colour sensor, film and screen splits or mixes light along the spectrum Newton described.