David Brewster

David Brewster was born in Jedburgh in 1781, the son of a schoolmaster. He was educated at the University of Edinburgh and licensed as a Church of Scotland minister — the expected path for a bookish Scottish boy of his generation — but he never held a parish. A severe stage-fright in front of a congregation cured him of the ambition before his first sermon. He turned to optical research, where the only audience was the experimental apparatus on his bench. Between 1812 and 1815 he ran a systematic programme of reflection measurements: light of various polarisations striking glass, water, diamond, mica, and two dozen other materials at varying angles, measuring how much reflected. In 1815 he published what is now called Brewster's law — at the angle θ_B = arctan(n₂/n₁), the reflected ray is purely s-polarised (perpendicular to the plane of incidence), because the p-polarised reflection coefficient passes through zero. This is the principle behind polarising sunglasses for killing road glare, the Brewster-angle microscope for studying monolayers on water, and the Brewster windows in gas-laser cavities.

In 1816 he invented the kaleidoscope. The optical principle — two or three mirrors at shallow angles producing multiply-reflected symmetric patterns — was elementary, but the product was irresistible. He patented it in 1817. Within months of the patent's issue, London optical hawkers had reverse-engineered the device and were selling thousands of unauthorised copies on street corners; the patent proved legally unenforceable because the prior art was too thin and too widely distributed. Brewster spent years chasing infringement cases and lost most of the manufacturing rights. His notebooks show he estimated he would have been wealthy had the patent held. It did not. He later co-invented the lenticular stereoscope (the binocular-like viewing device that sold in the hundreds of thousands in the 1850s–60s photography craze) and improved the polarimeter for chemical analysis.

His institutional legacy is as large as his experimental one. He edited the *Edinburgh Encyclopaedia* for its full 18-volume run between 1808 and 1830. He co-founded the British Association for the Advancement of Science (BAAS) in 1831 — the first large-audience science organisation in the world; its annual regional meetings established the modern template for scientific congresses that every discipline still follows. He was twice president of the Royal Society of Edinburgh. Knighted 1832. Late in life he became fascinated by photography, corresponded with Daguerre and Talbot, and wrote popular science books — *Letters on Natural Magic* was a best-seller explaining stage magic and optical illusions to the Victorian reading public. He died in 1868 at eighty-six, having outlived most of his correspondents.