Opticks

Written in English rather than Latin, and built from experiments a reader can follow one at a time, this is Newton’s most approachable book. Published in 1704, it reports what he found by letting a beam of sunlight through a prism in a darkened room: white light is not pure but a mixture, and the colours are already present before the glass ever touches them. Newton moves on to the rings of colour in thin films, the bending of light at edges, and the rainbow, then closes with the Queries, a set of open questions about light, matter, and heat that kept physicists busy for the next two centuries. Free to read and download in PDF and EPUB.