Newton's reputation rests overwhelmingly on the Principia, but two other threads of his working life reveal a stranger and more contested figure. His 1704 book Opticks, published in English rather than the era's usual scholarly Latin, reported years of experiments showing that white light is not pure but a mixture of colors that a prism separates and a second prism can recombine, overturning the older idea that prisms themselves added color to light. Alongside this public science, Newton spent decades on alchemy, filling more than a million words of private manuscripts with attempts to transmute metals and recover what he believed was an ancient, lost body of natural knowledge. He kept almost none of it published in his lifetime, and historians now treat it as a genuine, serious part of his intellectual project rather than an embarrassing sideline, even though it produced no results resembling the chemistry that followed.
The most bitter episode of Newton's career was his dispute with the German philosopher and mathematician Gottfried Wilhelm Leibniz over who had invented calculus. Newton had developed his method of fluxions in the 1660s but published almost nothing about it for decades, Leibniz developed an independent version in the 1670s and published first, in 1684, with a notation that proved easier to use and is largely what is taught today. When the priority question became a public accusation of plagiarism, the Royal Society, of which Newton was president, appointed a supposedly independent committee to investigate, Newton wrote much of its 1712 report himself without disclosing his authorship, and it ruled in his favor. The two men never reconciled, and the fight split British and continental mathematics into rival camps for a generation, with British mathematicians clinging to Newton's clumsier notation partly out of loyalty and falling behind continental developments as a result. Modern historians of mathematics regard the two as having reached calculus independently, by different routes, at nearly the same time.