Diagram from a letter written by Isaac Newton (1642-1727) to the Royal Society, & published in the Philosophical Transactions in 1672. BC, DE are boards which have holes at at x & y. Two prisms (A & F) are placed near the holes. The experiment is to move the prism A. This causes the colour of the light at GH to change because a different part of the spectrum reaches the prism F; but it also causes the light at GH to move because light of different wavelength is refracted to a different extent by the glass of the prism. Newton's major work on light and his prism experiments, Opticks, was not written until 1704.