“I procured me a Triangular glass-Prisme to try therewith the celebrated Phaenomena of Colours. And in order thereto having darkened my chamber, and made a small hole in my window-shuts, to let in a convenient quantity of the Sun’s light, I placed my Prisme at his entrance, that it might be thereby refracted to the opposite wall. It was at first a very pleasing divertisement, to view the vivid and intense colours produced thereby; but after a while applying my self to consider them more circumspectly, I became surprised to see them in an oblong form; which, according to the received laws of Refraction, I expected should have been circular.

And I saw . . . that the light, tending to [one] end of the Image did suffer a Refraction considerably greater then the light tending to the other. And so the true cause of the length of that Image was detected to be no other, then the Light consists of Rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.

Then I placed another Prisme . . . so that the light . . . might pass through that also, and be again refacted before it arrived at the wall. This done, I took the first Prisme in my hand and turned it to and fro slowly about its Axis, so much as to make the several parts of the Image . . . successively pass through . . . that I might observe to what places on the wall the second Prisme would refract them.

When any one sort of Rays hath been parted from those of other kinds, it hath afterwards obstinately retained its colour, notwithstanding my utmost endeavours to change it.”

—Sir Isaac Newton, from Optics, written during the plague years of 1665-66, first published in 1704.