XIV. An account of some cases of the production of colours, not hitherto described

Abstract

Whatever opinion may be entertained of the theory of light and colours which I have lately had the honour of submitting to the Royal Society, it must at any rate be allowed that it has given birth to the discovery of a simple and general law, capable of explaining a number of the phenomena of coloured light, which, without this law, would remain insulated and unintelligible. The law is, that “wherever two portions of the same "light arrive at the eye by different routes, either exactly or “very nearly in the same direction, the light becomes most “intense when the difference of the routes is any multiple of a “certain length, and least intense in the intermediate state of “the interfering portions; and this length is different for light "of different colours.” I have already shown in detail, the sufficiency of this law for explaining all the phenomena described in the second and third books of Newton's Optics, as well as some others not mentioned by Newton. But it is still more satisfactory to observe its conformity to other facts, which constitute new and distinct classes of phenomena, and which could scarcely have agreed so well with any anterior law, if that law had been erroneous or imaginary: these are, the colours of fibres, and the colours of mixed plates.