Young’s Musical Optics 175
Young presented new evidence to the Royal Society in “ An Account of Some Cases of
the Production of Colours not Hitherto Described, ” delivered in July 1802.^42 In it, he further
distills the content of his principle of superposition from its somewhat less quantitative
form in his Proposition VIII to what he now calls “ a simple and general law ” : “ 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. ”^43 With
his general law in hand, Young returns to simple experiments mentioned by Newton
and Grimaldi, from which he can now deduce the exact wavelengths they themselves did
not calculate. Observing the “ fine parallel lines of light which are seen upon the margin
of an object held near the eye, ” Young notes that “ they were sometimes accompanied by
coloured fringes, much broader and more distinct. ” To make them more distinct, he
observes a horse hair, then a wool fiber, then a single strand of silk, which gave the clear-
est, broadest pattern. Young made a rectangular hole in a card and bent its edges to support
a hair parallel to the sides of the hole, a stabilizing mounting that allowed him to measureFigure 11.5
Young ’ s table showing the wavelengths and frequencies of different colors of light, as he calculated from New-
ton ’ s experiments on thin plates. From “ On the Theory of Light and Colours ” (1801).