C 387 1 
XIV. An Account of some Cases of the Production of Colours, not 
hitherto described. By Thomas Young, M. D. F. R. 8, 
F. L. S. Professor of Natural Philosophy in the Royal Insti- 
tution. 
Read July 1, 1802. 
w hatever 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 unintelli- 
gible. 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 men- 
tioned 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. 
