180 



DR. YOUNG'S ACCOUNT OF SOME CASES 



IX. 



Simple and 

 general law re« 

 fpecting two 



An Account of fome Cafes of the Produftion of Colours, not 

 hitherto defcribed. By Thomas Young, M. D. F.R.S. 

 K L. S. Profejftr of Natural Philofophj/, in the Royal 

 Infiitution *. 



VV HATE VER opinion may be entertained of the theory of 

 light and colours which I have lately had the honour of fubmit- 

 fSllTght arS?. in S t0 the R °y al Society, it rauft at any rate be allowed that 

 ing by different it has given birth to the difcovery of a fimple and general law, 

 routes j capable of explaining a number of the phenomena of coloured 



light, which, without this law, would remain infulated and 

 unintelligible. The law is, that " wherever two portions o^ 

 •' the fame light arrive at the eye by different routes, either 

 *' exactly or very nearly in the fame direction, the fight be- 

 " comes moil intenfe when the difference of the routes is any 

 *' multiple of a certain length, and leaft intenfe in the inter- 

 " mediate flate of the interfering portions ; and this length is 

 " different for light of different colours." 



I have already mown in detail, the fuificiency of this lav* 

 for explaining all the phenomena defcribed in the fecond and 

 third books of Newton's Optics, as well as fome others not 

 mentioned by Newton. But it is (till more fatisfactory to 

 obferve its conformity to other facts, which conftitute new 

 and diftinct claffes of phenomena, and which could fcarcelj 

 have agreed fo well with any anterior law, if that Jaw had 

 been erroneous or imaginary : thefe are, the colours of fibres* 

 and the colours of mixed plates. 



As I was obferving the appearance oPthe fine parallel lines 

 of light which are feen upon the margin of an object held 

 near the edge of near the eye, fo as to intercept the greater part of the light of 

 an obftacle in- a diffont luminous object, and which are produced by the fringes 

 &S. caufed by the inflection of light already known, I obferved 



that they were fometimes accompanied by coloured fringes, 

 much broader and more diftinct ; and I foon found, that thefe 

 broader fringes were oecafioned by the accidental interpofition 

 of a hair. In order to make them more diftinct, I employed 

 a, horfe-hair ; but they were then no longer vifible. With a 



which explains 

 »any pheno- 



ijid new facts. 



Production of 

 colours by a 

 minute fibre 



* Philos, Trans. 1802. 



fibre 



