VIII. AN ACCOUNT OF SOME CASES 



OP THE 



PRODUCTION OF COLOURS, 



NOT HITHERTO DESCRIBED. 



BY 



THOMAS YOUNG, M. D. F. R. S. 



PROFESSOR OF NATURAL PHILOSOPHY IN THE ROYAL INSTITUTION. 



FROM THE PHILOSOPHICAL TRANSACTIONS. 

 Read before the Royal Society, Juli/ 1, 1802.~^^ 



W HATEVER opinion may be entertained 

 of the theory of light and colours which I 

 have hitely had the honour of submitting to 

 the Royal Society, it must at any rate be al- 

 lowed, that it has given birth to tlie discovery 

 of a simple and general law, capable of ex- 

 plaining a number of the phenomena of co- 

 loured 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 in- 

 tense 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 suf- 

 ficiency of this law, for explaining all the 

 phenomena described in the second and third 



VOL. II. 



books of Newton's Optics, as well as some 

 others not mentioned by Newton. But it is 

 still more satisfactory to observe its con- 

 formity to other facts, which constitute new 

 and distinct classes of phenomena, and which 

 could scarcely have agreed so well with an}' 

 anterior law, if that law had been erroneous 

 or imaginary : these are, the colours of fibres, 

 and the colours of mixed plates. 



As I was observing the appearance of the 

 fine parallel lines of light which are seen upon 

 the margin of an olyect held near the eye, so 

 as to intercept the greater part of the light of a 

 distant luminous object, and which are pro- 

 duced by the fringes caused by the inflection 

 of light already known, I observed that they 

 were sometimes accompanied by coloured 

 fringes, much broader and tnoredistinct; and 

 I soon found, that these broader fringes were 

 occasioned by the accidental interposition of a 

 hair. In order to make them more distinct. 

 4 M. 



