442 HISTORY OF OPTICS. 



though he would apply it in combination with the 

 emission of material light. In July, 1802, Young 

 explained, on the same principle, some facts in 

 indistinct vision, and other similar appearances. 

 And in 1803 \ he speaks more positively still. "In 

 making," he says, "some experiments on the fringes 

 of colours accompanying shadows, I have found so 

 simple and so demonstrative a proof of the general 

 law of interference of two portions of light, which 

 I have already endeavoured to establish, that I 

 think it right to lay before the Royal Society a 

 short statement of the facts which appear to me 

 to be thus decisive." The two papers just men- 

 tioned certainly ought to have convinced all scien- 

 tific men of the truth of the doctrine thus urged ; 

 for the number and exactness of the explanations 

 is very remarkable. They include the coloured 

 fringes which are seen with fibres ; the colours pro- 

 duced by a dew between two pieces of glass, which, 

 according to the theory, should appear when the 

 thickness is six times that of thin plates, and which 

 do so ; the changes resulting from the employment 

 of other fluids than water ; the effect of inclining 

 the plates; also the fringes and bands which ac- 

 company shadows, the phenomena observed by 

 Grimaldi, Newton, Maraldi, and others, and hitherto 

 never at all reduced to rule. Young observes, 

 very justly, "whatever may be thought of the 

 theory, we have got a simple and general law" of 



1 Phil. Trans. Memoir, read Nov. 24. 



