04: HISTORY OF OPTICS. 



micrometer?, in which instrument lines were drawn on glass at a dis- 

 tance of l-500th of an inch. The interference of the undulations of 

 the rays reflected from the two sides of these fine lines, produced peri- 

 odical colors. In the same manner, he accounts for the colors of thin 

 plates, by the interference of the light partially reflected from the two 

 surfaces of the plates. We have already seen that Hooke had long 

 before suggested the same explanation ; and Young says at the end of 

 his paper, " It was not till I had satisfied myself respecting all these 

 phenomena, that I found in Hooke's MicrograpMa a passage which 

 might have led me earlier to a similar opinion." He also quotes from 

 Newton many passages which assume the existence of an ether ; of 

 which, as we have already seen, Newton suggests the necessity in these 

 very phenomena, 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 appear- 

 ances. And in 1S03, 1 he speaks more positively still. "In making," 

 he says, " some experiments on the fringes of colors 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 endeavored 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 mentioned certainly ought to 

 have convinced all scientific men of the truth of the doctrine thus 

 urged ; for the number and exactness of the explanations is very 

 remarkable. They include the colored fringes which are seen with the 

 shaclow r s of fibres ; the colors produced by a dew between two pieces 

 of glass, which, according to the theory, should appear when the thick- 

 ness of the plate 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 

 accompany 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 the phenomena. He moreover 

 calculated the length of an undulation from the measurements of 

 fringes of shadows, as he had done before from the colors of thin 

 plates; and found a very close accordance of the results of the various 

 cases with one another. 



Phil. Trans. Memoir, read Xov. 24. 



