328 FOURTH REPORT — 1834. 



terial accession was made to the knowledge of their hiws until 

 the principles of the wave-theory were applied to their expla- 

 nation by Young. The exterior fringes, formed without the 

 shadows of bodies, were ascribed by Young to the interference 

 of two portions of light, one of which passed by the body and 

 was more or less inflected, while the other was obliquely re- 

 flected from its edge, the latter losing half an undulation at the 

 instant of reflexion*. The fringes formed by narrow apertures 

 were, in like manner, supposed to arise from the interference 

 of the two pencils i-eflected from the opposite edges ; while the 

 interior fringes, within the sliadows of narrow bodies, were ac- 

 counted for by the interference of the pencils which passed on 

 either side of the body at an insensible distance, and were in- 

 flected into the shadow. The observed facts closely correspond 

 with the calculated results of this theory ; and in the case last 

 mentioned Young proved that the phsenoniena admitted of no 

 other explanation. Placing a small opaque screen on either 

 side of the diff"racting body, so as to intercept the portion of 

 light which passed by one of its edges, the bands immediately 

 disappeared, although the light passing by the other edge was 

 unmodified. The same effect Avas produced, and by the same 

 means, upon the crested fringes of Grimaldi, formed within the 

 shadows of bodies having a rectangular terminationf. Thus the 

 phenomena of the fringes, or the alternations of light and dark- 

 ness, were shown to be cases of the more general principle of 

 interference ; and the connexion is now admitted by some of 

 the warmest advocates of the Newtonian theory^. The bending 

 of the light hito the shadow, or the fact of inflexion itself, was 

 at first ascribed by Young to the refraction of an ethereal at- 

 mosphere encompassing bodies and decreasing in density with 

 the distance. He afterwards, however, adopted the simpler 

 doctrine of Huygens and Grimaldi, and referred the pheno- 

 menon to the fundamental jjroperty of waves. 



But perhaps the most important of the labours of Young on 

 ' his subject is that in which he descends into numerical details, 

 and, takhig the observations of Newton, as well as his own, 

 calculates the differences of the lengths of the paths traversed by 

 the two pencils, when they destroy or reinforce one another by 

 interference. These intervals he found to constitute an arith- 

 metical progression for the successive bands ; the first term of 

 which was the same in the same species of light, whatever be 



* "On the Theory of Light and Colours," P/zi/. Traw*. 1802. 

 •|- " Experiments and Calculations relative to Physical Optics," Phil. 

 Trans. 1804. 

 X Biot, Precis elemenlaire, vol. ii. p. 472., o'"^ Edit. 



