DIFFRACTION. 55 



could be conceived capable of modifying the attractive force of the 

 body, or the density of the imagined atmosphere, and without 

 effect. The metallic wires and plates which produced the fringes 

 were heated to redness, and cooled down below the freezing-point ; 

 they were traversed by voltaic currents, and the charges of power- 

 ful batteries transmitted through them ; but in whatever manner the 

 condition of the diffracting body was varied, no change whatever 

 was perceived either in the intensity or dimensions of the diffracted 

 fringes.* 



Although the phenomena of diffraction were studied by many 

 diligent observers! after the publication of the Optics, no material 

 accession was made to the knowledge of their laws until the prin- 

 ciples of the wave-theory were applied to their explanation 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 reflected from its edge, the 

 latter losing half an undulation at the instant of reflexion.* The 

 fringes formed by narrow apertures were, in like manner, sup- 

 posed to arise from the interference of the two pencils reflected 

 from the opposite edges ; while the interior fringes, within the 

 shadows of narrow bodies, were accounted for by the interference 

 of the pencils which passed on either side of the body at an insen- 

 sible distance, and were inflected 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 phenomena 

 admitted of no other explanation. Placing a small opaque screen 

 on either side of the diffracting 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 un- 

 modified. The same effect was produced, and by the same means, 

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

 bodies having a rectangular termination. Thus the phenomena 



* " Sur les Causes de la Diffraction," Annales de Chimie, torn. xli. Similar ex- 

 periments had been made some time before by Mayer, and with the same result. 

 (jt;iqen Memoir*, vol. iv. 



t Maraldi (Mem. Acad. Par. 1723), Mairan (Ibid. 1738), Du Tour (JMM*tt 

 xentcs, torn, v.), Mr. Brougham (Phil. Trans. 1796-7), and Mr. Jordan (New Obterta. 

 tions concerning the Inflexion of Light. London, 1795). 



t " On the Theory of Light and Colours." TV'tf. Trans., 1802. 



" Experiments and Calculations relative to Physical Optics." PAH. Tram*. 180*. 



