DIFFRACTION. 107 



in the neighbourhood of that edge ; so that the magnitude 

 and position of the fringes, in this hypothesis, cannot vary 

 with the distance of the inflecting edge from the luminous 

 point. But this conclusion is the reverse of fact. The 

 fringes dilate, and their mutual inclination is increased, as 

 the obstacle approaches the luminous origin. 



The phenomena of diffraction, therefore, do not arise from 

 the operation of attractive and repulsive forces, exerted by 

 the molecules of bodies upon those of light. 



(122) The same objections apply to the hypothesis of 

 Mairan and Du Tour, which ascribes these eifects to the re- 

 fraction of small atmospheres encompassing the bodies, and 

 of a different refractive power from the surrounding medium. 

 For, if such an atmosphere be retained by the attraction of 

 the body which it encompasses, and this seems to be the only 

 intelligible mode of accounting for its presence, its den- 

 sity, and its form, must vary with those of the body itself ; 

 and, consequently, its effects upon the rays of light must vary 

 also. 



We are forced, then, to conclude, that the phenomena of 

 diffraction are inexplicable in the system of emission ; and we 

 proceed to examine in what manner, and with what success, 

 the principles of the wave-theory have been applied to their 

 explanation. 



(123) This important step in Physical Optics was made 

 by Young, and all the complicated phenomena of diffraction 

 are now reduced to the simple principle of Interference. 



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 deviated, while the other was obliquely reflected from its 

 edge. The fringes formed by narrow apertures Mjere, in like 



