REPORT ON PHYSICAL, OPTICS. 327 



the fringes, in this hypothesis, cannot vary in any way with 

 the distance of the inflecting edge from the luminous point. 

 But this conclusion is the reverse of fact : the fringes dilate in 

 breadth, and their mutual inclination is increased, as the screen 

 approaches the luminous origin. There seems to be but one 

 way of avoiding the inference drawn from this fact against the 

 theory of emission. It may be supposed that the bands have 

 their origin at some sensible distance from the edge of the body, 

 and thus that the obliquity of the incident ray varies as the edge 

 approaches the luminous point. Such was the conjecture of 

 Dutour, who noticed the fact. Fresnel has calculated the 

 breadth of the fringes according to this supposition, and found 

 that the computed and experimental results do not agree*. But, 

 in point of fact, the bands may be supposed without sensible 

 error to have their origin at the edge itself. Fresnel found by 

 direct measurement that the distance of the third band from the 

 edge of tlie shadow at its origin was less than the 100th part of 

 a millimetre. 



The objections just considered seem to apply equally to the 

 hypothesis of Mairan and Dutour, in which the phenomena of 

 diffraction are referred to the reflexions and refractions of an 

 atmosphere supposed to encompass all bodies. For if such an 

 atmosphere be retained around the body by its attraction, (and 

 this seems to be the only mode of accounting for its presence,) 

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

 and consequently its effects upon the rays of light must vary 

 also. But the experiments of M. Haldat seem to leave no 

 tenable ground for these hypotheses. Every agent has been tried 

 which 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 pro- 

 duced the fringes were heated to redness, and cooled down be- 

 low the freezing-point ; they were traversed by voltaic currents, 

 and the charges of powerful 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 fringesf. 



Although the phenomena of diffraction were studied by many 

 diligent observers % after the publication of the Optics, no mu- 



* " Memoive sur laDiffraction de la Lumiere," Mem. de l'InsUtut,iom. v. p. 353. 



t " Sur les Causes de la DifFraction," Annales de Chiviie, torn. xli. Similar ex- 

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

 Gottingen Memoirs, vol. iv. 



X Maraldi {Mem. Acad. Par. 1723.), Mairan {Thkl. 1738.), Dutour {Me- 

 moires prcseiifes, tom. v.), Mr. Brougham (P/iil. Trans., 179G — 7.), and 

 Mr. Joi-dan {Nnv Olservullons concernliKj tlie InJIemon ofLiyld, London 1795). 



