54 REPORT ON PHYSICAL OPTICS. 



Again, the inflecting forces though they must be supposed to 

 vary in intensity, with the form and mass of the body, and with 

 the distance of the luminous molecule from the edge cannot be 

 conceived to depend in any way upon the distance previously tra- 

 versed by the molecule, before it arrives in the neighbourhood of 

 that edge ; so that the magnitude and position of 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 Du Tour, who noticed the fact. Fresnel 

 calculated the breadth of the fringes according to this suppo- 

 sition, 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 the 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 Du Tour, 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 



founded two experiments of Fresnel which were instituted with different views, and 

 differently reasoned upon. Fresnel's object, in this experiment, was simply to show 

 diffract o f ^ Pr dUCed D GffeCt UPOD the Mn Ses, as * 5 to do if 



from boles'"! r^r a v tiVe ^ rePUldVe fOTCeS eXtendi "S to Se ^ ^stances 



fe intke ml JeCtl DS *** * *" Same " a ^ ainst the wave-theory 



rise, in lite manner, in misconception. 



Memoire sur la Diffraction de la Lumiere."-^ m . de PItutiMf tom> y< p ^ 



