104 DIFFRACTION. 



of diffraction may be endlessly varied, by "varying the form of 

 the diffracting edge. The preceding cases have been se- 

 lected as the most elementary. They are abundantly sufficient, 

 when pursued into numerical details, to test the truth of 

 any theory which may be applied to this class of pheno- 

 mena ; and such a theory being once established, the laws of 

 the more complex appearances are best sought for in its de- 

 ductions, We shall proceed, therefore, to consider the pre- 

 ceding phenomena in their relation to the two theories of 



light. 



(120) Newton conceived the rays of light to be inflected in 

 passing by the edges of bodies, by the operation of the at- 

 tractive and repulsive forces, which the molecules of bodies 

 were supposed to exert upon those of light before they ar- 

 rived in actual contact. By the operation of such forces, 

 Newton was enabled to explain the laws of reflexion and re- 

 fraction ; and it was reasonable to suppose that the same 

 forces played an important part in the phenomena now under 

 consideration. 



Thus, the rays passing by the edges of a narrow opaque 

 body, such as a hair or fine wire, are supposed to be turned 

 aside by its repulsion ; and, as this force decreases rapidly as 

 the distance increases, it follows that the rays which pass at a 

 distance from the body will be less deflected than those which 

 pass close to it, as is shown 

 in the annexed diagram. 

 The caustic formed by the 

 intersection of these de- 

 flected rays will be con- 

 cave inwards ; and as none 

 of the rays pass within it, 

 it will form the boundary 

 of the visible shadow. Thus this supposition explains satisfac- 



