1 06 DIFFRACTION. 



ascribed in the theory of emission, it will follow that they 

 must exist in different bodies in very different degrees ; so that 

 the amount of bending of the rays, and therefore the breadth 

 of the diffracted fringes, should vary with the mass, the nature, 

 and the form of the inflecting body. Now it is clearly as- 

 certained, on the contrary, that all bodies, whatever be their 

 nature or the form of their edge, produce under the same 

 circumstances fringes identically the same ; and, in fact, the 

 partial interruption of light, caused by the interposition of 

 an obstacle of any kind, appears to be the only condition 

 essential to the phenomenon. 



Gravesende seems to have first observed that the nature 

 or density of the body had no effect upon the magnitude 

 of the diffracted images ; and the fact has since been con- 

 firmed in the fullest manner by almost every inquirer in this 

 branch of experimental science. It is now admitted that the 

 inflecting forces, if such exist, must be independent of the 

 nature of the inflecting body, and altogether different from 

 those to which, in the theory of emission, the phenomena 

 of reflexion and refraction are ascribed. 



To ascertain whether the form of the edge had any effect 

 upon the fringes, Eresneltook two plates of steel, the edge of 

 each of which was rounded in one half of its length, and 

 sharp in the remaining half, and placed the rounded portion 

 of each edge opposite the angular part of the other. If then 

 the position of the fringes depended on the form of the edge, 

 the effect would thus be doubled, and the fringes appear 

 broken in the midst. They were found, on the contrary, to 

 be perfectly straight throughout their entire length. 



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 traversed by the molecule, before it arrives 



