&*r * 



ftri.iv 



DIFFRACTION. 



99 



fringes were observed, under favourable circumstances, within 

 the shadows of narrow bodies. 



The phenomena of diffraction were subsequently examined 

 by Hooke and by Newton ; and, lastly, in the hands of 

 Young and Fresnel, they have been forced to furnish evi- 

 dence in favour of the wave- theory, which few who impar- 

 tially examine it can withstand. We shall first describe the 

 most important of these phenomena, and afterwards examine 

 them in their bearing upon the two theories. 



(114) The most obvious of these phenomena are the mo- 

 difications which light undergoes in passing by the edge of 

 an obstacle of any kind. 



Let a beam of homogeneous light, entering a dark cham- 

 ber, fall on a lens of short focal length, MN, by which it is 



brought to a focus at 0, and thence diverges. Let an ob- 

 stacle, PP', be placed in the diverging beam, and let the 

 shadow which it casts be received upon a sheet of white paper 

 at Q, or on a piece of roughened glass. "We shall then ob- 

 serve the following phenomena : 



I. The line OPQ, which is the boundary of the geometric 

 shadow, is not the actual boundary of light and shade. 



II. The space below this line, Q,S, is not absolutely dark, 

 but is enlightened by a faint light, which -extends to a sen- 

 sible distance within the geometric shadow, and gradually 

 fades away as it recedes from the edge of this shadow at Q,. 



III. On the other side of the boundary of the geometric 



