Refracted and Diffracted Light. 423 



then decreasing in the same gradual manner, until the 

 light commences its rectilineal course, at an equal distance 

 from the surface within the medium. 



The refractive force, then, within the sphere of refrac- 

 tion, decreases as the distance from the surface of the re- 

 fractory body increases, and this might naturally be 

 expected; when, therefore, light passes through a small 

 round aperture, the central ray, whether beyond the sphere 

 of refraction, or under the influence of equal and oppo- 

 site forces, must proceed in its rectilineal course ; and the 

 light surrounding it, must be gradually more and more 

 refracted as it recedes from it, until, upon its passing 

 close to the edge of the aperture, its divergence arrives 

 at its maximum. 



The arrangement which must be thus produced, is so 

 precisely similar to that produced by a lens, as already 

 described, that we are justified in concluding the refract- 

 ing and diffracting forces are identically the same ; and 

 that the difference arising from the different applications 

 of it, proceeds from the light passing through the whole 

 sphere of the refractive force, when it enters the medium, 

 and only through a part of it, varying in degree with the 

 distance, when it merely passes the edges; in the first 

 case the light is uniformly refracted, in the last it is re- 

 fracted in proportion to the force to which it has been 

 exposed. 



The following experiment corroborates these state- 

 ments : — 



If we place an object with a straight edge, nearly close 

 to the eye, and when at some distance from a window, 

 bring the edge parallel to one of the bars that separate the 

 pane's of glass, so that the light from both sides of it, in 

 the transmission to the eye, may pass close to the edge of 

 the object, we shall perceive fringes of colours on each 

 side of the bar ; if it be a horizontal bar, and we look over 

 the edge, the fringes above the bar will be red and yellow, 

 and below the bar blue and violet, similar to what would be 

 produced by a prism with a very small refracting angle, held 

 upward ; and if we look under the edge, the fringes above 

 the bar will be blue and violet, and under the bar red and 

 yellow, similar to what would be produced by the same 



