116 DIFFRACTION. 



Among the most striking of these effects are those pro- 

 duced by light diverging from a luminous origin, and trans- 

 mitted through a small circular aperture; as, for example, 

 that formed by a pin in a sheet of lead. When the trans- 

 mitted light is viewed through a lens, the image of the aper- 

 ture appears as a brilliant spot, surrounded by coloured rings 

 of great vividness ; and these vary in the most beautiful 

 manner, as the distance of the aperture from the luminous 

 origin, or from the eye, is altered. When the latter distance 

 is considerable, the central spot is white, and the coloured 

 rings follow the order observed in thin plates. As the eye 

 approaches the aperture, the central white spot contracts to 

 a point, and then vanishes. The rings then close in on it in 

 order ; and the centre assumes in succession the most vivid 

 and beautiful hues, altogether similar to those of the reflected 

 rings of thin plates. 



This remarkable coincidence has been shown to be an 

 exact result of theory. It has been demonstrated that the in- 

 tensity of the light of any simple colour, at the central spot, 

 and the compound tint in the case of white light, will be 

 the same as that reflected from a plate of air, whose thickness 

 bears a certain simple relation to the radius of the aperture, 

 and to its distances from the luminous origin and from the 

 eye. 



The points of maximum and minimum intensity are easily 

 determined. \ 



Let be the luminous I 



point, and AB the line drawn 

 from it through the centre of 

 the aperture PP X ; then the 

 interval of retardation, S, of 



the ray which reaches any point B on this line, coming from* 

 the edge of the aperture, is OP + PB - OB. Let 



