SECT, xx.] DIFFRACTION OF LIGHT. 195 



tance of the screen from the hole. As the distance of the 

 screen diminishes, the white central spot contracts to a point 

 and vanishes ; and, on approaching still nearer, the rings 

 gradually close in upon it, so that the centre assumes succes- 

 sively the most intense and vivid hues. When the light is 

 homogeneous, red, for example, the rings are alternately 

 red and black, and more numerous ; and their breadth varies 

 with the colour, being broadest in red light and narrowest in 

 violet. The tints of the coloured fringes from white light, 

 and their obliteration after the seventh ring, arise from the 

 superposition of the different sets of fringes of all the coloured 

 rays. The shadows of objects are also bordered by coloured 

 fringes when held in this slender beam of light. If the edge 

 of a knife or hair, for example, be held in it, the rays, instead 

 of proceeding in straight lines past its edge, are bent when 

 quite close to it, and proceed from thence to the screen in 

 curved lines called hyperbolas ; so that the shadow of the 

 object is enlarged, and, instead of being at once bounded by 

 light, is surrounded or edged with coloured fringes alternating 

 with black bands, which are more distinct the smaller the 

 pin-hole (N. 197). The fringes are altogether independent 

 of the form or density of the object, being the same when it 

 is round or pointed, when of glass or platina. When the rays 

 which form the fringes arrive at the screen, they are of dif- 

 ferent lengths, in consequence of the curved path they follow 

 after passing the edge of the object. The waves are there- 

 fore in different phases or states of vibration, and either 

 conspire to form coloured fringes, or destroy one another in 

 the obscure intervals. The coloured fringes bordering the 

 shadows of objects were first described by Grimaldi in 1665; 

 but besides these he noticed that there are others within the 

 shadows of slender bodies exposed to a small sunbeam, a phe- 

 nomenon which has already been mentioned to have afforded 

 Dr. Young the means of proving, beyond all controversy, that 

 coloured rings are produced by the interference of light. 

 It may be concluded, that material substances derive their 



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