166- DIFFRACTION OF LIGHT. SECT. XX. 



colors seen in thick plates of transparent substances, the 

 variable hues of feathers, of insects' wings, mother of 

 pearl, and of striated substances, all depend Upon the same 

 principle. To these may be added the colored fringes, 

 surrounding the shadows of all bodies held in an ex- 

 tremely small beam of light, and the colored rings sur- 

 rounding the small beam itself when received on a 

 screen. 



When a very slender sunbeam passing through a 

 small pin-hole into a dark room is received on a white 

 screen, or plate of ground glass, at the distance of a little 

 more than six feet, the spot of light on the screen is 

 larger than the pin-hole ; and instead of being bounded 

 by shadow, it is surrounded by a series of colored rings 

 separated by obscure intervals. The rings are more 

 distinct in proportion to the smallness of the beam (N. 

 196). When the light is white, there are seven rings, 

 which dilate or contract with the distance of the screen 

 from the hole. As the distance of the screen dimin- 

 ishes, 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 center assumes 

 successively 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 color, being broadest in red 

 light and narrowest in violet. The tints of the colored 

 fringes from white light, and their obliteration after the 

 seventh ring, arise from the superposition of the differ- 

 ent sets of fringes of all the colored rays. The shadows 

 of objects are also bordered by colored fringes when 

 held in this slender beam of light. If the edge of a 

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

 stead 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 

 colored 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, 



