420 THE PKINCIPLES OF SCIENCE. [CHAP. 



be said of the colours exhibited by thin plates and films. 

 The rings and lines of colour will be nearly the same in 

 character whatever may be the nature of the substance ; 

 nay, a void space, such as a crack in glass, would produce 

 them even though the air were withdrawn by an air-pump. 

 The conditions are simply the existence of two reflecting 

 surfaces separated by a very small space, though it should 

 be added that the refractive index of the intervening sub- 

 stance has some influence on the exact nature of the colour 

 produced. 



When a ray of light passes close to the edge of an opaque 

 body, a portion of the light appears to be bent towards it, 

 and produces coloured fringes within the shadow of the 

 body. Newton attributed this inflexion of light to the 

 attraction of the opaque body for the supposed particles of 

 light, although he was aware that the nature of the sur- 

 rounding medium, whether air or other pellucid substance, 

 exercised no apparent influence on the phenomena. 

 Gravesande proved, however, that the character of the 

 fringes is exactly the same, whether the body be dense or 

 rare, compound or elementary. A wire produces exactly 

 the same fringes as a hair of the same thickness. Even the 

 form of the obstructing edge was subsequently shown to 

 be a matter of indifference by Fresnel, and the interfer- 

 ence spectrum, or the spectrum seen when light passes 

 through a fine grating, is absolutely the same whatever be 

 the form or chemical nature of the bars making the 

 grating. Thus it appears that the stoppage of a portion of 

 a beam of light is the sole necessary condition for the 

 diffraction or inflexion of light, and the phenomenon is 

 shown to bear no analogy to the refraction of light, in 

 which the form and nature of the substance are all impor- 

 tant. 



It is interesting to observe how carefully Newton, in his 

 researches on the spectrum, ascertained the indifference 

 of many circumstances by actual trial. He says : * " Now 

 the different magnitude of the hole in the window-shut, 

 and different thickness of the prism where the rays parsed 

 through it, and different inclinations of the prism to the 

 horizon, made no sensible changes in the length of the 



1 Opticks, 3rd. ed. p. 25. 



