EXPERIMENT. 27 



of indifference, and the form of the surface is the condition 

 of such colours 3 . 



Nearly the same may be said of the colours exhibited 

 by thin plates and films. The rings and lines of colour 

 will be of the same 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 withdrawn by an air-pump.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 substance has some influence on 

 the exact nature of the colour produced at any point. 



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 surrounding medium, whether air or other pellucid 

 substance, exercised no apparent influence on the pheno- 

 mena. 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 has exactly the 

 same effect 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 interference 

 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 forming 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 reflection and refraction of light, 



a ' Treatise on Optics/ by Sir D. Brewster, Cabinet Cyclopaedia, p. 117. 



