DIFFRACTION. 103 



beam diverges considerably after passing the aperture, so that 

 the space which it occupies on the screen, or roughened glass, 

 is much wider than the geometric projection of the aperture. 

 Secondly, the entire of this space is covered with parallel bands, 

 or fringes, alternately bright and dark, distributed symmetri- 

 cally on either side of the line passing through the luminous 

 point and the centre of the aperture. 



If we trace these fringes, from their origin at the aper- 

 ture to any distance, we shall find that they are propagated 

 in hyperbolas, like the former. The curvature of these 

 hyperbolic branches, and their inclination to one another, 

 depend on the breadth of the aperture, and on its distance 

 from the luminous point. Fraunhofer, who observed this 

 class of phenomena with great attention and care, found that 

 the angular distances of the successive bands of any given 

 colour from the central line formed an arithmetical progres- 

 sion, whose common difference was equal to its first term ; 

 and that, when different apertures were used, the distances 

 of one and the same band from the central line were inversely 

 as the breadths of the apertures. These fringes are broadest 

 and most widely separated in red light ; they are narrowest 

 and closest in violet light, and of intermediate magnitudes in 

 the intermediate rays of the spectrum. In white light, there- 

 fore, they present the succession of colours observed in other 

 cases. 



When the aperture is formed by two straight edges 

 slightly inclined, Newton observed that the fringes were not 

 accurately parallel to the edges, but became broader as they 

 approached ; and that they finally crossed, and formed two 

 hyperbolic branches, one of whose asymptots is perpendicular 

 to the line bisecting the angle of the edges, while the others 

 are parallel to the edges themselves. 



(119) Itis scarcely necessary to observe that the phenomena 



