118 DIFFRACTION. 



ness in compound light, but a succession of points, at which 

 the centre of the aperture is richly coloured. 



(130) The theory of Fresnel is not only in exact accord- 

 ance with facts already known, but it has also led to many 

 new and unexpected conclusions, and predicted consequences 

 which have been afterwards verified on trial. One of the 

 most remarkable of these is the phenomenon of diffraction by 

 an opaque circular disc. Poisson applied Fresnel's integrals 

 to this case ; and he was led to the startling result, that the 

 illumination of the centre of the shadow was precisely the 

 same as if the disc had been altogether removed. The prin- 

 ciples already laid down will enable the reader to satisfy 

 himself of the theoretical truth of this conclusion. Arago 

 was the first to show that it was in accordance with fact, 

 and his experiment may be repeated without much diffi- 

 culty. 



(131) We have seen that when light diverging from a 

 luminous point passes by the edges of a fine hair or wire, a 

 succession of coloured bands will be formed parallel to the 

 edge of the shadow ; and the distances of these bands from 

 the shadow, and from one another, will be greater, the 

 less the diameter of the ' wire. If many such wires be ex- 

 posed to the diverging beam, and if (instead of being pa- 

 rallel) they are crossed and interlaced in every possible 

 direction, it is easy to conceive that the coloured bands 

 will be disposed in concentric circles, whose centre is the 

 luminous point. These circles resemble the halos visible 

 round the Sun and Moon in hazy weather. Their dia- 

 meters vary in the inverse ratio of the thickness of the wires 

 or fibres. 



This law was applied by Young, in a very ingenious man- 

 ner, to the comparison of the diameters of fibres, or small 

 particles of any kind. 



