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Mr. W. B. Croft : Some 



distinction to a circular area. The play of light-waves in a 

 shadow gives rise to infinite paradox. The most notorious 

 is Arago's bright spot at the centre of the shadow of a cir- 

 cular disk, with which he astonished Napoleon Buonaparte: 

 squares and triangles turn about and point inwards : needles 

 seem to be split down with the light and the two parts point 

 outwards, the thinner needle having the broader central cleft: 

 lastly, lines inclined to one another have the effect of re- 

 solving one another into transverse flakes. 



Simple figures may be drawn to give a partial explanation 

 of these curious results. 



Fig. a. 



OOCOOOCiOOOOOOOOcy/ 



c 



, lt~~ 



H G ' 



Fig. a illustrates the ordinary principle of Huygens, that 

 a wave-front, AB, goes on as a similar wave-front, C, if it 

 be supposed that each point gives rise to a secondary wave : 

 when this strikes upon an obstacle, DE, the waves at the 

 edge, being isolated and not supported by similar waves on 

 the one side, may pass forward so that in directions F, G, H 

 one is always J, § , f of a wave-length in front of the other, 

 causing interference or darkness : these are the exterior 

 shadows : also there can be imagined the action of waves 

 from D and E about a position K which gives the interior 

 bands. 



Fig. b. 



F.y 



GH 



Fig. b shows t^at waves from A and B must always reach E 

 in the same phase, so at the centre there is always light ; at F, 



