(52 REPORT ON PHYSICAL OPTICS. 



and left a dark space in the centre * The experiment has been 

 repeated by Mr. Barton, and with a similar result. t These ex- 

 periments, however, were made with curved edges; and as Professor 

 Powell has observed, we have no ground for supposing that the 

 phenomenon may not be modified by this change in the conditions 

 under which it is presented. The theory of Fresnel has not been 

 applied to the more complex problem of an aperture with curvi- 

 linear edges, and the analytical difficulties of the problem seem to 

 be insuperable. There seems to be some uncertainty, however, 

 with respect to the phenomenon itself. Professor Powell repeated 

 the experiment with edges of various curvatures, and always 

 found that the centre was a point of relative brightness, as com- 

 pared with other points in the line perpendicular to the length of 

 the aperture. J As to Newton's experiment, it seems certain, as 

 the same writer has observed, that we are not acquainted with all 

 its conditions ; and it is apparent from many passages that the 

 illustrious observer himself was far from being assured with respect 

 to the real nature and circumstances of these phenomena.! 



But there is another essential circumstance to be taken into 

 account, in comparing the experiments of Newton with the results 

 of Fresnel's theory. In that theory the origin of light is supposed 

 to be a point ; and this condition is practically fulfilled by making 

 the light to diverge from the focus of a lens of high power, the 

 origin of the light in that case being (by the principles of the 

 wave-theory) the minute image of the sun in the focus. In 

 Newton's experiments, however, the sun's light was made to pass 

 through a hole of sensible magnitude; and in the remarkable 

 experiment now referred to, that hole was a quarter of an inch in 

 diameter. The problem of diffraction in this case is one of much 

 greater complexity. It is necessary to determine the joint effect 

 produced at any point of the diffracting aperture by the several 

 indefinitely small portions of a wave transmitted through the ex- 



* Optics, Book iii., Obs. vi. and vii. 



t- Phil. Mag., vol. ii. p. 268. 



J Ibid., p. 429, &c. 



" The subject of the third book I have also left imperfect, not having tried all 

 the experiments which I intended when I was about these matters, nor repeated some 

 of those I did try until I had satisfied myself about all their circumstances. To com- 

 municate what I have tried, and leave the rest to others for further inquiry, is all my 

 design in publishing these papers." Optics, Advertisement 1. See also latter part of 

 Obs. 11, Book iii. 



