334 FOURTH REPORT — 1834. 



been adduced in opposition to these results. Newton found 

 that when the distance of tliese edges was the 400th part of an 

 inch, the light which passed between the knives parted in the 

 middle, and left a dark space in the centre*. The experiment 

 has been repeated by Mr. Barton, and with a similar resultf. 

 These experiments, however, were made with curved edges ; and 

 as Professor Powell has observed, we have no ground for sup- 

 posing that the phenomenon may not be modified by this ghange 

 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 curvilinear edges, and the analytical difficul- 

 ties of the problem seem to be insuperable. There seems to be 

 some uncertainty, hovvever, 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 i-elative brightness, as compared with other points in the line 

 perpendicular to the length of the aperture]:. 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 him- 

 self 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 re- 

 sults 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 princi- 

 ples 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 deter- 

 mine the joint effect produced at any point of the diffracting 

 aperture by the several indefinitely small portions of a wave 



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



t Phil. Mag., vol. ii. p. 268. t Jl^id; 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 re- 

 peated some of those I did try until 1 had satisfied myself about all their circum- 

 stances. To communicate what I have tried, and leave the rest to others for 

 further inquiry, is all my design in publishing these papers." Optics, Adver- 

 tisement 1. See also latter part of Obs. 11. IJook iii. 



