respecting the Inflection of Light, 415 



from the calculated numbers given in my former paper, where, 

 with a very narrow aperture, the centre, though a point of 

 relative brightness compared with other points in the same 

 horizontal line, is yet a point of relative darkness compared 

 with greater breadths of aperture. I am of course quite aware 

 that, strictly speaking, the expression is incorrect, and though, 

 perhaps, it may not inaptly facilitate the primary apprehen- 

 sion of the phenomenon, yet it undoubtedly involves a theory, 

 and would therefore be better avoided. The idea that each 

 edge carried with it its own fringes, which, as the two edges 

 appi*oached, crossed and overlapped, and thus gave rise to 

 the bands actually observed, was the theory conceived by 

 Biot in his analysis of the phenomenon. FresnePs formula 

 resolves the whole into an expression for the brightness of 

 the light at points measured on a screen in a line at right 

 angles to the length of the slit. 



As to Mr. Barton's estimate of the comparative accuracy of 

 the experiments of Newton, Biot and Fresnel, every reader 

 will form his own judgement from the careful consideration 

 of all the circumstances which ought to be taken into account. 

 With respect to Biot's experiments, I see nothing to alter the 

 conclusion at which I before arrived, by showing the probable 

 amount of error which would bring the results into accord- 

 ance ; and I apprehend that in most cases of this nature the 

 presumption would be in favour of the later experimenters, 

 without any real disparagement of the earlier. With respect to 

 Newton's results there is one circumstance to be remarked 

 which, I believe, escaped me when writing my former paper. 

 The particular experiment of Newton in question is that of 

 the i nclined knife-edges. The formula with which it is com- 

 pared is that of Fresnel for parallel edges. This, as I have 

 above observed, is a most essential difference'; and thus, 

 whether the error lies with me or with Mr. Barton, the whole 

 computation, from first to last, is altogether nugatory. 



The difficulty which Mr. Barton has now more fully stated 

 respecting the origin of the rays, does not appear to me to 

 require more than a careful reference to what is laid down 

 in the best treatises for its elucidation. The effect of waves 

 propagated through a narrow aperture is ably explained 

 either in Professor Airy's tract (art. 27), or in Sir J. Her- 

 schel's treatise on Light (art. 607. 628)*. But the author's 

 view of the subject seems to be suggested as introductory to 

 the original theory which he proposes; and it appears to me 

 that it would be a far better course if, instead of encumbering 



• 

 * Encyc. Metr., 2nd Div. vol. ii. 



