412 The Rev. B. Powell's Remarks on Mr. Barton's Reply, 



and stifled, L e, dispersed, by the agency of the colouring mat- 

 ter which acts the part of the air in Mr. Wheatstone's expe- 

 riment, and self-neutralized by the opposition of its subdivided 

 portions as above explained. 

 Slough, October 19, 1833. 



LXI V. Remarks on Mr. Barton's Reply, respecting the Inflec- 

 tion of Light. By the Rev. B. Powell, M.A. KR.S. Savi- 

 lian Professor of Geometry, Oxford. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 I DID not see your Number for September last, containing 

 *■ Mr. Barton's reply to my former paper, till very lately, 

 and now hasten to send a few brief observations, which that 

 reply seems to call for, and which I trust you will favour me 

 by inserting in your Journal. 



In the first place allow me to say that the courteous tenour 

 of Mr. Barton's paper assures me that he will regard my pre- 

 sent communication with the same candour as he has done 

 the former ; and in that spirit of candour I will proceed at 

 once to the essential questions at issue. 



The important and conclusive experiment is that in which 

 the aperture has straight parallel edges. Here Fresnel's for- 

 mula applies directly, and accords most exactly with the phae- 

 nomena. This is evident both from what I have stated (Lond. 

 and Edinb. Phil. Mag., vol. ii. p. 431-2), and from the exact 

 experiments of Professor Airy, described in my postscript (lb. 

 p. 433). On this part of the question 1 do not perceive that 

 Mr. Barton alleges any result of his own of an opposite kind. 

 The only difficulty is about an experiment or' Newton's. 

 (Optics, book iii. obs. 5.) Now this experiment, as I before 

 observed, is involved in considerable ambiguity. I am not 

 aware whether Mr. Barton has succeeded in reproducing it 

 with all the concomitant circumstances as described by New- 

 ton, viz. the " long trains of light" which he speaks of, &c. 

 These are as essential to be explained as the appearance of a 

 dark space in the centre. I have repeatedly tried to verify 

 this experiment, but entirely without success; and I am much 

 inclined to believe that there were some circumstances in the 

 conditions of the case of which we are not fully informed. It 

 is surely, then, most imperatively incumbent on us to ascertain 

 carefully all the conditions, before we allege it in opposition 

 to the united testimony of all other experiments. 



But with respect not only to this, but also to the other ex - 



