[ H2 ] 



XXXII. Oti the Inflection of Light, in Reply to Professor 

 Powell. By John Barton, Esq.* 

 f" AM much indebted to Professor Powell for the candid 

 ■*■ and friendly manner in which he has commented on the 

 objections to the undulatory theory of light, which I adduced 

 in my former communication. On the present occasion I 

 propose, first, to make a few remarks in explanation or justifica- 

 tion of what I before advanced ; and then to suggest another 

 hypothesis, which I persuade myself is capable of explaining 

 the phenomena of inflection more completely and satisfactorily 

 than that of Fresnel. 



The first question at issue between the undulationists and 

 myself relates to a matter of fact. Newton found that when 

 a beam of light is suffered to fall on a very narrow slit, the 

 beam parts in the middle, leaving a dark space between the 

 two portions into which it is divided. This result agrees with 

 my own observation. Professor Airy, on the contrary, finds 

 the centre of the spectrum always relatively bright, however 

 narrow the slit through which the light passes. I have no 

 wish to disparage Professor Airy's authority ; but I must be 

 permitted to prefer the testimony of my own senses, sup- 

 ported as it is by the observations of Newton, and even, I 

 think I may add, by the observations of Professor Powell. 

 For although this gentleman has not succeeded in reproducing 

 the phenomena under the exact form described by me, and 

 delineated in my last communication, he has said enough to 

 convince every one who reflects on the subject, of the possibi- 

 lity of so succeeding when the experiment is repeated with due 

 precaution. "When," says he, "the edges approached very 

 nearly, I observed something like the appearance described 

 by Mr. Barton, but with this difference, that the boundaries 

 of the dark space, instead of being continuous, diverged either 

 way into the shadow in hyperbolic lines." Now, I am sure that 

 if Professor Powell will reconsider the question, he will see 

 that, from the nature of the hyperbola, there must, on his own 

 showing, be a certain width of interval between the two knife- 

 blades, giving a result such as I have described. In fact, it 

 is easy, from the hyperbolic form of the lines, combined with 

 Newton's observation that the dark space begins to appear 

 when the interval between the two blades is less than the four- 

 hundredth part of an inch, to assign the precise distance at 

 which the blades must be placed, and the degree of curvature 

 they must possess, in order to give a dark space of any re- 

 quired figure and dimensions. I need not dwell on this point, 



* Communicated by the Author : see our preceding volume, p. 424. 



