Mr. Barton on the Inflection of Light. 173 



since the details of the computation will be readily supplied 

 by any one versed in mathematics. But I may add, that if 

 Professor Powell will employ blades of less curvature, he will 

 find the experiment succeed more readily. Those employed 

 by him in repeating my experiment had, if I do not mistake, 

 so inconsiderable a radius of curvature as to reduce the di- 

 mensions of the dark space within a limit too small to be con- 

 veniently observed. 



It would seem, indeed, from his subsequent observations, 

 that Professor Powell is not quite sure whether to admit or 

 not the correctness of Newton's observation ; for after saying 

 that he cannot succeed in obtaining the results described by 

 me, he goes on to assign certain considerations which may 

 serve, he thinks, to reconcile the phaenomena with the undu- 

 latory hypothesis, even sicpposing the facts to be such as I have 

 asserted. The formulae of Fresnel, he says, always suppose 

 the opening through which the light enters the darkened 

 chamber to be a mathematical point: whereas Newton em- 

 ployed for this purpose a hole a quarter of an inch in breadth. 

 But the experiment succeeds with me equally well, whether 

 the opening in question be large or small, while it fails equally 

 in either case with Professor Airy ; this cannot, therefore, be 

 the source of the different results obtained by us. Indeed it 

 appears quite incredible that the enlargement or contraction 

 of the aperture by which the light enters the chamber can 

 produce a difference of such a kind as supposed by Professor 

 Powell. That a change in the dimensions of the aperture 

 should affect the breadth or intensity of the dark space may 

 be readily imagined ; but that such a change should convert 

 light into darkness appears inconceivable. 



The second observation of Professor Powell under this 

 head I am apprehensive I do not fully comprehend. He says, 

 " It results from the well-known fact of the enlargement of the 

 shadows of the two edges beyond their geometrical boundaries, 



that these shadows will coalesce before the edges meet; and 



this seems very likely to have been the real result observed by 

 Newton." Surely the formulae of Fresnel are intended to 

 comprise the whole of the phenomena, including among the 

 rest that enlargement of the shadow to which Professor Powell 

 alludes. If so, it cannot be allowable to employ this enlarge- 

 ment as an after correction to the results of calculations de- 

 duced from those formulae. 



With regard to another objection advanced by me against 

 the theory of Fresnel, — its inconsistency with the observa- 

 tions of Newton and Biot on the distance at which the first of 

 the dark bands cross one another in the centre of the spec- 



