THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



FEBRUARY 1845. 



IX. On Fresnel's Theory ofDiffraction. By R. Moon, M.A.^ 



Fellow of QueerHs College^ Cambridge^ and of the Cambridge 

 Philosophical Society*. 



''I^HE Fresnelian theory of diffraction has so long received 

 A the sanction of the scientific world, that it may appear 

 almost presumptuous to attempt to controvert it in any mate 

 rial point. I shall so far show deference to an opinion very 

 generally and very strongly pronounced, as not to enter upon 

 the consideration of the antecedent probability or improba- 

 bility of the fundamental hypothesis connected with it, that of 

 secondary waves emanating from the general front, since it 

 might be difficult, however much I may be impressed with its 

 unphilosophicalness, to advance anything very positive against 

 it. But the mathematics of the subject are less m.atters of 

 opinion and more matters of fact, and upon this ground I 

 enter with more confidence; for I believe that, unless I am 

 very blind or very ignorant, it can be shown that in this part 

 of the subject very grave errors have been committed. With 

 this much of explanation, I have to crave the indulgence of my 

 readers while I enter upon the discussion of the first expe- 

 riment treated of by Fresnel in his Theory of Diffraction ; 

 and for the clearer elucidation of my views on the subject, I 

 shall transcribe up to a certain point Sir John Herschei's 

 investigation of that experiment contained in his treatise on 

 Light (art. 718), presuming it to be substantially the same as 

 that of Fresnel. 



" Let us consider a wave A M F propagated from O, and 

 of which all that part to the right of A is intercepted by an 

 opake body A G ; and let us consider a point P in a screen at 

 the distance A B behind A as illuminated by the undulations 

 emanating simultaneously from every point of the portion 

 ♦ Communicated by the Author. 

 Phil. Mag. S. 3. Vol. 26. No. 171. Feb. 1845. H 



