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II. Reply to Mr. Tovey's Remarks on a Paper on the Appli- 

 cation o/' Huyghens's Pi-inciple in Physical Optics, insei'ted 

 in the Lond. Edinb. 8^ Dtihlin Phil. Mag. for October last', 

 vol. xvii. p. 24'3. By Richard Potter, Esq., B.A. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



I BEG to return Mr. Tovey my thanks for keeping alive the 

 discussion on the capability of the undulatory to explain 

 facts ; by such discussions we may entertain hopes that some 

 sound progress will eventually be made in physical optics. 



Mr, Tovey says I have mistaken a luminous line for a lu- 

 minous space, and consequently my conclusions have, in 

 reality, no foundation. He ought unquestionably to have 

 given us his proof. 



In the investigations for a small opake disc and a small 

 circular aperture, it was considered, by the most eminent ma- 

 thematicians who have adopted the undulatory theory of light, 

 as quite sufficient to find the intensity along a line drawn per- 

 pendicularly through the centre of the aperture or disc, and 

 their results were brought forward as strong proofs of the truth 

 of the theory. When, in the same way, I have applied a more 

 correct analysis, for all magnitudes of circular discs and aper- 

 tures, it is objected to, without showing where the limit be- 

 tween a large and a small magnitude lies, and at what point 

 the method becomes inaccurate. 



Our knowledge of the phaenomena of diffraction leads us to 

 conclude that there can be no maxima and minima in the line 

 before-mentioned without accompanying ones in the plane 

 perpendicular to that line, and such has tacitly been received 

 with respect to former investigations. With this the undula- 

 tory theory also accords. How Mr. Tovey could suppose 

 that there is in reality in any case only a luminous line, it is 

 not easy to account for ; if he had been acquainted with the 

 phaenomena I think he would not have made the objection. 



The case of the fixed stars shows that the smallness of the 

 angular magnitude of the luminous body at the eye is no bar 

 to light being perceived when there is adequate intensity. 



The following analysis shows that the undulatory theory 

 indicates maxima and minima in the plane perpendicular to 

 the line, for which my former results were investigated. 



Taking now a point out of the line (C B) passing perpendi- 

 cularly through the centre of the circular aperture or annulus, 

 let its distance from that line be small and equal to x. 



Let 6 be the angle which any radius makes with the plane 



