C 235 ] 



XXIX. Professor Powell and Dr. Whewell. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN a paper lately published by me among the Memoirs of 

 the Oxford Ashmolean Society entitled " On Necessary 

 and Contingent Truth," I have entered somewhat on the 

 questions which have been so much discussed as to the origin 

 of our ideas and foundation of our reasonings in mathematical 

 and mechanical science ; and in so doing have, of course, ad- 

 verted, as their importance justly demands, to the speculations 

 of Dr. Whewell on these points. 1 find, however, by a letter 

 which 1 have since received from that eminent writer, that in 

 the reference I make throughout my paper to his opinions I 

 have unfortunately mistaken them, in ascribing to him the 

 belief in i?ifiate, inherent, or intuitive ideas; or (as at p. 6) in 

 " ideas dependent upon intuitive conviction." Such belief^ 

 Dr. Whewell, in his letter to me, distinctly disclaims. He 

 has spoken of certain " fundamental ideas " appropriate to 

 different branches of science, but has nowhere discussed the 

 question of their origin ; contending only that they are results 

 of the active, not of the passive constitution of the mind. 



I am anxious therefore to take the opportunity afforded by 

 your Journal to express my regret at having been led into a 

 misconception of Dr. Whewell's meaning ; while I rejoice to 

 find that, by the explanation thus given, the difference between 

 the views which I have upheld and those of Dr. Whewell is 

 materially diminished. 



I remain, 



Very truly yours, 



Baden Powell. 



XXX. Proceedings of Learned Societies. 



' CAMBRIDGE PHILOSOPHICAL SOCIETY. 

 [Continued from vol. xxxv. p. 392.] 

 Nov. 26, /^N the Dynamical Theory of Diffraction. By Professor 

 1849. ^^ Stokes. 

 The problem of diffraction is treated mathematically by conceiving 

 each wave of a series incident on a small aperture, or passing the 

 edge of a diffracting body, broken up on arriving at the aperture or 

 diffracting edge, regarding each element of the wave as the centre 

 of an elementary disturbance, which diverges spherically from that 

 element, and finding by integration the aggregate disturbance at any 

 point in front of the primary wave. With the exception of one case 



