Xvi THIRD REPORT — 1833. 



mote facts of the precession of the equinoxes and the tides. 

 To all this there is nothing corresponding in the history of the 

 theory of emission ; and no one, I think, well acquainted with 

 the subject, would now assert, that if this latter theory had 

 been as much cultivated as the other, it might have had a simi- 

 lar brilliant fortune in these respects. 



" But if the undulatory theory be true, there must be solu- 

 tions to all the apparent difficulties and contradictions which 

 may occur in particular cases ; and moreover the doctrine will 

 probably gain general acceptance, in proportion as these solu- 

 tions are propounded and understood, and as prophecies of 

 untried results are delivered and fulfilled. In the way of such 

 prophecies few things have been more remarkable than the 

 prediction, that under particular circumstances a ray of light 

 must be refracted into a conical pencil, deduced from the theory 

 by Professor Hamilton of Dublin, and afterwards verified ex- 

 perimentally by Professor Lloyd. In the way of special diffi- 

 culties, Mr. Potter proposed an ingenious experiment which 

 appeared to him inconsistent with the theory. Professor Airy, 

 from a mathematical examination of this case, asserted that the 

 facts, which are indeed difficult to observe, must be somewhat 

 different from what they appeai-ed to Mr. Potter ; and having 

 myself been present at Professor Airy's experiments, I can 

 venture to say, that the appearances agree exactly with the 

 results which he has deduced from the theory. Another gen- 

 tleman, Mr. Barton, proposed other difficulties founded upon 

 the calculation of certain experiments of Biot and Newton ; 

 and Professor Powell of Oxford has pointed out that the data 

 so referred to cannot safely be made the basis of such calcula- 

 tions, for mathematical reasons. There is indeed here, also, 

 one question of fact concerning an experiment stated in New- 

 ton's Optics : In a part of the image of an aperture where 

 Newton's statement places a dark line, in which Mr. Barton has 

 followed him, Professors Airy, Powell, and others, have been 

 able to see only a bright space, as the theory would require. 

 Probably the experiments giving the two different results have 

 not been made under precisely the same circumstances ; and 

 the admirers of Newton are the persons who will least of all 

 consider his immoveable fame as exposed to any shock by these 

 discussions. 



" Perhaps, while the undulationist will conceive that his 

 opinions have gained no small accession of evidence by this ex- 

 emplification of what they will account for, those who think the 

 advocates of the theory have advanced its claims too far, will 

 be in some degree conciliated by having a distinct acknow- 



