96 Mr Wliewe!i''s Address to the British Association 



assert that if this latter theory had been as much cuhivated as 

 the other, it might have had a similar brilliant fortune in these 

 respects. 



But if the undulatory theory be true, there must be solutions 

 to all the apparent difficulties and contradictions which may oc- 

 cur in particular cases : and moreover, the doctrine will probably 

 gain general acceptance, in proportion as these solutions are pro- 

 pounded and understood, and as prophecies of untried results 

 are delivered and fulfilled. In the way of such prophecies few 

 things liave been more remarkable than the prediction, that un- 

 der 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 experimentally by 

 Professor Lloyd. In the way of special difficulties, Mr Potter 

 proposed an ingenious experiment which appeared to him incon- 

 sistent with the theory. Professor Airy, from a mathematical 

 examination of this case, asserted that the facts, which are in- 

 deed difficult to observe, must be somewhat different from what 

 they appeared 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 gentleman, Mr Barton, proposed 

 other difficulties founded upon calculation of certain experiments 

 of Biot and Newton ; and Professor Powell of Oxford has point- 

 ed out that the data so referred to cannot safely be made the 

 basis of such calculations, for mathematical reasons. There is 

 indeed here also one question of fact concerning an experiment 

 stated in Nev/ton''s Optics : in a part of the image of an aperture 

 where Newton'^s statement places a dark line, in which Mr Bar- 

 ton has followed him, Professors Airy, Powell, and others have 

 been able to see only a bright space, as the theory would re- 

 quire. Probably the experiments giving the two different re- 

 sults have not been made under precisely the same circum- 

 stances; and the admirers of Newton are the persons who will 

 least of ail consider his immoveable fame as exposed to any 

 shock by these discussions. 



Perhaps, while the undulationist will conceive that his opi- 

 nions have gained no small accession of evidence by this exem- 

 plification of what they will account for, those who think the ad- 



