426 Prof. Powell's Remarks on Mr. Barton's Paper 



small waves belonging to the original waves from Rli'&c, will 

 be equally strong in all directions, and A in this case is the 

 real centre and origin from which the new undulations com- 

 mence. 



But for all accurate experiments, the luminous point made 

 use of is the focus of a lens of short focal length ; in which 

 case the theory is equally clear ; and the waves after converg- 

 ing to this point diverge from it in the same manner as if 

 that point were a centre of excitation, or source of light. (See 

 Professor Airy's Tract, p. 289.) Indeed (unless I mistake Mr. 

 Barton's meaning,) it is difficult to conceive how such a point 

 could have occasioned any embarrassment or ground of objec- 

 tion. 



(2.) The second observation is of a more important charac- 

 ter, and brings us to an objection of a more tangible kind. 



The author admits at the outset that " the theory of Fresnel 

 agrees pretty well with the results of his own experiments." 

 Now this admission appears to me to involve very nearly a 

 concession of the whole question. Fresnel's researches were 

 conducted with a delicacy and a precision far exceeding those 

 of any previous experimenters, and keeping pace with the in- 

 creased and elaborate refinements of the theoretical analysis. 

 These coincidences of observation and theory are, in fact, such 

 as no one can examine without an irresistible conviction of 

 the truth of the laws which the formulae express. These con- 

 siderations alone might almost suffice to render superfluous 

 any further argument; since all that Mr. Barton alleges is 

 grounded on a comparison of the theory with older, and there- 

 fore probably far less accurate experiments. The observations 

 of an older date, however valuable for the period at which 

 they were made, are not, in researches of this kind, to be put 

 in comparison with those obtained by the philosopher of the 

 present day with all " the appliances and means to boot," 

 which the improvements of recent science place at his disposal : 

 if a discordance should be found, the more precise results of 

 the recent investigation would be fairly entitled to the pre- 

 ference. 



The observations of Newton on what has been called in- 

 flexion of light, were indeed remarkable instances of accuracy 

 and skill ; and when we consider that their illustrious author 

 had not only to discover the facts, but almost to invent the art 

 of experimenting, it would be no disparagement to his unpa- 

 ralleled preeminence, if modern research, enjoying superior fa- 

 cilities of instrumental precision, should have produced expe- 

 rimental determinations of such a character, as to supersede, 

 or even to invalidate, his earlier results. 



