174 Mr. Barton on the Inflection of Light, 



trum, — Professor Powell thinks these observations not to be 

 relied on, as being of earlier date, and performed with ap- 

 paratus less susceptible of accuracy than that employed by 

 Fresnel. I must observe, however, that the question at issue 

 does not rest on the comparative accuracy of the methods of 

 observation employed. Fresnel has not recorded any experi- 

 ments made under circumstances similar to those which gave 

 to Newton and Biot results so much at variance with his 

 theory. He examined in two cases only the distance at which 

 the first dark bands are found to intersect. In these two cases 

 the width of the slit was, respectively, two millimetres, and one 

 millimetre and a half*; whereas the discordant results given 

 by the observations of Newton and Biot relate to widths be- 

 low one millimetre, or the twenty-fifth part of an inch. Even, 

 therefore, if the observations of Fresnel w r ere as far superior 

 in accuracy to those of his predecessors as Professor Powell 

 assumes, it would not be decisive in favour of his theory. 

 But I confess I cannot see any sufficient ground for attri- 

 buting to the observations of Fresnel such unmeasured supe- 

 riority as has been supposed. I cannot think that if Fresnel 

 deserves all confidence to the hundredth part of a millimetre, 

 Newton and Biot are unworthy of being trusted to the tenth 

 part of a millimetre, as Professor Powell's reasoning implies. 

 I cannot think that the accuracy of different series of ob- 

 servations is to be estimated solely by their relative antiquity. 

 Fresnel certainly introduced one important improvement into 

 the method of observation : — he received the rays on a lens, 

 instead of receiving them on a sheet of paper, like Newton, or 

 on a piece of roughened glass, like Biot: he was thus enabled 

 to measure the intervals between the fringes by a micrometer, 

 — a method which admits of greater accuracy than the earlier 

 method of taking off the distance by a pair of compasses. 

 But let it be observed that in the particular class of pheno- 

 mena at present under consideration, the advantage gained 

 by this improvement in the method of observation ceases al- 

 together ; since the question relates, not to the width of the 

 interval between two fringes, but to the place of their inter- 

 section. With regard to another important point, the mea- 

 surement of the width of the slit, there is some reason to think 

 Newton's method of observation preferable to Fresnel's. I do 

 not, indeed, find in Fresnel's memoir any account of the 

 manner in which this was accomplished in his experiments ; 

 but Newton has described the method employed by himself. 

 Having placed his knife-blades with their edges (which were 



* Memoire* de I'lnstitut, tome v. pp. 398 ami 437. 



