2 Prof. Powell's Remarks on Lord Brougham's 



repeating tlie experiments with the utmost care, for all the most 

 material cases considered ; since which time various causes have 

 delayed the publication of my results. 



The whole of the author's investigations are expressed with 

 reference to his peculiar hypothesis of certain forces of " de- 

 flexion " and " inflexion " supposed to be exerted upon the rays 

 of light by the action of the edge of an opake body near which 

 they pass : nor is it always an easy matter to disentangle the 

 actual facts from the language of this theory, so as to see to 

 what the experimental e\ndence really amounts. 



Of those of the author's propositions which refer solely to the 

 exposition of his theoretical views, I do not propose to enter on 

 any discussion. There are also other portions of the investiga- 

 tions, which, though of a more experimental character, will not 

 call for much observation, as they either tend to establish phse- 

 uomena in exact conformity wath well-kno^vTi results, or are of a 

 natm-e not having much bearing on theory either way. 



Of this class are the prehminary experiments (Prop. I. Exp. 1, 

 2, 3) ; though with respect to the last it ought to be remarked, 

 that Newton by no means limits the number of fringes to three, 

 and in one modification of the experiment expressly mentions 

 that four or five were rendered visible*. "When (as in Exp. 4) 

 the origin of light is not the single point absolutely requisite in 

 all accurate investigations, but an extended object, such as a flame, 

 the moon, &c., it may be questioned how far the fringes may be 

 properly termed images of it. In Prop. II. Exp. 2, that the nature 

 or form of the edge makes no difference in the result, accords 

 exactly with the long-known experiments of Biot, Haldat, and 

 others. Indeed, as is equally well understood, the fringes may 

 be produced without any opake edge at all, as at the junction of 

 two faces cut on a glass, slightly inclined to each other. Again, 

 the hyperbolic fringes of an acute angle (in Prop. V. Exp. 3), as 

 well as the measures of the fringes at successive distances from 

 the edge determining tbe locus of any given fringe (in Prop. X., 

 and additional remarks, (2) p. 252), appear to agree with previous 

 observations; though, according to the author's theory, each 

 fringe seems to be regarded as an individual ray, while in the 

 interference theory it is the locus of the intersections of a series 

 of rays. 



At another part of his discussion the author assumes (Prop. XI.) 

 an aggressive position, and endeavours to refute the application 

 of the interference theory. In reply I think it will suffice to 

 remark, under the several heads, — (1) the theory of interference 

 explains perfectly both the internal and external fringes of a 

 shadow ; (2) the breadth of the fringes has no dependence on 

 * See Opticks, book 3. part 2. obs. 2. 



