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



suit : indeed it would clearly be iuconsisteut with the former 

 experiments that it should be otherwise. 



Prop. VI. appears precisely to express FresnePs conclusion 

 (above referred to), that with two edges at unequal distances from 

 the origin, the fringes will be broader on the side towards the 

 edge most remote from the origin, which is again more precisely 

 exhibited in Prop. YIII., when the aperture is sufficiently narrow 

 to give a white centred image ; the same regard being had to the 

 limits in chstance as before. 



In Prop. VII. the meaning is by no means obvious ; but it seems 

 to amount experimentally to this, — that with one edge only, the 

 fringe nearest that edge is the broadest ; and that when a second 

 edge acts opposite to it at some distance along the ray, but so as 

 to give the fringes of an aperture, then among the fringes of each 

 set, those towards the middle of the aperture are the broadest : 

 the first being obviously the case of the external fringes ; the 

 second easily verified, and agreeing with the ordinary case of an 

 aperture with edges at the same distance ; while as to the appli- 

 cation of the undulatory theory, we can oijjy make tlie same re- 

 marks as before. 



In the Additional Observations, (3), p. 254, the truth of the 

 general assertion, that when fringes are formed by two edges, a 

 third can afiect them only when parallel and not when at right 

 angles to them, is indeed obvious ; but the precise conditions of the 

 experiment mentioned are difficult to understand. It would seem 

 to consist in first forming the fringes of a narrow apertm-e with 

 a broad white centre in the ordinary way ; and then in that white 

 centre producing new fringes by a third edge nearer to the screen : 

 these, however, the author affirms, will be formed only when the 

 third edge is parallel to the aperture and not when at right angles 

 to it ; they are also described as brighter and narrower than the 

 ordinary fringes. The author cautions us against confounding 

 them with the ordinary external fringes, and proceeds to argue 

 that they are of a different nature, for several reasons, but 

 chiefly because (Exp. 1) they do not increase in breadth when 

 the aperture is narrowed, and (Exp. 2) because their breadth 

 increases as the distance of the third edge from the aperture is 

 diminished, the third edge remaining at the same distance from 

 the screen. 



The last results (which I have fully verified) do not appear 

 to me to evince any peculiarity : relatively to the third edge, 

 the aperture may be regarded as a new origin of light, in which 

 light the third edge gives its external fringes. 



But with resjjcct to the first part of the proposition, viz. that 

 these fringes are only formed parallel to the aperture, on re- 

 peatedly trying the experiment, I have uniformly found them 



