154 EXPERIMENTS AND INVESTIGATIONS 



disc's light being diminished by the flexion of the first two 

 edges, or not, for the present I stop not to inquire. This is 

 certain, that if the third edge be placed across the beam, and 

 at right angles to the two first edges, you no longer have the 

 small fringes. They are not formed in the direction A .9, 

 parallel to the edges as now placed. If the double edges are 

 changed, and are placed in the direction h'y', you again have 

 the bright fringes ; but then, if the third edge is now placed 

 parallel to li e', you cease to have them. Care must, however, 

 be taken in this experiment not to mistake for these bright 

 fringes the ordinary deflexion fringes made by one flexion 

 without disposition, as described in Proposition I. For these 

 may be perceived, and even somewhat more distinctly in the 

 disc than in the full light of the white pencil or beam. 



Now are these bright fringes only the flexion fringes, that 

 is fringes by simple flexion without disposition ? To ascer- 

 tain this I made these experiments. 



Ex p. 1. If they are the common fringes, and only enlarged 

 by the greater divergence of the rays after flexion, and more 

 bright by the dimness of the distended disc, then it will 

 follow that the greater the distension, and the greater the 

 divergence of the rays, the broader will be the bright fringes 

 in question. I repeatedly have tried the thing by this test, 

 and I uniformly find that increasing the divergence, by ap- 

 proaching the edges of the instrument, has no effect whatever 

 in increasing the breadth of the fringes in question. 



Exp. 2. If these fringes are not connected with disposition, 

 it will follow that the distance of the edge which forms them 

 from the double-edged instrument cannot affect them. But I 

 have distinctly ascertained that their breadth does depend on 

 that distance, and in order .to remove all doubt as to the 

 distance between the chart and the third edge which forms 

 them, I allowed that edge to remain fixed, and varied its 

 distance from the other two by bringing the double-edge 

 instrument nearer the third edge. The breadths of the bright 

 fringes varied most remarkably, being in some inverse power 

 of that distance. Thus, to take one measurement as an 



