UPON THE properties OF LIGHT. 
255 
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 example 
of the rest, at 4 feet from the third edge the chart was fixed and the third edge kept 
constantly at that distance from it. Then the double-edge instrument was placed suc- 
cessively at 14^, at 9 and at4| eighths of an inch from the third edge. The breadths 
were respectively 2, 3f and 4^ twentieths of an inch. In some experiments these mea- 
sures approached more nearly the hyperbolic values of y, but I give the experiment 
now only for the important and indeed decisive evidence which it affords, that these 
fringes are caused by disposition, and are wholly different from those formed with- 
out previous flexion. 
Exp. 3. If the greater breadth of these fringes is owing to dispersion, then they 
should be formed more in the rays of the prismatic spectrum than in white light, or 
even in light bent by flexion. Yet we find it more difficult to trace fringes across the 
prismatic spectrum than in white light, and more difficult across the spectrum when 
there is divergence, than when formed parallel to its sides when there is no diver- 
gence. There are fringes formed, but of the narrow kind, which are described in 
Prop. I. 
Exp. 4. I have tried the effect on the fringes in question of the curvilinear edge 
described in the first article of these observations, and the effect of which is repre- 
sented in fig. 18. It is certain that at a distance from the double-edge instrument 
the third edge seems only to form fringes rectilinear, or of its own form. But when 
placed very near, as half an inch from the instrument, plainly there is a curvilinear 
form given to the fringes in question; and this is most easily perceived, when, by 
moving the third edge towards the side of the pencil, you form the smaller fringes so 
as to be drawn across or along the greater ones made by the two first edges. 
I think, without pursuing this subject further, it must be admitted that these 
fringes in light, which is bent and disposed, lend an important confirmation to the 
doctrine of disposition. It is clear that the rays are affected only on two of their 
four sides, or a h and c d, if these are parallel to the bending body’s edge, and not at 
all on the sides ch and da-, that, on the other hand, cb and da are affected when the 
