UPON THE PROPERTIES OF LIGHT. 
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made by the second edge inflecting the rays which the first had deflected. These 
can only be seen when the second edge is near the first, because the rays cannot pass 
on so as to form the images on the chart, if the second is distant from the first. The 
pencils diverge both by the deflexion and by the inflexion of the first edge. But we 
can always, when the inflected rays pass too far from the second edge, bring this so 
near them as to act on them, whereas we in so doing intercept the deflected rays. 
However, after this is explained, we find no difficulty in examining the effects of the 
inflexion by the second edge, only we must place it near the first, and thus we have 
two sets of fringes, one extending into the shadow of the first edge at an inch distance 
between the two edges ; but at three-fourths of an inch, nay, at two inches, or even 
more, this experiment can well be made. 
Exp. 2. At these distances 1 examined repeatedly the comparative breadths of the 
two sets. In fig. 16, a 6 is the white disc, on each side of which are fringes ; those 
on the one side, he, cd, are by the inflexion of the second edge ; those on the opposite 
side, af, fe, are by the deflexion of that second edge. I repeatedly measured these 
sets of fringes, and at various distances from the second edge ; and I always found 
them much broader on the side of the second edge than on the opposite side. Thus 
a b being the breadth of 5, & c was 3, and cd 4|, while, on the opposite side, a f was 
= 1 and fe only f or The fringes by inflexion of the second edge also uniformly 
increased as they receded from a h, the direct rays, whereas the opposite fringes as 
constantly decreased. 
Exp. 3. If however the distance between the two edges be reduced, it is observed 
that the disparity between the two sets of fringes decreases, and they become gradu- 
ally nearly equal ; and when the edges are quite opposite each other there is no dif- 
ference observable in the two sets. Each ray is disposed and polarized alike and 
affected alike by the two edges, and no difference can be perceived between the two 
sets. 
Exp. 4. The experiments also agree entirely with the calculus in respect of the 
relative values of r and v affecting the result. It appears that the fringes by the second 
edge’s inflexion are broader than those by that edge’s deflexion, whether we use white 
or homogeneous light. In the latter, however, the difference is not so considerable. 
This I have repeatedly tried and made others try, whose sight was better than my 
own. I may take the liberty of mentioning my friend Lord Douro, who has, I believe, 
hereditarily, great acuteness of vision. 
Proposition IX. 
The joint action of two bodies situated similarly with respect to the rays which 
pass between them so near as to be affected by both bodies, must, whatever be the 
law of their action, provided it be inversely as some power of the distance, produce 
fringes or images which increase with the distance from the direct rays. 
Let (fig. 17) A and B be the two bodies, and AC=CB=a be their spheres of 
2 K 
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