ON LIGHT AND COLOUES. 



145 



fa. i c 



white disc, on each side of which are fringes ; those on the one 

 side, be, c d, are by the inflexion of the second edge ; those on 

 the opposite side, a/, fe, are by the de- 

 flexion of that second edge. I repeat- 

 edly measured these sets of fringes, and 

 at various distances from the second 



edge ; and I always found them much RgA9. 



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, af 

 was = 1 and fe only -f or ^. The fringes by inflexion of the 

 second edge also uniformly increased as they receded from a b, 

 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 gradually nearly 

 equal ; and when the edges are quite opposite each other 

 there is no difference 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 mention- 

 ing my friend Lord DOURO, who has, I believe, heredi- 

 tarily, 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 



