Refracted and Diffracted Light. 421 



situated, with the more refrangible colours which proceed 

 from a part of the lens nearer its centre ; the greater re- 

 fraction of the former, compensating for the greater refran- 

 gibility of the latter, so as to bring the rays of the differ- 

 ent colours to the same point. 



This will appear more evident, perhaps, if we suppose 

 the object B to be fixed on a part of the central ray corre- 

 sponding in distance with a certain part of the lens, so that, 

 supposing light to be equally refrangible, the light from 

 this circle of the lens would pass the edges of the object, 

 and form the circle of light nearest the shadow ; and then, 

 removing the condition of equal refrangibility, in confor- 

 mity to the known law of refraction, consider, that at 

 whatever distance from the centre of the lens this circle 

 may be situated, the more refrangible rays belonging to it, 

 from their greater divergence, cannot possibly arrive at the 

 edge of the object, with the less refrangible rays, so as to 

 perform the conditions required; for either the latter must 

 fall upon the object, and be intercepted, or the former 

 must be at such a distance from the edge as to allow other 

 rays to pass from circles nearer the centre. In either case 

 the light of different colours, which passes the edge of the 

 object to form the fringes, must consist of rays, the most 

 refrangible of* which proceed from a part of the lens nearer 

 the centre, than the less refrangible rays which meet them 

 at this point ; and as they cross here, and open at the same 

 angle as that by which they approached, the more refran- 

 gible rays, which were previously outside, will now be 

 inside, or nearest the shadow : every fringe is, in fact, the 

 boundary of the shadow of the object, in the light of the 

 colour in which it is seen. 



I do not mean to deny the action of the interposed body 

 upon the light which passes its edges ; but in the present 

 experiments, the resulting appearances are in a great de- 

 gree independent of it, the fringes being in every respect 

 the same, according to the experiments of Sir Isaac New- 

 ton and Sir David Brewster, whatever may be its density 

 or refractive power. 



Hitherto the developement of the theory has produced a 

 literal explanation of the facts ; but we have only yet ac- 

 counted for one set of fringes ; and if the theory has im- 



