354 Mr. P. Cooper, on the Connexion between 



from the object which forms the shadow, are proportioned 

 to the degree of refraction; in order, then, to account for 

 the greater breadth of the less refrangible colours, we have 

 only to shew, that, whatever may be the position of the 

 object B, with regard to its distance from the point F, the 

 less refrangible colours that pass its edges, must necessarily 

 proceed from a part of the lens where they are more highly 

 refracted, than the more refrangible colours which pass 

 with them. 



In whatever position we fix the object B, on the line R, 

 formed by the central ray, a little attention will discover to 

 us, that the white light immediately adjoining its edges, 

 which, on its arrival at the screen, forms the fringes, is 

 composed of rays of the different colours, which proceed 

 from different parts of the lens ; for every compound ray 

 transmitted by the lens, is refracted in the order of the 

 refrangibility of the different colours of which it is formed ; 

 and its component parts must be separated to distances 

 from each other, in proportion to their degree of refraction, 

 and the distance from the centre of divergence ; but, as 

 there is a succession of rays, supplied by the breadth of 

 light which falls on the surface of the lens, gradually dif- 

 fering from each other in their degrees of refraction, the 

 more refrangible colours which are separated by greater 

 divergence are replaced by similar colours proceeding from 

 a part of the lens where they are less refracted : the white 

 light, therefore, which surrounds the central ray, is every 

 where formed by the intersection of different colours, the 

 least refrangible of which are the most highly refracted. 

 The fringes, consequently, under circumstances in other 

 respects similar, if observed apart from each other, as in 

 the case which is the object of inquiry, will be broadest in 

 red light, smallest in violet, and of intermediate size in the 

 intermediate colour. 



That white diffracted light is formed by the intersection 

 of coloured rays, the least refrangible of which pass 

 through a part of the lens of higher refracting power than 

 the more refrangible rays which cross them, may be shewn 

 from the following consideration : — The surface of white 

 light upon the screen C D, is formed by the superposition of 

 enlarged images of the lens L L, in red, green, and violet- 



