420 Mr. P. Cooper on the Connexion between 



stated, that the same fringe is not formed by the same 

 light, at all distances from the body, but by the intersec- 

 tions of different rays. 



This will be rendered more evident by the following 

 considerations, with a view to the explanation of the fourth 

 observation. 



As all the rays of light which cover the triangular space 

 F C D, cross somewhere near F, it necessarily follows, 

 that their divergence is unequal ; that the central ray must 

 be unrefracted ; and that from the line formed by it, the 

 refraction must gradually increase to C and D. 



If, then, we place an object upon the line R, at different 

 distances from F, or the focus of divergence, its edges will be 

 surrounded by light differently refracted ; if, for instance, 

 we place the object so near to F, that only the rays which 

 fall on the screen near C and D, are allowed to pass, the 

 boundaries of its shadow will be formed of the most highly 

 refracted light ; if the object is taken to B, its shadow will 

 be bounded by rays which having passed through the lens 

 near the central ray, are little refracted ; if we take it to 

 b, its shadow will be bounded by the intermediate rays, 

 and be refracted accordingly. In all these cases, the fringes 

 indicate the degree of refraction by their different breadths. 

 M. Fresnel has given the result of some experiments on 

 this subject, from which it appears, that when the distance 

 of the object, was four inches from the focus F, the angular 

 inflexion of the red rays of the first fringe was 12' 6"; 

 and when the distance was increased to twenty feet, the 

 angular inflexion of the corresponding rays was only 3' 55"; 

 the screens, C D, c d, which receive the shadows, being, 

 in both experiments, placed at 39 inches from the object. 

 The breadths of the fringes, in these experiments, vary at 

 different distances, from the focus, as they would in sur- 

 faces of light refracted by prisms of different refracting 

 angles. 



The fringes commence with the most refrangible rays 

 reckoning from the shadow. 



The light, which at the moment of its passing the edge 

 of the object, forms white light, is composed of rays trans- 

 mitted by different parts of the lens, the least refrangible 

 colours of which meet at this point, wherever it may be 



