Refracted and Diffracted Light. 353 



light, smallest in violet, and of intermediate sizes in the 

 intermediate colours. 



3. The body B continuing fixed, let us either bring the 

 screen C D nearer to B, or bring the lens with which we 

 view the fringes nearer to B, so as to see them at different 

 distances behind B. It will be found that they grow less 

 and less as they approach the edge of B, from which they 

 take their rise. But if we measure the distances of any 

 one fringe from the shadow at different distances behind 

 B, we shall find that the line joining the same point of the 

 fringe is not a straight line, but a hyperbola, whose vertex 

 is at the edge of the body ; so that the same fringe is not 

 formed by the same light at all distances from the body, 

 but resembles a caustic curve formed by the intersection of 

 different rays. 



4. Hitherto we have supposed that B has been held at 

 the same distance from F ; but let it now be brought to C, 

 much nearer F, and let the screen C D be brought to c d, 

 so that b g is equal to B G. In this new position, where 

 nothing has been changed but the distance from F, the 

 fringes will be found greatly increased in breadth, their 

 relative distances from each other and from the margin of 

 the shadow remaining the same." 



These observations, descriptive of the character of dif- 

 fracted light, are extracted from Sir David Brewster's 

 Optics, Cabinet Cyclopaedia, chapter the 11th. 



With regard to the first observation : It has already been 

 shewn, that the fringes formed on the edges of the shadows 

 of bodies in refracted light, proceed from the different 

 direction of the rays of different colours, arising from their 

 difference of refrangibility ; and that these fringes are 

 totally independent of the refractive power of the body 

 which forms the shadow. 



In explaining the second of these observations, it will be 

 necessary to keep in view, that the rays of light of different 

 colours are quite independent of each other ; and, conse- 

 quently, that, when only one colour is transmitted, it will 

 take precisely the position it would occupy if other rays of 

 different colours were present. 



It has been before observed, with reference to refracted 

 light, that the breadth of the fringes at equal distances 



VOL. III. 2 A 



