qdletitiortl EibttAow.^s in chetpa(i|t,witbinfiti(<3ififlice'ri>€interfered€ie 

 Wiih which we arc now dealing 'occurs without the shadow), 

 can only take phice thl'oUgh h change in the velocity of tranV 

 misisionfiw^ier^as.1 it iiiisii^eufeitaHy been considered that a» 

 vacuo all waves are transmitted with the same uniform veld>»- 

 city. Bub, .thpygU disposed to consider it as approximately 

 true, that in vncuo, ^aTl contunious waves are propagated with 

 'the'sfAWib'un'iforrii velbci^y, I am by no means inclined to admit 

 that the rlimarkable case' of broken waves is amenable to the 

 same rule. I have elsewhere observed in reference to this 

 subject (see the paper last referred to), that "thepr^gnt is a 

 case of wave-motion altogether peculiar, and one of which no 

 example has been hitherto subjected to investigation." All 

 cases of v/ave-motion hitherto investigated algebraically (I 

 might have, added 'or otherwise,' unless Fresnel's luminous 

 theory be thought an exception to the remark) resolve them- 

 selves into the simple case of the propagation in the direction 

 iof the axis of a cylindrical tube, of a wave whose front is per- 

 pendicular to its edge. The motion of a wave after diffrac- 

 tion may be assimilated to motion along a tube of which part 

 of the side has been cut away." Now, though perfectly true 

 that in the former case the wave is propagated with the same 

 uniform velocity, it seems very inconceivable, I might say 

 perfectly incredible, that in the latter the wave, attenuated by 

 its lateral extension (for it viust extend itself laterally), should 

 present an even front with the undiffracted wave, and so far, 

 therefore, from the change of form of the waves being contrary 

 to received principles, it is just what in the nature of things must 

 take place. , , ,,j : . 



/But that npp^je may suppose Fresnel's theory to be fri^e 

 from an objection, real or imaginary, which presses upon min4 

 I shall proceed to show, that even according to it the form of 

 the wave is changed after diflPraction. 'Jfljis woidd readily ap- 

 pear from a considera- ^ -,■^uLv\^^ ^'^ ,n:a .^luiuuikJ 

 tion of the formula wh.ch,,,„;^,,^f .^^ gnijdiioL ladjia/l 



rresnel gives tor the di- / ° 



sturbance in the example a. J, 'fi'^ ovodu adi ^^nKKUoAh t\\ * 

 above discussed, but it is i ' "* 



manifested with singular / _ 'l 



clearness in the following ^1 .rj.ilJoli 



investigation, taken sub- 

 stantially from Mr. Airy's 

 tract on the Undulatory 

 Theory, p. 270 et seq. 



A B a section of the umij 

 diffi-acting body by the 

 plane of the paper which 



