ON LIGHT. 323 



terior, are observed to dilate in breadth, according to a 

 certain law which it is not necessary here to state, but 

 whose agreement with the result of calculation affords a 

 very satisfactory verification of the theory adopted for 

 the explanation of the whole series of these phenomena 

 on the undulatory hypothesis : while, on the other hand, 

 it is very obvious that no such dilatation could possibly 

 take place were the fringes produced by any kind ot 

 action on the rays in their passage by or near to the 

 edges of the object, in the nature of attractive or repul- 

 sive forces originating in the material substance of which 

 it consists, and deflecting them from their rectilinear 

 paths ; inasmuch as such action couhi neither be increased 

 nor diminished, or in any way modified, by the greater 

 or less distance traversed by the rays before their arrival 

 within its sphere. 



(106.) The appearances exhibited when the light is 

 transmitted through a narrow rectangular slit, are even 

 more curious. In this case a bright image of the open- 

 ing is thrown on the screen, but instead of being an 

 evenly illuminated narrow band, bounded on either side 

 by uniform darkness, it is bordered both externally and 

 internally by parallel fringes. The external ones are 

 bright and highly coloured, and vary in breadth, but not 

 materially in brightness, as the screen is withdrawn from 

 the slit ; but the interior fringes undergo singular changes 

 as the distance increases. At near distances they are 

 narrow and close, and leave a medial space of uniform 

 light; but as the distance increases they enlarge in 

 breadth, and close in on the illuminated space, so that 



