ON LIGHT AND COLOURS. 



149 



though much narrower than when the bending bodies are 

 more near to one another. 



3. The breadth of the interference fringes is said to be in 

 some inverse proportion to the difference in length of the 

 interfering rays. It is commonly said to be inversely as that 

 difference. 



In fig. 20, A is the first and B the second edge. By inter- 

 ference the fringe at C should be 

 broadest and at D narrowest, be- 

 cause AC BC = AO is less 

 than AD-BD = AP; and so 

 as you recede from D, the fringes 

 should become broader and 

 broader, because the two rays 

 become more nearly equal. But 

 the very reverse is notoriously 

 the case, the breadth of the fringes 

 decreasing with their distance 

 from the direct rays. 



4. In the case of the fringes formed by the second body 

 inflecting and the first deflecting, there can be no interference 

 at all ; for the whole action is on one and the same pencil or 

 beam. A deflects and then B inflects the same ray ; and 

 when a third edge is placed on the opposite side to B, it 

 only deflects the same ray, which is thus twice bent further 

 from the direct rays, the last bending increasing that 

 distance. 



5. Let A be the first and B the second edge as before (fig. 

 20). Suppose B to be moveable, and find the equation to the 

 disposing force at different distances of the two edges, we 



shall find this to be ?/ = = , a being 



Va. 2 +^ 2 - A /(a -a:/ + 6* 



= A E, b = E D, and A B = x. But all the experiments show 

 it to be y = , a wholly different curve. 



Again, let B be fixed, or the distance of the two edges be 



