ON LIGHT AXD COLOURS. 



129 



the four surfaces. It is found that the image or fringe 1 1', 

 made by abcde (or E S y B K), is of the same breadth and in the 



Kg*. 



1 



t- 



same position throughout its whole length. So if directly op- 

 posite to this edge another straight edge is placed, and acts to- 

 gether with abcde on the light passing, the breadth of the fringe 

 I is increased, and its distance is increased from the direct rays, 

 but it has the exact same breadth from I to I' ; its portion 1' q 

 answering to a b, q P answering to b c, P answering to c d, 

 and I answering to d e, are of the same breadth, provided 

 care be taken that the second edge is exactly parallel to the 

 edge E K. And this experiment may be made with the 

 second edge behind abode, as in Exp. 1 of this proposition; 

 also it may be usefully varied by having the second edge 

 composed of four surfaces like the first, only it becomes the 

 more necessary to see that this compound edge is accurately 

 made and kept quite parallel to the first, any deviation, how- 

 ever minute, greatly affecting the result. "When care is thus 

 used the fringes are as in r v, v' r', quite the same in breadth 

 and in position through their whole length ; and not the least 

 difference is to be discerned in them, whether made by a 

 second edge, which is one sharp edge, or by a compound 

 second edge, similar to abcde. 



Hence I conclude that the beam passing by the compound 

 edge, or compound edges, is exactly as much distended by 

 the different flexibility of the rays, and is exactly as much 

 ,bent from its direct course when the flexion is performed by 

 a sharp edge, by a plane surface, by a very flat cylinder, or by 

 a very convex cylinder ; and therefore that all the action of 

 the body on the rays is exercised by one line, or one particle, 



