ON LIGHT AND COLOURS. 139 



all the points of B, from D to E, are equidistant from C ; 

 therefore nothing can be ascribed to 

 the divergence of the bent rays. 

 B bends the rays disposed by A at 

 different distances D D' and E E' from 



the point of disposition. The fringe . _ tg ' <3< 



is now of various breadths from dd' 

 to e, the broadest part being that 

 answering to the smallest distance of D, the point of flexion, 

 from D' the point of disposition ; the narrowest part, e, 

 answering to EE', or the greatest distance of the point 

 of flexion from the point of disposition. Moreover, the 

 whole fringe is now inclined ; it is in the form of a curve 

 from dd' to e, and the broad part dd', formed by the flexion 

 nearest the disposition, is furthest removed from the direct 

 rays ; the narrowest part, e, is nearest these direct rays. 

 It is thus quite clear that the flexion by B is in some in- 

 verse proportion to the distance at which the rays are bent 

 by B from the point where they were disposed by A. I 

 repeatedly examined the curve de, and found it certainly to 

 be the conic hyperbola. Therefore m = 1, and the equation 



to the force of disposition is y = . 



30 



In order to ascertain the value of m, I was not satisfied 

 with ordinary admeasurements, but had an instrument made 

 of great accuracy and even delicacy. It consisted of two 

 plates, A and B (Plate VI.), with sharp rectilinear edges, 

 one, A, horizontal, the other, B, moving vertically on a 

 pivot, and both nicely graduated. The angle at which the 

 second plate was vertically inclined to the first, was likewise 

 ascertained by a vertical graduated quadrant E. Moreover 

 the edges moved also horizontally, and their angle with each 

 other was measured by a horizontal graduated quadrant K. 

 There was a fine micrometer F to ascertain the distances of 

 the two edges from each other, and another to measure the 

 breadth of the fringes on the chart. The observations made 

 with this instrument gave me undoubted assurance that the 



