UPON THE PROPERTIES OF LIGHT. 
239 
But (fig. 6) place B anywhere between A and y r on the side of the ray opposite to 
A, and the breadth of rv is increased, and also its distance from the direct ray RR', 
as v' r ' ; and in like manner (fig. 7) the deflected rays A i;, A r are both more separated, 
making a broader image at v", and are further removed from RR' by B’s inflexion. 
Exp. 2. If you bend the rays either by a single edge, or by the joint action of two 
edges, it makes not the least difference either in the breadth or in the distance from 
the direct rays of the images, or in the distension or elongation of the luminous body’s 
disc, whether the bending body is a perfectly sharp edge (which in regard to the rays 
of light is a surface, though a narrow one), or is a plane (that is, a broader surface), 
or is a curve surface of a very small, or of a very large radius of curvature. 
In fig. 8, ae is an instrument composed of four pieces of different forms, but all in 
a perfectly straight line; ah is an extremely sharp edge; he ^ flat surface; erZ a 
cylindrical or circular surface of a great radius of curvature ; d e one of a small radius 
of curvature. But all these pieces are so placed that ESy is a tangent to ed, dc, 
and is a continuation of y|3 K, that is, of ch, ha. So the light passing by the whole 
ah ede, passes by one straight line EK, uniting or joining the four surfaces. It is 
found that the image or fringe II', made by ahede (or E^y|8 K), is of the same 
breadth and in the same position throughout its whole length. So if directly opposite 
to this edge another straight edge is placed, and acts together with ah c de 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 V q answer- 
ing to a&, answering to he, PO answering to ed, and OI 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 
ah e de, ns \n 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, however minute, greatly affecting the result. When care is 
thus used the fringes are as in rv, v'r', quite the same in breadth and in position 
through their whole length ; and not the least difference is to be discerned in thetn, 
whether made by a second edge, which is one sharp edge, or by a compound second 
edge, similar to ahede. 
Hence I conclude that the beam passing by the compound edge, or comj)ound 
edges, is exactly as much distended by the different flexibility of the rays, and is ex- 
actly 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, and not first by one and then by others in succession ; and this clearly 
proves that after a first flexion takes place, no other flexion is made by the body 
on the same side of the rays. This is easily shown. 
For a plane surface is a series or succession of edges infinitely near each other ; 
