63 



A POPULAR ACCOUNT 



Upon measuring the breadths of 

 these fringes at their brightest parts, and 

 also the breadths of their intervals, 

 Newton found them nearly in the same 

 proportion, and so that the fringes and 

 intervals taken in regular succession 

 were as the following quantities : 



i, L---L 



V 2 V 3 V 4 V 5 



His next experiments were on the 

 shadows produced by the edges of sharp 

 knives placed across the aperture through 

 which the light was transmitted. The 

 light admitted between the edges being 

 received upon a screen at some distance, 

 the edges were moved towards one ano- 

 ther, until their distance did not exceed 

 the four hundredth part of an inch. The 

 light upon the screen now parted in the 

 middle, and formed two parallel lines. 

 The intermediate shadow was so black 

 and dark that all the light which passed 

 between the knives seemed to be bent, 

 and turned to the one side or the other. 

 As the edges approached, the shadow 

 grew broader, and the lines of light nar- 

 rower, until at length, when the edges 

 were in actual contact, the light wholly 

 vanished. This experiment Newton 

 considered conclusive of the greater de- 

 flection of the rays which were nearer 

 the body. 



From these experiments, he concluded 

 that the light which formed the first 

 fringe passed the edge of the knife at a 

 distance not less than the eight hun- 

 dredth part of an inch ; that the light 

 of the second fringe passed the edge of 

 the knife at a greater distance than that 

 of the first, and the light of the third 

 fringe at a still greater distance. 



From these and other circumstances, 

 Newton concluded that the distances at 

 which the light forming the fringes 

 passed the knives are not altered by the 

 approach of the knives, but that the 

 angles at which the light is inflected 

 are increased by their approach ; the 

 knife which is nearer each ray deter- 

 mining the direction in which it is bent, 

 and the other knife increasing the deflec- 

 tion. 



When the rays fell very obliquely 

 on the ruler, at the distance of the third 

 of an inch from the knives, the dark lines 

 between the first and second fringes 

 bounding each shadow, intersected each 

 other at the distance of the fifth of an 

 inch from the termination of the light. 

 Hence Newton computed the distance 

 between the edges, at the concourse of 



these lines, to be the hundred and six- 

 tieth part of an inch. Since this con- 

 course is in the middle of the light which 

 passes between the edges, it follows that 

 one-half of the light passes each edge 

 at a distance not greater than the three 

 hundred and twentieth part of an inch. 



Upon increasing the distance of the 

 ruler from the knives, he found that the 

 distance of the concourse of the dark 

 lines before mentioned, from the termi- 

 nation of the light, was more than the 

 fifth part of an inch. Hence it appeared 

 that the light, which in this case passed 

 between the knives at the concourse of 

 the dark lines, passed at a greater dis- 

 tance from the edges than the hundred 

 and sixtieth part of an inch. 



From these experiments Newton 

 concluded that " the light which makes 

 the fringes, is not the same light at all 

 distances from the knives; but when 

 the ruler is held near the knives, the 

 fringes are made by light which passes 

 the edges at a less distance, and is more 

 bent than that which forms the same 

 fringes at a greater distance." It will 

 be perceived that this is consistent with 

 the explanation of the phenomena of in- 

 flection already given. 



When the shadows were received upon 

 paper at a great distance from the 

 knives, the fringes assumed the form 

 of hyperbolic curves, being nearly 

 straight where the distance between the 

 edges was considerable, but bending 

 into arches after intersecting. Let C A, 

 C B,y7j\ 55, represent the projections of 

 the edges of the knives upon the paper. 

 The entire light would fall within the 

 angle A C B, were there no inflection. 

 Through C draw D E equally inclined 

 to CA and C B. The curve eis re- 

 presents the boundary of the shadow of 

 the blade A C ; fk t represents the dark 

 line which separates the first and se- 

 cond fringe, and glo the dark line 

 which separates the second and third 

 fringe. The lines xip, y kg, zlr, are 

 similarly related to the shadow of the 

 other edge. 



The two systems of curves, which are 

 perfectly similar, intersect at the points 

 i, k, I ; so that the shadows of the edges 

 are marked by the lines et>and#t/?, 

 until the intersection of the fringes, 

 and then each of those lines crosses the 

 fringe corresponding to the other edge. 

 Then those lines cross the fringes, dis- 

 tinguishing them from another light 

 which begins to appear at *, and illu- 

 minates the triangular space ip DEs. 



