464 HISTORY OF SCIENCE. 



receiving the shadow on a piece of ground glass, and when looking 

 at these through a magnifying-glass, he observed that fringes were 

 equally visible beyond the area of the piece of ground glass, and that, 

 in fact, the interposition of this screen was useless, as the coloured 

 fringes could be directly viewed through the magnifying-glass. The 

 first use he made of this mode of direct observation was to trace the 

 fringes to their origin : that is, as in approaching the wire the fringes 

 and the intervals between them continually narrowed, the question was 

 from what part of the opaque body did they originate. Fresnel traced 

 the exterior bands quite up to the very margin of the opaque body, 

 or, at least, the most powerful magnifying-glass failed to detect any 

 interval. Newton's theory, which sought to explain these fringes by 

 repulsive action exercised on the ray of light in passing near the opaque 

 body, required an appreciable interval between the origin of the bands 

 and the margin of the opaque body. That for the production of the 

 bands within the shadow rays emanating from both margins were 

 necessary, Fresnel proved conclusively by simply attaching a little 

 piece of black paper to one or the other side of the wire, when the 

 interior bands opposite to that part of the wire disappeared. It may 

 easily be understood that by carefully measuring the actual intervals 

 between the fringes in places at various known distances from the 

 wire, the shape of the paths of the fringes could be determined. The 

 intervals, small as they are, could be very accurately measured by a 

 micrometer constructed on the same principle as the instrument used 

 in telescopes (page 212). 



In order to place clearly before the reader the manner in which 

 Fresnel conceived the undulations to operate in producing diffractive 

 phenomena, we reproduce in Fig. 208 the diagram which accom- 

 panies his paper, s is the radiant point, A and E the extreme points 

 of the body producing the shadow. From the points s, A, and B as 

 centres, series of circles are described, with radii increasing successively 

 by the same amount, which is supposed to represent half a wave-length. 

 The circles traced in plain lines may be taken to represent the crests, 

 and those in dotted lines the hollows of the waves. [We use here the 

 words crests and hollows, but the original has the terms which are 

 applied to waves of sound.] The intersections of the two different 

 kinds of circles are points where the undulations arrive in opposite 

 phases, and are therefore points in the darkest bands. Lines traced 

 through these points are hyperbolas (page 42), and the position of 

 the dark bands is determined by the places in which the screen 

 intersects these hyperbolas, F' and/', gg 1 . Again, the intersection of 

 circles of the same kind will give the positions of bright bands, and 

 these occupy the places of the hyperbolas L L' and //'. An inspec- 

 tion of the figure will explain one of the most notable facts in the 

 experiment, namely, that the number of bands within the shadow is 

 greater as the shadow is examined at positions nearer to the wire. It is 



