DIFFRACTION. 51 



ber through a very small aperture, its shadow was much larger 

 than its geometric projection, so that the light suffered some devia- 

 tion from its rectilinear course in passing by the edge. Observing 

 these shadows more attentively, he found that they were bordered 

 with three iris-coloured fringes, which decreased in breadth and 

 intensity in the order of their distances from the edge of the sha- 

 dow, preserving the same distance from the edge throughout its 

 entire extent, unless where the body terminated in a sharp angle. 

 Similar fringes were observed under favourable circumstances 

 within the shadows of narrow bodies.* 



The phenomena of diffraction were subsequently examined by 

 Hooke, and by Newton. The first observations of Newton were 

 but repetitions of those of Grrimaldi ; and it is remarkable that he 

 altogether overlooked the important phenomenon of the interior 

 fringes noticed by the Italian philosopher. But to Newton we 

 owe the analysis of the phenomena, so far as they depended on the 

 nature of the light. When the different species of simple light, 

 into which the sun's rays were divided by a prism, were cast in suc- 

 cession on the diffracting body, Newton observed that the fringes 

 formed were broadest in the red light, narrowest in the violet, 

 and of intermediate magnitude in the light of mean refrangibility, 

 so that the iris-coloured fringes which are formed in white light 

 are but the fringes of different colours superposed. But the ob- 

 servations of Newton most closely connected with his physical 

 theory are those in which the light is made to pass between two 

 near knife-edges, whether parallel or inclined. From these obser- 

 vations Newton concluded that the light of the first fringe passed 

 by the edge, at a distance greater than the 800th of an inch, that 

 of the second and third fringes passing at still greater distances. 

 These distances, however, were not the same wherever the fringes 

 were formed ; and it appeared to follow from the experiments that 

 the light of the same fringe was not the same light at all distances, 

 but that each fringe was, as it were, a caustic formed by the in- 

 tersection of the rays passing at different distances from the edge ; 

 the portion of the fringe near the knives being formed of light 

 which passed nearest to the edge and was most bent.f 



To account for these phenomena, Newton supposed the rays of 



* Physieo-Mathesis de Luminc, Bologna, 1665. 

 f Optics, Book iii. 



E2 



