3j?4 FOURTH REPORT — 1S34. 



to investigate their laws in the simple case in which the mag- 

 nitude of tlie hnniiious body is reduced to a point. The eflects 

 thus presented were first observed bj^ Grimaldi, and they have 

 been since studied, as a separate branch of optical science, under 

 the title of diffraction or injiexioii. Grimaldi found that when 

 a small opaque body was placed in the cone of light, admitted 

 into a dark chamber through a very small aperture, its shadow 

 was much larger than its geometric projection, so that the light 

 suftered some deviation 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 shadow, 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 Grimaldi ; 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 succession on the diffracting body, Newton observed 

 that the fringes formed were broadest in 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 observations of Newton most closely connected with his 

 physical theory are those in which the light is made to pass be- 

 tween two near knife-edges, whether parallel or inclined. From 

 these observations 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 experiment, 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 intersection of the rays passing at dif- 

 ferent 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 bentf. 



* P/ij/sicO'Mathesh de Ltwiine, Bologna, 1G65. 

 t Optics, Book iii. 



