Diffraction Experiments. 



123 



pin-hole into a dark chamber, or, still "better, through an 

 opening of greater size, behind which a lens of short focus is 

 placed so as to form an extremely minute and brilliant image 

 of the sun, from which rays diverge in all directions, its shadow 

 is observed to be bordered externally by a series of coloured 

 fringes, which are more distinct the smaller the angular dia- 

 meter of the luminous point, as seen from the object. ... To 

 see the fringes in question, they may be received on a smooth 

 white surface." M. Fresnel, Sir John tells us, first used an 

 emeried glass plate to receive the fringes, and looked at them 

 through it from behind. He then found that when he had 

 focussed them with a magnifier they continued visible, as if 

 drawn on the air, when the plate was removed. The first 

 thing to be noticed is " that the distance (of the fringes) from 

 each other, and from the border of the shadow, diminishes 

 as the screen on which they are received approaches the border 

 of the opaque body, and ultimately coincides with it." The 

 second important thing to be noticed is, " That they are not 

 propagated in straight lines from the edge of the body to a 

 distance, but in hyperbolic curves, having their vertices at that 

 edge, and therefore it is not one and the same light which forms 

 one and the same fringe at all distances from the opaque 

 body." In the diagram subjoined, Fig. 1, ' ' is 

 the luminous point, A the edge of the (opaque) 

 body, and G H a screen perpendicular to the 

 straight line OA; the border of the visible 

 shadow, and D, E, F, the places of the succes- 

 sive minima of the fringes in a line at right 

 angles to the edge of the shadow. If the 

 screen be brought nearer to A as at g h, and if 

 c, d, e,f, be the points corresponding to 0, D, 

 B, F, their loci will be the hyperbolas Ac 0, 

 Ad D," etc. The diagram shows that the rays 

 behave as if they were so repelled from A as to 

 oblige them to continue their path in a curve 

 instead of going straight on from A to B. The 

 result of this is that. shadows are seen where 

 they would not occur if the light did move quite 

 straight from A to B. "Newton found the 

 shadow of a hair, ^-g-y-th of an inch in diameter, placed at 

 twelve feet distance from the luminous point, to measure at 

 four inches from the hair -jjVth pf an inch, or upwards of four 

 diameters of the hair, at two feet TrVth of an inch, or ten diame- 

 ters, while at ten feet it measured only ^th of an inch, or 

 thirty-five diameters, instead of 120, which it would have done 

 if the rays terminating the shadow had proceeded in straight 

 lines, or rather, to speak more correctly, if the shadow were 



FIC:1. 



