290 Mr. S. K. Mitra on the Large-Angle Diffraction 



such large areas to depend on the precise form of tlie 

 aperture used in a much more striking manner than is 

 possible within the limits of the geometrical region ot: trans- 

 mission. If: the analogy could be stretched a little further, 

 it might indeed be said that we have to proceed outside tlie 

 limits of this region in order that the true " waves of dif- 

 fraction " having their origin at the boundaries of the 

 aperture might be separately observed. An investigation 

 of the distribution of luminosity in the fainter outlying- 

 regions in diffraction-patterns of the Fresnei class is thus 

 evidently of considerable interest and importance ; and the 

 present work was undertaken with a view to studying a few 

 typical cases of the kind, and thus ascertaining the general 

 features of the phenomenon. 



In order that the best results might be obtained, it is 

 necessary that the edges of the apertures used should be 

 smooth and highly polished. The cases for which the large- 

 angle diffraction have been studied in the present investi- 

 gation are the following : — (a) Elliptic apertures ; (b) semi- 

 circular apertures, and other forms of aperture bounded by 

 arcs of circles and straight lines ; (c) apertures or screens 

 with undulating or corrugated boundaries. The experi- 

 mental method adopted was to use a brilliantly illuminated 

 point source, the light from which after passing through 

 the diffracting aperture falls upon a photographic plate 

 held at a suitable distance from it. 



By using prolonged exposures, the fainter regions of 

 luminosity surrounding the Fresnei pattern may be recorded 

 on the plate, which is then studied at leisure. The brighter 

 parts of the field of course become greatly over-exposed and 

 cease to show any detail. A few selected photographs from 

 among those obtained are reproduced in Plate IV. figs. a-l. 



2. Elliptic Apertures. 



The case of the elliptic aperture was the first to be studied. 

 If the excentricity of the ellipse be small, nothing of special 

 interest is noticeable outside the limits of the geometrical 

 region of transmission. But as the excentricity is gradually 

 increased till the ratio of the major to the minor axis exceeds 

 \/2, some interesting features develop which become more 

 and more striking as the ratio is increased. Fig. a in the 

 Plate shows the effect. It will be noticed that the luminosity 

 is particularly marked along four curved arcs, which mea- 

 surements show to coincide exactly with the evolute of the 

 elliptic boundary in which the plane of observation intersects 



