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



this, we have numerous streamers of light, one of which is 

 parallel to the diameter and the others form roughly a system 

 of hyperbolic arcs symmetrical about the horizontal axis. 



The streamers are brightest to the left of the diameter, but 

 they may also be seen Yery faintly to the right of it. The 

 pattern thus shows many striking resemblances with the 

 Fraunhofer pattern at the focus of the heliometer objective, 

 which is described by Everitt * in the following words : — 



" The image is symmetrical about two perpendicular axes. 

 There is a central approximately oval spot which is roughly 

 a little less than twice as long as it is broad. This central 

 spot is bisected along the major axis by a long bright ray 

 which fades away as it recedes from the centre. Around 

 the spot and following its outline are a series of alternate 

 bright and dark rings, w T hich, at the minor axis, are identical 

 with those of the diffraction image of a circular aperture, 

 and, along the major axis, terminate at an angle in a series 

 of bright spots. Further, these rings are crossed and modified 

 by a series of alternate bright and dark rays of approximately 

 hyperbolic shape, having the major axis of the figure for their 

 geometric axis, which they cut at its maxima and minima, 

 with the exception that the first one of the series passes 

 through a point in the central bright spot." 



Fig. d in the Plate represents the large-angle diffraction 

 pattern due to a circular segment smaller than a semi- 

 circle, and is instructive in the comparison it affords with 

 the case just considered. The two cases have many features 

 in common. The transverse streamers obtained in the case 

 of the segment are, however, much clearer and cover a 

 smaller part of the field than in the case of the semicircle. 



On one side they may be clearly seen radiating from a 

 point which lies outside the strongly illuminated part of the 

 field, and is in fact the centre of the circular boundary. 



Figs. /'and e in the Plate show the large-angle diffraction 

 due to apertures bounded by three and four arcs of circles 

 respectively, the centres of which lie outside the area of the 

 apertures. The streamers of light starting out normally from 

 the projection of the boundaries are clearly visible in the 

 photographs. On one side of each they diverge, and on 

 the other side they converge to foci at the respective centres, 

 and then diverge again. The streamers are not all equally 

 bright, the fluctuations of intensity near the margin of each 

 group being particularly noticeable. 



* P. F. Everitt, Proceedings Royal Societ}^, vol. lxxxiii. (1910). 



