10 



Mr. T. K. Chinmayam on the Flow of 



the diffraction-fringes due to a straight edge. Some photo- 

 graphs taken with the arrangement described above are 

 shown in Plate L, the figures (a), (b), (c) corresponding 

 to the phenomena in planes at increasing distances from the 

 " edge." 



2. The Form of the Illumination Curves. 



Debye * has shown from the electromagnetic theory that 

 at a great distance from a cylinder (assumed to be of a large 

 radius) on which plane waves are incident, the disturbance 

 due to it is practically the same as that to be expected from 

 the principles of geometrical optics, this statement, however, 

 not being taken as correct in respect of points lying in a 

 direction very nearly the same as that of the incident rays. 

 Debye's results suggest a simple method of finding the 

 distribution of intensity at points lying within the region of 



Fig. 1. 



\ 





K 



M M ' 





\ 



1 



\ 



\ P 







 / Q,' 



\ 



\ 



p' _._. 











light in the immediate neighbourhood of the cylinder. Let 

 QO (fig. 1) represent the section of the cylinder and KOY 

 the direction of the incident rays. We may assume that the 

 fringes observed in the plane OX containing the " edge " 



* Debye: Pki/s. Zeitschr. ix. 

 Abstracts,' 1909, p. 88. 



pp. 



775-778, Nov. 1908; also 'Science 



