﻿Diffraction-bands due to a Rectangular Aperture. 495 

 reflected according to the usual law. If is not small, 



. + 6 . 6—0 

 •la sin . t 9 ~ sin ^ - = +n\ 



C 



and the diffraction-pattern is identical with that produced by 

 the effective aperture of the prism-face, and is symmetrical. 

 If is small, (2) is no longer true, and a reference to 

 the Tables shows that if the angle is small, for equal 

 increments of its cosine, the increments of the angle are 

 large and by no means equal. This shows that the bands are 

 fairly broad, and that the minima are not at equal angular 

 distances from one another. 



I give this example worked out from the following data : — 



a = 3 cms. \=7000 A.U. 0=1° 9'. 



Angular Distance from 



The 4th minimum to the 3rd 202" 



3rd „ to the 2nd 212" 



2nd .. to the lsr 223" 



1st „ to the central band ... 233" 



Central band to the 1st minimum 247'' 



1st minimum to the 2nd 266 v 



2nd „ to the 3rd 2S7" 



3rd „ to the 4th 311" 



Further, the smallest value of <j> admissible is zero. There 

 is therefore a limit to the number of bands possible on one 



