﻿Diffraction-bands due to a Rectangular Aperture. 497 



This was verified by experiment. An aperture 2 cms. wide was 

 cut in a thin sheet of zinc, which was then bent into the shape 



Fisr. 3. 



shown (fig. 3). The side-vanes served to cut off the light from 

 the other portions of the collimator and to support the sheet on 

 the table of the spectrometer. It was found that the diffraction- 

 bands were sensibly symmetrical when the incidence was 

 moderate, and asymmetrical when it approached 90°. The 

 minima in this case are of course given by the usual equation 

 «(cos —cos (f>)— ± n\. 



The angle 6 could not be measured ; only the relative positions 

 of the bands could be determined. The table gives some 

 measurements. 



a=2 cms, X = 6500 A.U. unknown. 



From 



The 2nd minimum to the 1st 



1st „ to the central band .. 



Central band to the 1st .., , 



1st minimum to the 2nd 



306" 

 322" 

 364" 



422" 



The intensity of illumination at any point of the diffraction- 

 pattern is given by the expression 



sin 2 — (cos d — cos </>) 



\ —(cos 0— cos <j> v 

 Phil Mag. S. 6. Vol. 12. No. 71. Nov. 1006, 



2K 



