﻿Clianges in Diffraction Spectra. 



27 



has been calculated for a number of special cases. The 

 method of calculation is represented geometrically in fig. 2, 



Fio 2. 



where vector diagrams arc drawn for a seventeen 

 aperture diffraction grating with \\ cubic aberration. 

 AOB represents the phases of the disturbances transmitted 

 in the direction of the normal to the wave-front at its centre, 

 ON, fig. 1, the dotted line AB representing the resultant 

 amplitude in this direction. Similarly A'B' represents the 

 amplitude in a direction ON', fig. 1, such that the difference 

 of phase between the light from the central and outside 

 apertures is increased to 180°, and AiBj the amplitude in a 



