28 Rheinberg: Common Basis oftheTheories ofMicroscopic Vision. XIX, 1. 



3) The brightness of a maximnm may be zero , if I may be 

 allowed such a paradoxical expressiou. For suppose that the Posi- 

 tion where a number of slots reinforce one another to form a maxinuim 

 happens to coincide with the position of a minimum on the iutensity 

 curve of the Single slots , then it is evident that Ave can have uo 

 light there, and the maximum is in fact missing. 



This occurs when the width of the interspaces are just an even 

 number of times as wide as the slots. In figure 24 and 22 where 

 the interspace is twice as wide as the slot, there is a missing maxi- 

 mum at R. 



4) The number of maxima is theoretically one less than the 

 number of wave lengths separating corresponding points in two slots. ^ 

 Figures 32 and 33 in which the distance is 1 and 4 wave lengths 



1 

 32. 



respectively, show this. It is seen that the direction of the tirst 

 maximum in figure 32 and the 4th. maximum in figure 33 would 

 be horizontal and in the plane of the slots themselves. 



Praetically we usually get less maxima, because if the distance 

 apart equals a large number of wave lengths, the light intensity of 

 all but the first few on each side is too small to be noticeable, 

 moreover they may fall too near togetber for the eye or Photographie 

 plate to resolve tliem. 



It has perhaps been remarked that throughout this chapter no 

 use has yet been made of the expression „diffraction grating", only 

 rows or series of slots having been spoken of. That has been done 

 intentionally. A diffraction grating is nothing other than a series 

 of slots, but it is ordinarily used in the restricted sense of a very 



1) Excepting the case that any spectra should be missing, as per 

 preceding paragraph. 



