24 Rheinberg: Common Basis ofthe Theories ofMicroscopic Vision. XIX, 1. 



Next let US consider the case of three slots , this being one 

 of special interest, and suppose the slots A, B and C to be equally 

 wlde and separated by a distance eqnal to their width. We can find 



the Joint eflfect of these three sh;)ts 



by regarding it as the elTect due 



A*ß to every combination of two of thera. 



ß^C There are firstly A and B^ which 



aäC would produee the intensity cnrve 



of figure 20 with one maxininm on 



each side of the central band. 



Then there are B and 6', which 



would produee the same curve. The 



addition of these two similar curves produee one with every point 



double as luminous , as shown in figure 26. Then there is the 



26. 



27. 



28. 



eflfect of the combination A and 6', viz. : the curve shown in 

 figure 25 with three maxima on each side. Adding this to the other 

 one, we see (fig. 26 and 27) that the position of the maxima 



the light increases, and this accounts for the double width of the central 

 band as eompared to the others. 



Although the point Q thus forms an exeeption to the general rule 

 in not being the brightest point of the band in which it is situated, yet it 

 is usual and convenient to designate as „maxima" all the points at which 

 rays arrive froui the two ends of the slot with an odd nuinber of half 

 wave lengths difterence, and in what foUows () will be spoken of as the 

 Position of first maximum formed by a single slot. 



