XIX, 1. Rheinberg: Common Basis ofthe Theories ofMicroscopic Vision 21 



fulrilled at the slant of h' r ffifi'. 2()f/) which follows a path one wave 

 lengtli longer tliau the parallel ray clrawn frora b. Not to confuse 

 the raain diagram with too many liues , this has beeu shown sepa- 

 rately, and we may now represent it on the main diagram by the 

 line BR drawn in the same direction. \Ve will also represent the 

 intensity curve of the central image which the slot gives rise to by 

 the cnrved line pqr (iig. 20 6)^ 



If the slots A and B were quite independent, and produced no 

 interf'erence etfects , doubliug the heiglit of the line pqr at every 

 point, as shown by the dotted curve p" 2 r^ would give the intensity 

 curve of the two together. But a reference to the diagram shows 



21. 



9>> 



23. 



that very marked interference effects take place , as rays from the 

 slot B in the direction of the lines jBI, i?II, jBIII, traverse a path 

 one two and tliree half wave lengths respectively, longer than rays 

 at the corresponding slants from slot A. Consequently it is only in 

 the direction B\l that we get the double intensity etfect from the 

 two slots, whilst at the slant B\ and Z>III they completely neutra- 

 lise one another. Intermediate slants of course give intermediate 

 effects. We now plot the curve p" 123 r showing the result, and 

 find that the general effect has been to divide up the single broadeued 



^) Care must be taken not to confiise the intensity curve with the 

 amplitude curve. The intensity is as the Square of the amplitude, and the 

 dotted line shown is a sufficient approximation for the purpose here in view. 



