XIX, 1. Rheinberg: Common Basis of the Tlieories ofMicroscopic Vision. 7 



to tlie case of the light at points in tlie vicinity of P', whicli, as 

 they concern iis more particiüarly, we will now consider. 



Take the point Q (lig. o). Assuming the distance YY' to be 

 half a wave length , it is seen from the construetion tliat an undii- 

 lation must reach Q from 1' half a wave length behind one arriving 

 there from Jsi. But we have already seen tliat a ditference of phase 

 of exactly half a wave length means that crest meets trough , or a 

 certain movement to is met by exactly the same amonnt of movement 

 fro, and the amplitude being the same, the etfect due to one is 



3. 



exactly nentralised by tliat dne to the other. So far then as the 

 points J\^ and I^ are concerned, the point Q would receive no light. 

 But a glance shows tliat on joining the points between X^ and Y 

 with points on the cnrve J^ Y' in the direction of (), tlie separating 

 distance varies gradually from nil to half a wave length , and con- 

 sequently these points on the wave front X. Y produce a corresponding 

 effect at Q varying from nil to the maximum amplitude. Their 

 Joint effect is therefore to produce a certain amount of light at Q 

 — as a matter of fact the amplitude there is a little less than |^"^- 

 that at P', and the intensity of tlie light varying as the Square of 

 the amplitude is about ^lo ^^^ ^^''^^ '^^ ^' • 



