410 



Transactions of the Society. 



we shall get a resulting amplitude nearly equal to the simple sum 

 of the two (fig. 78, a). But if we take a pair with a considerable 

 difference of phase, one being behind, the other an equal amount 

 in front of the light from the centre of the slit, then there will be 



a 



Fig. 78. 



interference. And an inspection of fig. 78, b, immediately shows a 

 striking peculiarity ; for as one wave-curve recedes just as much 

 from any of the nodes of the central light as the other exceeds it, 

 the displacements of the two waves at those nodes must always be 

 equal to each other, but in opposite directions ; on the principle of 



