20 Rlieinberg: Common Basis ofthe Theories ofMicroscopic Vision. XIX, 1. 



simplifies matters, for the eye can readily detect on the diagrams wliat 

 lines are parallel to each other. and without drawing the whole figures, 

 we have but to see with what ditference of pliase such parallel lines 

 Start, to be able to say what result they would produce when united 

 at the focus. In figure 19 it will be observed the parallel lines, 

 wbich are bronght to a focns at Q start from all poiuts ou slot CD 

 just one wave length later than from corresponding points on E FXj 

 and just two Avave lengths later than from GH. The light from 

 these slots therefore reinforce each other at that slant, and the etfect 

 due to the three is simply three times that dne to any one of them. 



20. 



Another convenience whieh we have is that so soon as we know 

 what the amplitude is which any number of parallel rays gives rise 

 to, we may represent it by a Single line in the same direction. 

 Now let ns first examine Avhat takes place when we have two slots 

 Ä and B of equal width, and to begin with we will suppose that the 

 distance separatiug them is of the same width as the slots (fig. 20). 



We have seen what sort of an intensity curve the slot B alone 

 would givc US from the previous chapter, and we wish to ascertain 

 the direction in which the first dark line limiting the broadened out 

 central Image of the slot falls. It is as we know, in that particular 

 direction where the ray from end of the slot traverses a path just 

 one wave length longer than from the other end. This condition is 



