THEORY OF ETHER 163 



the grating we should see the slit directly ; by means of the 

 grating the interfering portions are intercepted, and so more 

 than a direct view of the slit is obtained, and we see, in a 

 fashion, around a corner. 



Suppose that the distances of P from a 2 and from a- 3 differ 

 by a whole wave-length, then the light which would come 

 from the portion a 2 a 3 of the large wave may be considered to 



P 



Fig. 56. 



consist of two portions, which are nearly equal but in opposite 

 phases ; the line a. 2 6 2 , however, intercepts one portion com- 

 pletely, or incompletely, according to the relative size of the 

 opaque and transparent portions. A bright band will conse- 

 quently be seen at P a 3 , and this will be the first band. At 

 another position a 6 a- t on the grating, the line p 6 will differ 

 from P a- by two whole wave-lengths ; then the light which 



M 2 



