S3 



Fig. 2. 



Let Fig. 2 represent a highly magnified cross-section of a dif- 

 fraction grating with plane waves of light falling upon it nor- 

 mally, as indicated by the arrows. The wave motion will pass 

 through the apertures, and will continue as a series of plane 

 waves ; and if brought to a focus by a telescope will produce an 

 image of the slit source just as if no grating were present (save 

 that it is fainter^ and some of the light is cut off by the opaque 

 portions). This image may be considered as produced by the 

 concurrence of all the elementary waves from the separate aper- 

 tures meeting in the same phase of vibration, thus re-inforcing 

 each other. But this may also be true in an oblique direction, 

 as shown in the figure, if the retardation of the successive waves 

 is just one whole wave length (or any whole number), as is il- 

 lustrated in Fig. 3, 



Fig. 3- 



where the successive waves from apertures i, 2, 3, are shown 

 to re-inforce each other just as if they all belonged to a single 



