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wave- train. In this direction therefore there will also be an 

 image of the slit source ; and this direction is determined by the 

 relation 



sm (y=— - 



where / is the length of the light-wave of this particular color; 

 s, the distance between the apertures (the grating space) ; and 

 7n, the number of waves in the common retardation (i, 2, 3, 

 etc.) But even if the light thus diffracted be absolutely homo- 

 geneous (that is, consist of an infinite wave-train of constant 

 v/ave-length) it does not follow that the light is diffracted in the 

 given direction; there will be some light in directions dififering 

 slightly from this — growing less until the extreme diiference 

 of path is, say, n-l-1 waves (instead of n), when it is nil. 



In fact, if we divide the pencil having this new direction into 

 two equal parts, AC and CP>, the ray A A, will be n-f >4 waves 

 in advance of CC, and the two will be in opposite phases of 

 vibration and will therefore neutralize each other. The same 

 will be true of each pair of rays taken in the same manner over 

 the whole grating space, and the result is total darkness for this 

 direction. Let us suppose we are examining the double sodium 

 line. The difference between the components is about one-thous- 

 andth of the wave length. With a grating of n lines there will 



