270 On the Longitudinal Component in Liglit. 



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whirls whose length is —j- and breadth ~~ 7~^ — ™ • The length 

 is the same as the width of the lines of the grating, and the 



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breadth somewhat greater than the length of a wave = — . It 



o & 9 



is especially obvious in this case that some longitudinal com- 

 ponent exists. 



The existence of the terms depending on e~^m' 2l '-i^ 

 shows that there may be something analogous to total re- 

 flexion with its extinction wave in the case of a grating in 

 respect of the spectra that are of a higher order than can be 

 transmitted by the grating. It would seem, then, that the 

 whole energy of the wave might not be distributed over the 

 spectra unless the variation of opacity in each line be 

 judiciously made. This may also be connected with the 

 high absorbing and radiating powers of rough surfaces and 

 with the action of coherers. 



It is a matter for consideration whether it would not be 

 worth while manufacturing photographic gratings by causing 

 the two first spectra on each side of the central image, 

 together with this central image, or without it, to interfere on 

 the surface of a sensitive film. We might thereby produce a 

 grating which had such a distribution of opacity as to repro- 

 duce only these first order spectra and have all the light that 

 passed through concentrated in them. Similarly we might 

 manufacture a grating which would have the light concen- 

 trated in any desired pair of spectra, though this would 

 practically come to the same thing as the first proposal, with 

 the lines closer together. This comes to the same thing as 

 producing gratings by means of the interference of two beams 

 of parallel rays of monochromatic light in the manner that 

 Wiener has shown to be possible. 



In all these cises it is quite evident that a longitudinal 

 component of either electric or magnetic force is essential 

 to the existence of waves whose intensity is not constant all 

 over their surface, and that it is a practically universal con- 

 comitant of all waves of noncondensational type. That in the 

 case of short waves which vary slowly from point to point, 

 the intensity of the longitudinal component at any place will 

 be in general very small, because the area is very large over 

 wdiich the motion along the surface at one place has at its dis- 

 posal in which to turn and be continuous with the motion 

 back along the face of the next wave. This does not make it 

 unimportant, however. In a great many cases the total flow 

 along the face of a wave must somewhere flow longitudinally 



