Discontinuity in the Phenomena of Radiation 53 



state of affairs may be gathered from the next slide*, which 

 presents to you a diagram illustrating by thick and dotted lines 

 the crests and troughs of each wave-train as they would be 

 situated at a definite moment, were the other train absent. 

 Where crest cuts crest, or trough cuts trough you would expect 

 reinforced oscillation, and you will notice that such intersections 

 do actually lie on radial lines ; while where crest cuts trough you 

 Avould expect interference and absence of energy flow, and such 

 intersections, once more, lie on radial lines, which separate the 

 previous set of lines. Furthermore, you will experience no 

 difficulty, I presume, in appreciating the fact that if the frequency 

 of the oscillation of the needles were increased the channels of 

 energy flow would be more numerous and more closely packed ; 

 their angular separation would be less, for with increased fre- 

 quency the common wave length of each train would be decreased, 

 the thick and dotted circles of the second slide would be more 

 numerous, and the lines of intersections, therefore, more numerous 

 as well. Indeed, there is a very simple relation which connects 

 the wave-length of each ripple-train with the separation of the 

 lines of energy flow when the distance separating the needles is 

 known, and by its aid wave-length could be calculated if other 

 and more direct methods were not available. One limiting 

 condition must be observed before we leave this illustration ; the 

 frequency of the oscillation must neither be so great nor so small 

 as to make the wave-length too small a fraction or too great a 

 multiple of the distance apart of the needles. In the first case 

 the channels of flow would be so numerous and closely packed as 

 practically to obliterate the intervening quiescent avenues, while 

 in the second they might not exist at all. 



In applying this method to the problem of light radiation, 

 we naturally ask are there sources of light which send out 

 ethereal vibrations of such a simple character as that possessed 

 by our ripples, having the necessary uniformity of wave-length 



* Fig. 2, 



