Discontinuity in the Phenomena of Radiation 51 



one important distinguishing factor is wave-length, or its equiva- 

 lent, frequency. Another feature is the amplitude of the 

 oscillation of the wave particles, i.e., the extent of movement 

 between the highest and lowest point. This diminishes, no 

 doubt, as we recede from the needle, but at any one defined spot 

 it depends on the amplitude of movement of the needle, and may 

 be increased or decreased by variation of the extent of the 

 needle's oscillation. So we naturally discriminate between two 

 ripple-trains by the details — wave length (or frequency) and 

 amplitude. The water is, morever, the receptacle of energy which 

 is being imparted to it by the vibrating mechanism of the needle, 

 and which we readily apprehend as existing in one of the two 

 most familiar forms, kinetic energy or potential energy. At 

 places where the water is momentarily at its undisturbed level, 

 the energy is entirely that of motion ; for there each ])article is 

 passing through its mean position, upward or downward, with its 

 greatest speed. At the trough or crest the energy of each 

 particle is for the instant all potential, since in the foimer case 

 the pressure of the surrounding elevated water gises an excess 

 of upward force which will presently impart kinetic energy to the 

 drop against downward-acting gravity, while at the crest the 

 conditions are reversed, but with the same result — possession of 

 potential energy. At other phases of motion the energy is partly 

 kinetic and partly potential. But whatever the form it takes, 

 the existence of an energy in the water, which has been 

 " emitted " by the mechanism controlling the needle, is obvious. 



I have chosen this very simple and commonplace example of 

 wave-motion, because its very simplicity exhibits all the essentials 

 for a general grasp of radiation problems ; it is, in fact, a case of 

 actual radiation in the wide sense in which that word is used 

 to-day. Allow me to make a little further use of it, in order to 

 show you how physicists measure the wave-lengths of those 

 ethereal vibrations which possess the simple qualities which we 

 have attributed to the ripples. In the case of the ripples them- 

 selves the measurement of the wave-length would appear to be a 



