Discontinuity m the Phenomena of Radiation 61 



Let us return to the analogy of the water surface. Suppose 

 our pool or tank of water has not merely one or two needles 

 touching it, but millions. Conceive each one to be oscillating up 

 and down as before, with all varieties of frequency and form of 

 vibration, The surface of the water will present at any instant 

 a form which must be the synthesis of all the individual move- 

 ments which would exist alone were all needles but one at rest. 

 In the same sense as we say that there are red, green and blue in 

 the white sunlight, so we can say that there are in the complex 

 motion of the water surface all the individual wave motions 

 emitted by each needle, and a natural question presents itself — 

 can we from the actual wave surface determine the wave length 

 and intensity of each individual train"? Well that problem 

 certainly meets too many mathematical obstacles to be soluble in 

 any but a few limited cases. But another consideration equally 

 interesting crops up. Supposing we have some knowledge of the 

 movement of the needles, not necessarily a detailed one, but 

 something about their average behaviour, should we not then be 

 able to infer something about the resultant movement of the 

 water surface, again not detailed knowledge necessarily, but 

 behaviour on the average 1 Now turn from this two-dimensional 

 picture : think of the three-dimensional ether aud the analogues 

 of our needle-points, viz., the molecules, atoms and electrons of 

 matter, all in excessively rapid movement in it and all exciting 

 ethereal radiations whose frequencies and intensities are depend- 

 ent on these movements. To narrow down our problem to quite 

 manageable dimensions, let us abstract ourselves from open space 

 where we meet with bodies with all varieties of temperature. 

 This is a perfectly legitimate procedure in investigating the 

 elements of a problem. Let us think of this room — with our- 

 selves removed from it, as causing too much disturbance — having 

 walls, ceiling, floor, etc., all at one temperature, and quite imper- 

 vious to all radiation from external sources ; whether we leave 

 the air in it or not does not matter. The enclosed ether is 

 excited by radiations proceeding in all directions, emitted by the 



