soMi: coxi r.Mi'ou.ih') .ii'i'.i.Xiis i\ riivsics rii 323 



the total energy radiated shall be hv exactly. ArrordiiiK to tiic wave- 

 theory, it einerKes as a spherical wave-train, of which the wave- 

 fronts are a series of expanilinj; spheres, widening in all liirections 

 away from the atom at their common centre. Place another atom 

 of the same kind some little tiistance away. Apparently it can 

 absorb no radiant energy at all, unless it absorbs the whole anioiiiu 

 Ity radiated from the first atom. But how can it do this, seeing that 

 only a very small portion of each wavefront touched it f)r came any- 

 where near it, and much of the radiant energy went ofl from the first 

 atonj in a diametrically opposite direction? How can il reach and 

 suck up all the energy from the entire wavefront, so little of which it 

 actually intercepts? And the difficulty with the momentimi is (•\cn 

 greater. 



But, of course, this experiment is unreaii/able. In any laboralor\- 

 experinient, there are always great multitudes of radiating atoms 

 close together, and the atoms exposed to the radiation are bathed in 

 myriads of wave-trains proceeding from myriads of sources. Does 

 then the atom which absorbs the amount liv of energy take it in little 

 bits, one from this wavetrain and another from that, until the proper 

 capital is laid up? But if so, it surely would reciuire some appreciable 

 time to gather up the separate amounts. According to the classical 

 electromagnetic theory, a bound electron placed in a wavetrain of 

 wavelength X will gather up energy from an area of each wavefront. 

 of the order of magnitude of the quantity X-. Hence we should not 

 expect that the exposed atom would finish the task of assembling the 

 amount of energy hv from the various wavetrains which pass by it, 

 until the lapse of a time-interval sufificient for so much energy to flow 

 against a circle of the area X-, set up facing the rays at the point 

 where the atom stands. Set up a mercury arc, or better still, an 

 X-ray tube, and measure the intensity of the radiation from it at 

 various distances. You will easily find a position sufficiently near 

 to it for convenience, and yet sufficiently far from it, so that if a 

 circular target of this area were set in that position, the radiant energy 

 falling upon it would not mount up in one minute — nor in one day 

 — nor in one year, to the amount hv. Yet cover the source of rays 

 with a shutter, and then put a piece of matter in that position, and 

 then lift the shutter; and you will not have to wait a year, nor a day, 

 nor a minute, for the first electron which emerges from the matter 

 with a whole quantum of energy; it will come out so quickly that no 

 experimenter has, as yet, demonstrated a delay. What possible 

 assumptions about the structure of the alom can account for this? 



More and more the evidence is piled up to compel us to concede 



