May 30, 19 1 8] 



NATURE 



255 



Now the fact that the energy of emission is the same, 

 whether the body from which it is emitted is 

 held within an inch of the source, whore the Hght is 

 very intense, or a mile away, where it is very weak, 

 would seem to indicate that the light simply pulls a 

 trigger in the atom, v.hich itself furnishes all tne energy 

 with which the electron escapes, as was originally 

 suggested by Lenard in 1902 {Ann. d. Phys. [4J, vol. 

 viii. [1902J, p. 149), or else, if the light fiirnishes the 

 energy, that light itself must consist of bundles of 

 energy which keep together as they travel through 

 space, as suggested in the Thomson-Einstein theory. 



Yet the fact that the energy of emission is directly 

 proportional to the frequency v of the incident light 

 ^oils Lenard 's form of trigger theory, since, if the 

 atom furnishes the energy, it ought to make no dilTer- 

 ence what kind of wave-length pulls the trigger, while 

 it ought to make a difference what kind of gun— that 

 is, what kind of atom— is shot off. But both 

 these expectations are the exact opposite of the observed 

 facts. The energy of the escaping corpuscle must 

 come then, in some way or other, from the incident 

 light. 



When, however, we attempt to compute on the basis 

 of a spreading-wave theory how much energy a cor- 

 puscle can receive from a given source of light, we find 

 it diflficult to find anything more than a very minute 

 fraction of the amount which the corpuscle actually 

 acquires. 



Thus, the total luminous energy falling per second 

 from a standard candle on a square centimetre at a 

 distance of 3 m. is i erg.^ Hence the amount falling 

 per second on a body of the size of an atom, i.e. of 

 cross-section 10-** cm., is lo"'® ergs, but the energy /iv 

 with which a corpuscle is ejected .by light of wave- 

 length 500 fJLfi. (millionths millimetre) is 4X10-'^ ergs, 

 or 4000 times as much. Since not a third of the inci- 

 dent energy is in wave-lengths shorter than 500 fjLfx, a 

 surface of sodium or lithium which is sensitive up to 

 500 .fi/x should require, even if all this energy were in 

 one wave-length — which it is not — at least 12,000 

 seconds, or four hours, of illumination by a candle 3 m. 

 away before any of its atoms could have received, all 

 told, enough energy to discharge a corpuscle. Yet the 

 corpuscle is observed to shoot out the instant the light 

 is turned on. It is true that Lord Rayleigh has re- 

 cently shown {Phil. Mag., vol. xxxii. [1916], p. 188) 

 that an atom may conceivably absorb wave-energy 

 from a region of the order of magnitude of the square 

 of a wave-length of the incident light rather than of 

 the order of its own cross-section. This in no way 

 weakens, however, the cogency of the type of argument 

 just presented, for it is only necessary to apply the same 

 sort of analysis to the case of 7 rays, the wave-length 

 of which is of the order of magnitude of an atomic 

 diameter (10- * cm.), and the difficulty is found still 

 more pronounced. Thus Rutherford® estimates that 

 the total 7-ray energy radiated f>er second by one 

 gram of radium cannot possibly be more than 47 x 10* 

 ergs. Hence at a distance of 100 m., where the y rays 

 from a gram of radium would be easily detectable, the total 

 7-ray energy falling per second on a square millimetre 

 of surface, the area of which is ten-thousand billion 

 times greater than that either of an atom or of a disc 

 the radius of which is a wave-length, would be 

 47 X 10* -^4n- X To^'' = 4 X 10-^ ergs. This is very close to 

 the energy with which ^ rays are actually observed^ to 

 be ejected by these 7 rays, the velocity of ejection 

 being about nine-tenths that of light. Although, then, 

 it should take ten thousand billion seconds for the atom 

 to gather in this much energy from the 7 rays, on 

 the basis of classical theorv the /3 ray is observed to be 



* Dfude, " IxjHrbiidi (l»r Optik " (1006), p. 472. 



* "Radioactive Substances and their Radiations," p. 288. 



NO. 2535, VOL. lOl] 



ejected with this energy as soon as the radium is put 

 in place. This shows that if we are going to abandon 

 the Thomson-Einstein hypothesis of localised energy, 

 which is, of course, competent to satisfy these energy 

 relations, there is no alternative but to assume that at 

 some previous time the corpuscle had absorbed and 

 stored up from light of this or other wave-length 

 enough energy so that it needed only a minute addition 

 at the time of the experiment to be able to be ejected 

 from the atom with the energy hv. 



Now the corpuscle which is thus ejected by the 

 light cannot possibly be one of the free corpuscles of 

 the metal, for such a corpuscle, when set in motion 

 within a metal, constitutes an electric current, and we 

 know that such a current at once dissipates its energy 

 into heat. In other words, a free corpuscle can have 

 no mechanism for storing up energy and then jerking 

 itself up "by its boot straps" until it has the huge 

 speed of emission observed. 



The ejected corpuscle must then have come from the 

 inside of the atom, in which case it is necessary to 

 assume, if the Thomson-Einstein theory is rejected, 

 that within the atom there exists some mechanism 

 which will permit a corpuscle continually to absorb 

 and load itself up with energy of a given frequency 

 until a value at least as large as ^v is reached. What 

 sort of a mechanism this is we have at present no idea. 

 Further, if the absorption is due to resonance — and we 

 have as yet no other way in which to conceive it — it is 

 difficult to see how there can be, in the atoms of a 

 solid body, corpuscles having all kinds of natural fre- 

 quencies so that some are always found to absorb and 

 ultimately to be rejected by impressed light of any par- 

 ticular frequency. But apart from these difficulties, the 

 thing itself is impossible if these absorbing corpuscles, 

 when not exposed to radiation, are emitting^ any energy 

 at all ; for if they did so, they would in time lose all 

 their store, and we should be able, by keeping bodies in 

 the dark, to put them into a condition in which they 

 should show no. emission of corpuscles whatever until 

 after hours, or years, of illumination with a given 

 wave-length. Since this is contrary to experiment, we 

 are forced, even when we discard the Thomson-Einstein 

 theory of loralis^^d energy, to postulate electronic ab- 

 sorbers which, during the process of absorbing, do not 

 radiate at all until the absorbed energy has reached 

 a certain critical value when explosive emission occurs. 



However, then, we may interpret the phenomenon of 

 the emission of corpuscles under the influence of aether 

 waves, whether upon the basis of the Thomson- 

 Einstein assumption of bundles of localised energy 

 travelling' throug-h the aether, or upon the basis of a 

 peculiar property of the inside of an atom which 

 enables it to absorb continuously incident energy and 

 emit only explosively, the observed characteristics of 

 the effect seem to furnish proof that the emission of 

 energy hy an atom is a discontinuous or explosive pro- 

 cess. This was the fundamental assumption of 

 Planck's so-called quantum theory of radiation. The 

 Thomson-Einstein theory makes both the absorption 

 and the emission sudden or explosive, while the loading' 

 theory, first sugfgested bv Planck, thoue^h from another 

 viewpoint, makes the absorption continuous and only 

 the emission explosive. 



The h determined above with not more than one- 

 half of I per cent, ot uncertainty is the explosive con- 

 stant, i.e. it is the unchanging: ratio between the 

 energy of emission and the freqiiency of the incident 

 lierht. It is a constant the existence of which was first 

 discovered by Planck by an analysis of the facts of black- 

 body radiation, though the physical assumptions under- 

 lying Planck's analysis do not seem to be tenable any 

 lon£?er. For the American physicists Duane and Hunt 

 (Phys. Rev., vol. vi. [1915], p. 166) and Hull (ihid.. 



