August 25, 1923] 



NATURE 



279 



Letters to the Editor. 



The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, nor io correspond with 

 the writers of rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.^ 



Light and Electrons. 



Sir Oliver Lodge, in his survey of the problems 

 connected with Ether and Electrons (Nature 

 Supplement, August 4), propounds the interesting 

 question : " Does light generate an electron ? " 

 The hypothetical conversion of radiation into matter 

 may, as he points out, accord with observed results 

 as to the photo-electric emission of electrons. In 

 particular the striking reciprocal relation between 

 the energy of an electron and the energy of X-rays 

 seems to justify his statement : " It is as if the 

 same beta particle, that is, the same electron, had 

 gone out of existence at one place, and been recreated 

 at another, the intermediate link being constituted 

 by specific radiation of a perfectly definite wave- 

 length." Sir Oliver Lodge says further : "I know 

 that the Bohr Theory of the Atom seems at first 

 against these speculations. Electrons do appear to 

 jump from one oirbit to another, and thereby give 

 out a certain quantum of energy. But this may be 

 a supplementary, and not a contradictory statement." 



In this connexion I should like to direct attention 

 to a suggestion made by Prof. E. T. Whittaker in his 

 paper on the quantum mechanism in the atom (Proc. 

 Roy. Soc. Edin. vol. 42, p. 141, 1922). He points out 

 that Bohr's theory of series spectra can be assimilated 

 to the theory advanced in his paper in the following 

 way. " In Bohr's theory let a negative electron E 

 fall from an orbit of radius a^ (position P^) to an orbit 

 of radius Uf, (position Pq). Now in the initial state 

 of this system, which consists of the electron E at P^, 

 let us introduce two coincident electrons E' and E" 

 at Po, one positive and one negative, so that they 

 annul each other ; and let us replace Bohr's con- 

 ception of the fall of the electron from E at Pj to 

 E' at Po, by the conception of the discharge of a 

 condenser whose charges are E and E" ; the discharge 

 annihilates E and E", and so leaves E' surviving 

 alone at the end of the process, and is therefore 

 equivalent to Bohr's notion of a translation of E to 

 the position of E'." 



The suggestion is easier to visualise if instead of 

 the circling electrons of Bohr's theory we employ the 

 stationary electrons obtained by introducing Lang- 

 muir's " Quantum Force " (Phys. Rev., vol. 18, 

 p. 104, 1921). The conception of the discharge of a 

 condenser is not essential to the picture, and Sir 

 Oliver Lodge may prefer to replace it by a mechanical 

 vibration of the column of ether between E and E", 

 resulting in the production of what Silberstein has 

 called a " light-dart." In speaking of the discharge 

 of a condenser, as in speaking of the vibration of a 

 medium, we are using figurative language, which 

 is meant only to suggest an illustration of a process 

 which is beyond the range of our experience. 



One of the difficulties in Bohr's theory is to under- 

 stand how the frequency of the radiation emitted in 

 accordance with his fundamental frequency condition 

 can be fixed as soon as the electron quits the first 

 stationary state and before it has reached the final 

 state. As Silberstein puts it : " Needless to say the 

 founder of the new theory and his followers do not 

 attempt to describe the mechanism of such an extra- 

 ordinary performance, one, that is, that enables the 



atomic system to hit precisely upon the frequency 

 required." Again in a recent letter Prof. C. G, 

 Darwin (Nature, vol. iii, p. 771, June 9) refers to 

 the difficulty " that the quantum conditions deter- 

 mining the permissible Bohr orbits can only be 

 explained physically by attributing to the electrons 

 a knowledge of the future." 



This difficulty — and the similar one which arises in 

 connexion with absorption — seems to be diminished, 

 if not entirely removed, by the suggestions put for- 

 ward by Prof. Whittaker. On this view the emission 

 of light originates not so much at the position P^ as 

 at the position Pq, where we may imagine an incipient 

 crack in the ether developing under the influence of 

 some external disturbance, say the approach of some 

 other atomic system. There is here a suggestion of 

 a discrete structure for the electromagnetic field (or 

 ether) in the space surrounding an atom such as I 

 have previously attempted to indicate in speaking 

 of Faraday's magnetic lines as " Quanta." 



In the present stage of the development of physics, 

 when we seem forced to believe in two mutually 

 contradictory theories of light (the undulatory and 

 the corpuscular theory) at the same time, the wildest 

 guess at a solution may be permitted. This must be 

 my excuse for hazarding the suggestion that con- 

 ceivably the " head " of the disturbance (derived 

 from the negative electron E) spreads out as the light 

 advances — the amount of spreading involved being 

 a question requiring further investigation — whilst the 

 " tail " (derived from a positive electron) retains to 

 a greater extent its corpuscular character, and plays 

 the part of one of Sir J. J. Thomson's " specks " as 

 it follows the advancing wave-front. On this view 

 absorption of radiation takes place when an electron 

 grasps the light — in this revised version of Little 

 Bo-Peep — by its " tail " ! 



H. S. Allen. 



The University, St. Andrews. 



NO. 2808, VOL. I 12] 



Continental Drift and the Stressing of Africa. 



As one among many geologists who (so it would 

 seem) would welcome proof of an hypothesis of 

 continental drift, but who cannot accept Dr. Wegener's 

 peculiar opinions with regard to it, I recognise that 

 we owe a debt of gratitude to Dr. J. W. Evans for 

 showing us an ingenious way out of some of the 

 difficulties that Wegener, albeit unintentionally, 

 demonstrates rather than removes ; none the less, the 

 views of Dr. Evans on this subject appear to be 

 open to question. 



Dr. Evans states (Nature, March 24, p. 393) that 

 " there seems reason to believe that Africa is in the 

 main the centre of a region of tension, due to the 

 outward drift of continental masses," which, as he 

 points out, is explicable as " drift from a region of 

 comparatively low gravity to one of higher gravity." 

 Following Osmond Fisher and Pickering, Dr. Evans 

 sees no objection to the view that the Pacific depression 

 is the scar left by the separation of the moon from 

 the earth — a phenomenon which Sir George Darwin 

 attributed to tidal action — and is inclined to follow 

 Prof. Sollas in regarding the African protuberance 

 as an unsuccessful attempt on the part of the earth 

 to produce another satellite. 



The birth of the moon is a piece of extremely 

 ancient history, and the consequent stressing of 

 Africa, if indeed there be any such consequence, 

 must have started as soon as the moon's mass was 

 lost, or, in the event of excessive resistance of sima 

 to sial — an unlikely event if the postulated circum- 

 stances of the moon's origin be correct — as soon as 



