39: 



NATURE 



[May 26, 192 1 



Letters to the Editor. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



The Aurora of May 13-15. 



In pursuance of the programme outlined in my 

 letter to Nature of March 31, p. 137, I have been 

 photographing the spectrum of the night sky every 

 night. On the night of May 13-14, and also of 

 May 14-15, a strong auroral spectrum was obtained, 

 showing the aurora line A5578, and also the negative 

 bands of nitrogen, which were much stronger than 

 the aurora line. Ten heads of these 'bands were shown 

 on the photograph of May 14-15, though only six 

 appear .within the same spectral range in Vegard's 

 investigation made on a special expedition to the 

 Arctic. 



That these nitrogen bands should be actually 

 stronger than the aurora line is very remarkable. On 

 ordinary nights it is generally possible to photograph 

 the aurora line, while the nitrogen bands have never 

 been photographed in the course of about 100 nights 

 except on these two consecutive occasions^ 



It is of interest to note that the aurora apparently 

 ceased before the magnetic storm was over, according 

 to the data given by Dr. Chree in Nature of May 19, 

 p. 35q. On the night of May 15-16 the aurora line 

 was barely, if at all, visible on the plate, and below 

 its average intensity on ordinary nights. Nothing 

 could be seen of the nitrogen bands. 



Rayi.eigh. 



The Gravitational Field of an Electron. 



In the Proceedings of the Royal Society for 

 February last (vol. xcix., A, p. 123) is an interesting 

 paper by Dr. G. B. Jeffery, in which he applies an 

 extension of Einstein's theory to ascertain something 

 about the state of the aether close to an electron — 

 though perhaps he might not express it in that way. 

 He obtains independently, and then discusses, a result 

 recently published by Nordstrom, namely* an exten- 

 sion of the now well-known 7 of Schwarzschild's 

 equation, 



ds'' = y-'dr''+etc.-yc'dt', 



by adding to it a mixed gravitational and electrical 

 term, so that it becomes 



y=i- 



2Gm Ge* 

 7^ K^^' 



wherein, while the old second term involves the 

 gravitational potential, the new third term involves 

 the square of the electrostatic potential. 



I have introduced the dielectric constant of the 

 aether into the denominator so as to keep dimensions 

 right, and if we now choose to make use of the 

 familiar "J. J. T." (1881) expression for the inertia of 

 an electric charge, tn = 2/xe*/3a — which is reasonable, 

 inasmuch as the gravitation constant is dominant 

 over both the second and third terms — we may write 

 the above value of 7 thus : 



, 2Gw/ 3«"\ 



This expression attains a minimum value when 

 r=§a ; and it is unity both at infinity and at 

 r = |a. This last probably means that the electric and 

 mass terms just balance "at the surface of an electron 

 (for we may probably ignore the | factor as unlikely 

 NO. 2691, VOL. 107] 



to be accurate so close to a charge), or else it means- 

 that the gravitational effect at an electron-boundary 

 is reduced to one-quarter of Its normal value. The 

 formula does not apply in the interior of an electron, 

 if an electron has any interior ; though if It is a geo- 

 metrical point, as Dr. JefTery evidently thinks pos- 

 sible, then 7 inay rise to a very high value within 

 what is commonly thought of as the boundary. 



It is not to be thought that the new term is merely 

 the natural consequence of electromagnetic mass, for 

 it is opposed In sign ; the electrical and the mass 

 effects tend to neutralise each other ; but nowhere do 

 they succeed except at or near the boundary of an 

 electron. For all distances large compared with the 

 size of an electron 7 has its customary Einstein value, 

 so the third term has no astronomical significance 

 whatever. 



But a study of what happens to radiation when it 

 impinges on, or penetrates between, the ultimate 

 elements of matter — in fact, a studv of the whole 

 behaviour of a stream of radiation at its two ends, 

 the source and the sink — is obviously of great im- 

 portance. An immense amount of work has been 

 done on the emission end of radiation, but less on 

 the absorption end. By the two together we may 

 ultimately hope to get some information as to the 

 structure of an electron. It may be pardonable to- 

 mention some small papers of my own in this con- 

 nection, in the Philosophical Magazine for April and' 

 probably for June and July this year, though the full 

 working out Is not attempted. 



It must be emphasised that the e% above, means 

 the square of the unbalanced charge only — it is (2*)^ 

 not 2(>') — and that all neutralised charges are pro- 

 vided for by the second term. The conclusion that 

 unbalanced electric potential and neutralised or mass 

 potential oppose each other in their warping effect on 

 the aether appears rather suggestive. I confess I 

 should have expected a third term which did not 

 involve the gravitation constant ; and perhaps there 

 may be a reconsideration on this point, for the present 

 manner of obtaining the expression Is far from easy — 

 in fact, is abstruse. Oliver Lodge. 



The Magnetic Storm of May 13-17. 



The magnetic storm which began soon after i p.m. 

 on May 13 presented several unusual features beyond 

 that of almost unprecedented magnitude. As recorded 

 on the Eskdalemuir rhagnetographs. It showed a 

 "sudden commencement" at i3h. i^m. G.M.T. orb 

 May 13. The phenomenon known by this name 

 usually takes the form of a sharp change In value of 

 the horizontal components of terrestrial force, fre- 

 quently including as its first part a temporary drop 

 In value lasting for about a minute, but always ex- 

 hibiting a rise in value immediately thereafter. In 

 the vertical component the change, when appreciable,, 

 is very much slower and is in the direction of the 

 zenith. In the case of the storm now described the 

 change in the north component was too quick for the 

 photographic paper to receive a visible impression of 

 the light spot. On the west component a drop In 

 value is shown at first, amounting to 297 (000029 

 C.G.S.) and extending over two minutes. There then 

 followed a rise of 947, occupying 2| minutes. On the 

 vertical component the "sudden commencement"" 

 assumed an unusual form. There Is quite clear evi- 

 dence of a preliminary increase in value of the ver- 

 tically downward directed force amounting to 87, fol- 

 lowed immediately by a reverse change of 317, the 

 latter being much more rapid than is generally the 

 case. These preliminary phases having passed, the 

 main features of the disturbance were quickly de- 

 veloped. This Is quite in accord with the results of 



