446 



NA TURE 



[September 30, 1922 



Letters to the Editor. 



The Editor does not hold liimselj responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 • iters of, rejected manuscripts intended for 

 this or any other part of NATURE. No notice is 

 taken of anonymous communications.] 



Relativity and the Mther. 



While at Brighton recently I visited its excellent 

 Public Library, and there found the three supple- 

 lentarj volumes of the " Knew Brit." containing the 

 remarkably lucid article on relativity by Dr. Jeans. 

 .Many have written and multitudes have read about 

 this subject, but I venture to say that nowhere can 

 be found a more compact and accurate presentation 

 of the strict relativist position. There is nothing 

 anaemic or half-hearted about it ; and I imagine that 

 while non-physicists will find the article readable, 

 most physicists will find its crisp clearness instructive. 

 Very little can a pragmatic or anti - philosophic 

 relativist like myself find to disagree with in it, and 

 where I do disagree it is just possible that both or 

 neither may be right. 



Still I do want to quote and criticise one clear 

 and definite sentence ; into which I insert only 

 references to my comments which follow, so as not 

 to spoil it. 



On the electromagnetic theory of Faraday and 

 Maxwell, waves of light " were simply waves in the 

 aether and travelled with an absolute velocity c 

 determined once and for all by the structure of the 

 aether, (i) On this view it was quite certain (2) that 

 an observer moving through the aether with a velocity 

 u would measure the velocity of light travelling in 

 the same direction as himself as c-u. Relativity 

 teai lies (3) that this velocity is always precisely c, 

 and this in itself disposes (4) of the aether of Faraday 

 and Maxwell." 



On this pronouncement I comment thus : — 



(1) That is certainly true. 



(2) Not quite certain. It might have been thought 

 certain ; but the FitzGerald contraction is an indica- 

 tion of unexpected possibilities in the instruments of 

 measure. Still, no first order loophole has been 

 suggested, and the challenge is a legitimate one. 

 Would that the experiment could be tried ! I 

 assume that every one will agree that it has not yet 

 been tried, and that it is difficult to devise a sure 

 and certain way. 



(3) It certainly and very forcibly does so teach. 



(4) But a hypothesis, even the foundation hypo- 

 thesis of a developed theory, cannot logically be cited 

 as if it were an experimentally ascertained and con- 

 clusive fact ; nor can it be used to give a knock-out 

 blow to another theory. Opposing theories still 

 have to fight it out. Full-blown Relativity might 

 equally well be said to dispose of Matter, by reducing 

 it to the unevenness of a field, or to the tensor 

 - SvrT M ,., or to an algebraic expression like G^,.- |g7u.G, 

 which apparently is conserved like matter, is obedient 

 to our laws of motion, and vanishes in what we call 

 empty spai e 



I do not think that even Dr. Jeans will claim that 

 there is any ascertained fact to substantiate what 1 

 have marked as (3). It all depends upon ux/c 2 and 

 differing estimates of time. 



It may be said that conclusions of relativity have 

 been verified, and that thus the theory is established. 

 1 should prefer to say that some mathematical 

 deductions arrived at by the relativity method have 

 been brilliantly verified, and hence that the method 

 has pragmatically been proved to be sound. I should 



NO. 2761, VOL. I 10] 



not say that its philosophic foundations were es- 

 tablished ; still less that they have rendered all other 

 foundations rotten. 



It would seem rather that more than one mode of 

 expression is possible for even the simplest fact, and 

 that a criterion of absolute and exclusive truth in 

 any statement is increasingly difficult to find. A 

 theory which renders it uncertain whether the Fire 

 of London preceded or succeeded the outburst of 

 Nova Persei, whether the sun revolves round the 

 earth or vice versa, and whether a much-travelled 

 man's death preceded his birth, should not be too 

 positive when it leaves its own realm and enters the 

 region of fact and reality, whatever those possibly 

 question-begging terms may mean. Perhaps there 

 is no absolute truth. More probably absolute truth 

 exists, but is not easy to arrive at. 



Meanwhile we may be grateful that, thanks to the 

 new school, we are beginning to recognise the un- 

 certainty and contingency inseparable from all forms 

 of human statement. Let our geniuses not extinguish 

 but supplement each other. There is room not only 

 for Einstein and Weyl but also for Newton and 

 Maxwell. The reconciliation may not be obvious, 

 the connecting passage is difficult to find, but it 

 would be wise to keep the door ajar. 



Oliver Lodge. 



The Legal Equivalent of the Metre. 



May I correct in Nature an unfortunate error 

 which occurs on p. 580 of the " Dictionary of Applied 

 Physics " ? It is there stated that the equivalent of 

 the metre in inches is 39-37008, and the Order in 

 Council of May 19, 1898, is referred to as giving the 

 legalised value of this quantity. The information 

 was taken, by permission, from the " Computer's 

 Handbook," issued by the Meteorological Office in 

 1 92 1, and the inference is that the above figure is the 

 legalised value. 



Dr. Stratton, of the United States Bureau of 

 Standards, recently directed my attention to the 

 error. The legalised value given in the Order of 

 Council is 39-370113 inches. 



The " Computer's Handbook " states that : " The 

 most recent values for the metrical equivalent of the 

 fundamental British Units are those contained in the 

 Order of Council of 19th May 1898," and, after a 

 reference to earlier editions of the " Handbook," 

 continues : " Values in accordance with the Order in 

 Council of 1898 have now been substituted." 



I assumed this to mean that the actual legalised 

 values had been used and printed the figures without 

 further verification, but this is not the case. 



The figure given — 39-37008 — is deduced from the 

 relation 1 inch = 25-4 millimetres ; the legalised value 

 of the inch in millimetres is 25-399978. The differ- 

 ence, less than one part in a million, is negligible for 

 nearly all purposes and there would be many advan- 

 tages in accepting, as the legalised ratio, 1 inch =25-4 

 millimetres, but this has not been done. 



At present some confusion may easily arise ; for in 

 America the ratio 1 metre = 39-370000 inches has been 

 adopted. 



We thus have the equivalents given in the following 

 table : 



