February ig, 1920] 



NATURE 



659 



mechanics, and experienced teachers will agree 

 that "a thorough grounding in this most funda- 

 mental of all the sciences is the beginning of all 

 wisdom in physical knowledge." The treatment 

 of the various subjects follows conventional lines, 

 a short chapter on the discharge of electricity 

 through gases being the only one which deals 

 specifically with the results of modern research. The 

 style is lucid and interesting, and the explanation 

 of physical principles exceptionally clear. It is to 

 be regretted that the price of i6s. net should be 

 so high as to make it impossible for many 

 students to purchase the volume. 



(2) A smaller treatise on the elements of physics 

 has been written by Dr. Houstoun, who has 

 attempted to cover the same ground in less than 

 half the number of pages. The matter is con- 

 sequently somev^'hat compressed and the style 

 curt. The author has been successful in includ- 

 ing a section on simple harmonic motion, which 

 is so important in the study of vibrations ; and 

 another on the characteristic features of wave 

 motion, in which the difference between a 

 stationary and a progressive wave is well brought 

 out. The work should be useful as giving a com- 

 pact systematic treatment of the whole subject. 



Both books are furnished with useful collections 

 of questions and problems, and answers are pro- 

 vided for the numerical exarnples. 



H. S. Allen. 



OUR BOOKSHELF. 



Harmsworth's Universal Encyclopedia. Edited 

 by J. A. Hammerton. No. i. Pp. xix -1-128. 

 (London : The Amalgamated Press, Ltd., 1920.) 

 Price IS. 3d. 



This is, of course, a work of reference for the 

 general reader, no't the expert. The editor 

 claims that it possesses the three necessary 

 qualities, comprehfnsiveness, conciseness, and 

 accuracy. All three are relative terms, and there 

 is no absolute test by which his claim can be 

 judged. But, since Mr. Hammerton is the acknow- 

 ledged authority on What the Public Wants, the 

 first claim may be conceded without further 

 question. On the second the bare statement that 

 .\ — Afranius occupies 128 closely printed but well- 

 illustrated pages will enable the reader to judge 

 for himself. On the third it is sufficient for 

 Nature to record th;.t the scientific articles appear 

 all to be as completely accurate as the space 

 allotted to them will permit, and that one of the 

 introductory articles, by Lord Moulton on "Science 

 and the Future," is a model expression of the 

 obvious ; originality or profundity could not be 

 expected. 



We think the production is one of which the 

 .Amalgamated Press may well be proud. 



NO. 2625, VOL. 104] 



LETTERS TO THE EDITOR. 



[The Editor does not hold hhnself 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.'] 



Relativity and the Olsplacement of Fraunhofer Lines. 



In view of the uncertainly in the interpretation of 

 Einstein's equations in the matter of the displacement 

 of solar spectrum lineSj and of the hope which has 

 been expressed that experimental spectroscopic evi- 

 dence may be forthcoming which will settle the point 

 at issue, it may be of interest to give a brief account 

 of the present state of the problem from the experi- 

 mental point of view. There are really three ques- 

 tions to which answers are required : — (i) Is there 

 any means whereby the displacements of solar lines 

 relative to those of terrestrial origin may be dis- 

 entangled from such disturbing causes as pressure, 

 varying arc conditions, density gradients, etc.? (2) If 

 so, what do the outstanding displacements amount 

 to? (3) To what extent are they due to gravitation 

 and line-of-sight motion respectively? 



L'|)on (i) it is to be remarked that, since both 

 gravit.ition and motion displacements vary directly 

 with the wave-length, they are indistinguishable 

 spectroscopically ; moreover, the possibilitv of separa- 

 ting their sum from those due to other causes, which 

 in general displace spectrum lines (I have enumerated 

 some ten possible causes in a recent communication 

 to the Royal Astronomical Society), depends upon the 

 reputed invariability of the wave-lengths of the 

 cyanogen bands in different parts of the arc, under 

 pressure, under varying current densities, etc. Prac- 

 ticallv all other lines are affected by one or more of 

 these influences, and must be ruled out of account. 

 Further laboratory experiments are necessarv before 

 the cvanogen bands can be regarded as suitable 

 criteria, particularly in view of a recent statement 

 from the Bonn Laborator\- that thev are unsvni- 

 metricali which for well-founded reasons brings them 

 seriously under suspicion for astrophvsical purposes. 

 Even if we assume for the present that the cvanogen 

 bands are satisfactory standards, (2) presents a further 

 difficulty in that the Mount Wilson observers find dis- 

 placements varving from o to about one-third of that 

 predicted bv Einstein ; whereas Evershed and three 

 Continental observers find a displacement of about 

 one-half the reauired amount. It is possible that the 

 discrepancv is due to the observations being made on 

 different dates, since at Kodaikanal, on different occa- 

 sions, measures made at the pole of the sun varied 

 from one-half to the full predicted displacement. 

 .Simultaneous solar observations at Kodaikanal and 

 Mount Wilson are, unfortunately, impossible, and it 

 is a pity that the Australian Solar Observatory is not 

 yet in existence to link up the two. 



If Mr. Evershed and others prove correct, the 

 problem still remains to interpret the half-displace- 

 ment observed; obviously, this can be done onlv :f 

 bv some independent evidence the motions of the 

 vapours are known uoon that portion of the solnr 

 surface towards which the spectroscope is directed. 

 This involves a deeper knowledge of the currents in 

 the solar atmosphere than we at present possess. 

 There has been a disposition to regard displacements 

 at the nolar limb as free from motion effects, but It 

 is inevitable that there will be surface currents, and 

 these need not be of excessive violence in order to 

 rtive disniacements of the order of magnitude of those 

 observed. The problem, if we accept Mr. Evershed's 



