December 8, 192 1] 



NATURE 



467 



its secretary. For so wide a theme we shall need 

 the physicist and chemist and biochemist and physio- 

 logist and tuberculosis expert and sanitarian, at least. 



In the upshot it will appear, I believe, that "the 

 things men live by " (and for lack of which they die), 

 even in our complex and marvellous cities, are very 

 simple. The primary laws remain. As Bacon said, 

 • Nature can be commanded only by obeying her." 

 For the body we need little more than light and air 

 and water and fresh food — paH de joie gras^ and can 

 de Cologne and champagne are superfluities ; and 

 for the soul, as Dr. Cabot, of Boston, says, we need 

 no more (nor less) than work and play and love and 

 worship. C. W. S.^leeby. 



Royal Institution, November 26. 



Relativity and Materialism. 



My reply to my friend Mr. Hugh Elliot is quite 

 plainly that, so far as I can discover from his letter 

 in N.-^TLRE of December i, and from his book, and 

 from an article I have read of his, and from an essay 

 which I believe nearly won the five-thousand-dollar 

 prize, he has not understood the principle of relativity. 

 He discusses with very great ability and lucidity the 

 negative results of the experiments and all the illus- 

 trations of conflicting experience in relatively moving 

 systems of reference, and, for aught I know, he may 

 be quite familiar with the differential equations which 

 the relativist mathematicians use, but all that he does 

 is to offer a plausible explanation of the phenomena 

 on his materialistic hypothesis. That is not the prin- 

 ciple of relativity, yet, strangely enough, he seems 

 quite convinced that it is. The principle of relativity 

 is essentially the construction of the universe from 

 pure concrete experience without any causal theory of 

 experience whatever. This is the very antithesis of 

 materialism. To affirm the contrary is like saying 

 that Berkeley is a materialist. It is simply evidence 

 that words are being used without knowing their- 

 meaning. 



But let me bring the matter to the test. The uni- 

 verse, according to Minkowski, Einstein, and the other 

 relativists, is a four-dimensional continuum. In this 

 universe there is no simultaneity. This does not 

 mean that we have to calculate the simultaneity of 

 events on a new principle ; it means that simul- 

 taneity in the accepted meaning has lost all signi- 

 ficance, and, in fact, represents nothing ; no two 

 events are simultaneous in any absolute or universal 

 meaning. Also in this universe there is no universal 

 system of geometry, nothing which even takes the 

 place of the Euclidean geometry of the Newtonian 

 absolute space. Every point-event has its own geo- 

 metrical svstem. The whole rationalitv of these con- 

 cepts lies in the conception of a scientific reality con- 

 stituted whollv of concrete experience. Hence event' 

 point-instant in this universe the track of which forms 

 a world-line is taken primarily from its own point of 

 view, according to which it is central and its direction 

 straight. 



How can anyone accept this basis of scientific reality 

 and be a materialist? Materialism I? a metaphysical 

 theory which may be right, and relativity Is an anfi- 

 metaphysical theorv which may be wrong, but accept- 

 ance of the one is the rejection of the other. 



H. WiLDON Carr. 



King's College, London, December 2. 



Prof. Tyxdall, In his " Scientific Use of the 

 Imagination," allowed a fair play-room to this facultv 

 in scientific research. Dr. Norman Campbell would 



NO. 2719, VOL. 108] 



seemingly restrict Its use to a sphere in whictj 

 phenomena could be submitted to the check of ex- 

 periment. In his letter In Nature of November 24, 

 and In his contribution to the Einstein controversy In 

 the issue of February 17 of the present year, he 

 demurs to the use of arguments based on anything 

 that cannot possibly happen. 



Dr. Campbell would thus rule out, as scientifically 

 Invalid, Prof. Eddington's conception of a perceptive 

 being, travelling at the velocity of light, as having 

 no knowledge of time, and as living in a perpetual 

 present. Such a being and such a condition of things 

 could not be subjected to experiment, and therefore a 

 conclusion drawn from them would be futile. 



Einstein's argument based on an imaginary "lift" 

 hanging in space far removed from matter, in which 

 an observer draws deductions, from his experience, as 

 to his gravitational field, would also come under Dr. 

 Campbell's censure, as being outside the region of 

 experiment. 



. Dr. Campbell seems to me^ — but I may be quite 

 wrong — to lay down a new canon of scientific method. 



Edmund McClure. 



80 Eccleston Square, S.W.i, November 28. 



The Radiant Spectrum. 



Prof. Raman has recently directed attention 

 (Nature, September i, p. 12) to some observa- 

 tions by Brewster on what the latter called the 

 "radiant spectrum " (Phil. Mag., vol. 2, p. 202, 1867). 

 Brewster advanced the hypothesis that the phenomenon 

 was due to the granular surfaces of the eye rendering 

 the ultra-violet rays visible by fluorescence. Prof. 

 Raman proposes the alternative hypothesis that diffrac- 

 tion by the corneal corpuscles of the eye accourits for 

 the phenomenon. Brewster's view is at variance with 

 the fact that when a colour screen, opaque to ultra- 

 violet rays. Is placed in the optical path between the 

 source, the prism, and the eye, it does not render 

 the "radiant spectrum" appreciably less visible. 

 Fluorescence set up by ultra-violet rays can therefore 

 be safely excluded as a possible cause of the pheno- 

 menon. Prof. Raman's view that diffraction effects 

 by the corneal corpuscles of the eye are responsible Is 

 at variance with the following facts : — 



(i) That if the head of the observer be rotated, so 

 as to rotate the eye about its optical axis, and there- 

 fore cause the corneal corpuscles to take up new 

 meridians, then little or no change, such as might be 

 expected, Is seen to occur In the radiant spectrum. 



(2) That by placing a suitable screen between the 

 prism and the eye, it Is possible to exclude the 

 ordinary direct spectrum of the light source and yet 

 still to observe the so-called "radiant spectrum." If 

 the effect was produced by the eye, such a cutting off 

 of the direct spectrum should also have the effect of 

 destroying any diffraction effects produced, since all 

 light would thus be prevented from reaching the eye. 



(3) The "radiant spectrum " can be seen on the 

 ground glass of a photographic camera, and can pre- 

 sumably, therefore, be photographed. Since no cor- 

 neal corpuscles are present In a glass lens the 

 "radiant spectrum" should not, on Raman's hypo- 

 thesis, be visible In this case. 



The following hypothesis fits in with all the above 

 facts, viz. that the " radiant spectrum " originates 

 by diffraction principally at the prism surfaces them- 

 selves. Four observations are in favour of this 

 view : — 



(i) That a prism with ven,^ perfectly polished sides 

 gives a very weak radiant spectrum. 



