August 20, 1891] 



NATURE 



383 



principle.^ Any discrepancy between the observed and ihe cal 

 culated times of revolution of stars round each other can possibly 

 be explained by a relative motion between us and the pair of 

 bodies along the line of sight. 



If our text-books clearly recognized this, we should not so 

 often find examination candidates asserting that the apparent 

 time of revolution of a satellite of Jupiter depends on the dis- 

 tance of the earth from that planet, instead of on the speed. I 

 should indeed be sorry to be judged by the performance of my 

 own students, but I fear that many of the less obvious mistakes 

 made by reasonably trained examination candidates are more 

 directly traceable to their teachers than some of us as teachers 

 would like to admit. 



The change in the refrangibility of light by reason of the 

 motion of its source, though commonplace enough now, was at 

 first regarded as too sii all to be observed, and one or two at- 

 tempts directed to detecting the effect of this principle on the \ 

 spectra of the stars, or sometimes on sunlight reflected by a 

 45° mirror into the line of the earth's motion (which is not a 

 possible method), wholly failed. I take pleasure in remember- 

 ing that this effect was clearly observed for the first time by the 

 gentleman we this year honour as our President ; and that it is \ 

 by this very means that the latest sensational discovery in astro- i 

 nomy of the rapidly revolving twin star iS-Aurigae, by Prof. Picker- 

 ing and the staff connected with the Draper Memorial, was made. 

 The funds for the investigation that led to this result were 

 provided by Mrs. Draper, as a memorial 1;o her late husband ; 

 and if iS-Aurigas does not constitute a satisfaccory memorial, I 

 am at a loss to conceive the kind of tombstone which the 

 relations of a man of science would prefer. 



The fourth event to which it behoves me to refer is the practical 

 discovery of a physical method for colour photography. When 

 I say practical I do not mean commercial, nor do I know that it 

 will ever become applicable to the ordinary business of the 

 photographer. Whether it does or not, it is a sound achieve- 

 ment by physical means of a result which the chemical means 

 hitherto tried failed, some think necessarily failed, to produce. 

 I say practical, because already it had been suggested as possible 

 theoretically ; and a step toward it, indeed very near it, had 

 been actually made. The first suggestion of the method, so far 

 as I know, was made by Lord Rayleigh in the course of a 

 mathematical paper on the reflection of light, and with reference 

 to some results of Becquerel obtained on a totally different plan. 

 He said in a note that if by normal reflection waves of light 

 were converted into stationary waves, they could shake out silver 

 in strata half a wave length apart, and that such strata would 

 give selective reflection and show iridescence. 



The colour of certain crystals of chlorate of potash, described 

 in a precise manner by Sir George Stokes (Proc. Roy. Soc, 

 February 1885), and also the colours of opal and ancient glass, 

 had been elaborately and completely explained by Lord Rayleigh 

 on this theory of a periodic structure (the laminated structure in the I 

 case of chlorate of potash being caused by twinning) (Phil. Mag., j 

 September 1888, pp. 256 and 241) ; and he subsequently illus- | 

 trated it with sound and a series of muslin disks one behind the I 

 other on a set of lazy-tongs. Each membrane reflected an in- ! 

 appreciable amount, but successive equidistant membranes j 

 reinforced each other's action, and the entire set reflected , 

 distinctly one definite note, of wavelength twice the distance ' 

 between adjacent muslins. So also with any serie-^ of equidistant I 

 strata each very slightly refleciing. They should give selective 

 reflection, and the i-pectrum of their reflected beam should show i 

 a single line or narrow band, corresponding to a wave- length 

 twice the distance of the strata apart.* ! 



' Dr. Huggins has just pointed out to me a perfectly clear statement to ' 

 the above etfect in Professor Tait's little book on Light. 



" The footnote of Lord Rayleigh on page 158, Phil. Mag., 1887, vol. xxiv., j 

 is brief and forcible enough to quote in full ;— " A detailed experimental | 

 examination of the various cases in which a laminated structure leads to a 1 

 powerful but highly selected reflection would be of value. 1 he most frequent ; 

 examples are ii.et with in the organic world. It has occurred to me that j 

 Becquerel's reproduction of the spectrum in natural colours upon silver { 

 plates may perhaps be explicable in this manner. The various parts of the j 

 nlm of subchloride of silver with which the metal is coated may be conceived 

 to be subjected during exposure to stationary luminous waves of nearly I 

 definite wave-length, the effect of which might be to impress upon the i 

 substance a periodic structure occurring at intervals equal to half\.\i^ wave- I 

 length of light: just as a sensitive flame exposed to stationary sonorous 

 waves is influenced at the loops, but not at the n >des {Phil. Mag., March 

 1879, P- I53)- In 'his way the operation of any kind of light would be to pro- 

 duce just such a modificaticn of the film as would cause it to reflect copiously 

 that particular kind qf light. I abstain at present from developing this 

 suggestion, in the hope of soon finding an opportunity of making myself 

 experimentally acquamted with the subject." 



Independently of all this, Herr Otto Wiener, imitating Hertz's 

 experiments with ordinary light, in 1889 reflected a beam 

 directly back on itself, and, by interposing a very thin collodion 

 film at extraordinarily oblique incidence, succeeded in the difficult 

 experiment of so magnifying by the cosine of inclination the half 

 wave-length, as to get the silver deposited in strata of visible 

 width, and thus to photograph the interference nodes themselves 

 at the places where they were cut by the plane of the film 

 {Wiedemann's A nnalen, vol. xl., 1890). 



Then M. Lippmann, using a thicker film, not put obliquely but 

 normal to the light, obtained the strata within the thickness of 

 the film itself — hundreds of layers ; and so, employing incidence 

 light of definite wave-length, was able to produce a stratified 

 deposit, which reflected back at appropriate incidences the same 

 wave-length as produced it ; thus reproducing, of course, the 

 definite colour. 



It is probable that the silver is first shaken out at the ventral 

 segments, but that the strata so formed are thick and blurry. I 

 conjecture that by over-exposure this deposit is nearly all mopped 

 up again, traces being left only at the nodes, where the actioa is 

 very feeble and takes a long time to occur ; but that these residual 

 strata, being fairly sharp and definite, will be likely to give much 

 better effects. And so I suppose that these are what are actually 

 effective in obtaining M. Lippmann's very interesting, though 

 not yet practically useful, result. 



I now leave the retrospect of what has been done, although 

 many other topics might usefully detain us. and I proceed to 

 glance forward at the progress ahead and at the means we have 

 for effectively grappling with our due share of it. 



There is a subject which has long been in my mind, and 

 which I determined to bring forward whenever I had a cathedral 

 opportunity of doing so ; and now, if ever, is a suitable occasion. 

 It is to call attention to the fact that the further progress of 

 physical science in the somewhat haphazard and amateur fashion 

 in which it has been hitherto pursued in this country is becoming 

 increasingly difficult, and that the quantitative portion especially 

 should be undertaken in a permanent and publicly-supported 

 physical laboratory on a large scale. If such an establishment 

 were to weaken the sinews of private enterprise and individual 

 research it should be strenuously opposed ; but, in my opinion, 

 it would have the opposite effect, by relieving the private worker 

 of much which he can only with great difficulty, sacrifice, and 

 expense, undertake. To illustrate more precisely what I mean, 

 it is sufficient to recall the case of astronomy. The amateur as- 

 tronomer has much work lying ready to his hand, aud he grapples 

 with it manfully. To him is left the striking out of new lines 

 and the guerilla warfare of science. Skirmishing and brilliant 

 cavalry evolutions are his natural field, he should not be called upon 

 to take part in the general infantry advance. It is wasting his 

 energies, and he could not do it in the long run well. What, 

 for inst.ince, would have been the state of astronometry — the 

 nautical almanac department of astronomy — without the con- 

 secutive and systematic work of the National Observatory at 

 Greenwich? It may be that some enthusiastic amatturs would 

 have devoted their lives to this routine kind of work, and here 

 at one time and there at another a seriesof accurate observations 

 would have been kept for several years. Punued in that way, 

 however, not only would the effort be spasmodic and temporary, 

 but the energy and enthusiasm of those amateurs would have 

 been diverted from the pioneering more suited to them, and 

 have been cramped in the groove of routine, eminently adapted 

 to a permanent official staff, but not wholesome for an individual. 



Long-continued consecutive observations may be made by a 

 leader of science, as functions may be tabulated by an eminent 

 mathematician ; but if the work can be done almost equally 

 well (some would say belter) by a professional observer or com- 

 putator, how great an economy results. 



Now all this applies equally to physics. The ohm has been 

 determined with 4-figure, perhaps with 5-figure, accuracy ; but 

 think of the list of eminent men to whose severe personal labour 

 we owe this result, and ask if the spoil is worth the cost. Per- 

 haps in this case it is, as a specimen of a well-conducted 

 determination. We must have a few specimens, and our leaders 

 must show us the way to do things. But let us not continue to 

 use them for such purposes much longer. The quest of the fifth 

 or sixth decimal is a very legitimate, and may become a very 

 absorbing, quest, but there are plenty of the rank and file who 

 can undertake it if properly generalled and led : not as isolated 

 individuals, but as workers in a National Laboratory under a com- 

 petent head and a governing committee. By this means work far 



NO. 1 138, VOL. 44] 



