Early History of Fourier Transform Spectroscopy

EARLY HISTORY OF FOURIER TRANSFORM SPECTROSCOPY

PIERRE CONNES Service d’Akonomie du CNRS, B.P. No. 3, 91370 Verrieres-le-Buisson, France

(Received 31 October 1983)

Abstract-This is an attempt to explain how and why the stage was set for the appearance on the scene of Fourier transform spectroscopy (FTS) in the 1950s and not a whit before. The play begins 100 years earlier with Fizeau and Foucault who first produced high path-difference interference phenomena and used them for measuring solar spectrum wavelengths in the near IR. Next, the story unfolds with Michelson’s contribution, which led to important discoveries around 1890: the hyperfine structures and widths of atomic lines. Somewhat less well known is the Rubens interferometric technique, presented in 1910, because no such striking results were ever collected; still, it represented a distinct advance over the Michelson one. What is the reason why Michelson, Rubens and Lord Rayleigh (who made no experiments himself but understood all about them) never managed to get together and propose the modern form of FTS? Part of the responsibility we must ascribe to chance; however, sufficient motivation could not be felt as long as basic noise limitations had not been understood and closely approached.

INTRODUCTION

The Durham Conference has just demonstrated that Fourier transform spectroscopy (FTS) is now one of the most versatile and widely useful tools in the paraphernalia of modern optics. Will it become one of the permanent features of the landscape, just like-let us say-the Cathedral? Such a prediction would be risky of course; but any participant who briefly stole away from one of the less thrilling sessions and trespassed across the Close, surely discovered that all the great surviving medieval cathedrals are first of all things of the mind: destroyed, rebuilt, enlarged or mutilated, plastered all over and then joyfully restored and rediscovered. What truly endures is oft more the spirit than the stones. At the dissipation rate of all things modern, our present interferometers (some of them anyhow) will be museum pieces before the turn of the century-millenium, and likewise most of our analysis or recording techniques. But the basic principles involved might well exhibit far greater staying power, and earn a permanent niche on the shelves of scientific methodology (see Note l).* Where do they come from? All involved in FTS realize that the independent work of Fellgett and Jacquinot about 30 years ago is unquestionably at the origin of the whole development (see Note 2). They also know of the classical results of Michelson, a trace of which is still found in textbooks, but only vaguely. Few are even aware of the work of Rubens in the field, or realize that the modern form of FTS was almost discovered around 1910; if it had been, much of the development of twentieth century spectroscopy would have been different, particularly of course in the IR. Why was it not discovered in actual fact? Was the prime reason insufficient mathematical understanding, immature technology, absence of motivation, or just the lack of the right people (the last being of course no valid historical explanation)? How much of the basic advantage of FTS (as we understand it today) was already seen? How close to the modern devices were the turn of the century interferometers? And what is the essential change of scene that made possible-and practical-the technique that had failed 40 years before? The following brief (and amateurish) historical study will attempt to answer these questions. It will not start with Fourier, whose contribution is of course essential but had originally nothing to do with optics. For the purpose of our demonstration, the story begins by experimental advances around the middle of the last century.

*Notes 1-7 can be found at the end of the article.

69 70 PIERRECONNES

I. HIPPOLYTE FIZEAU (1819-1896) AND LEON FOUCAULT (1819.-1868)

Born within a few days of each other, Fizeau and Foucault at first tackled several fundamental problems of experimental optics together; after they split, in 1849, they went on in competition. and produced equally brilliant results, We are concerned here with a minor part of that work: their contribution to interference spectroscopy. Fizeau and Foucault initiated two distinct lines of research which were much later to converge and make present-day FTS possible: the production of interferences at high path d@erence, and the recording of IR int~~fi~renceJringes, leading to the first measurement of IR wavelengths. Prior to 1845, no interference phenomena seem to have been observed, or at least understood and properly analysed, for path differences greater than a few wavelengths. Then Fizeau and Foucault published together two brief Notes”’ “ Sur le phinomene des interferences entre deux rayons de lumiere dans le case des grandes differences de marche”. The authors introduce a new technique: the observation of channelled spectra (thanks to a much improved prism spectroscope) and its systematic use for the measurement of path differences. The source was the sun, and bright and dark bands were counted in between the Fraunhofer lines. Three different interference devices were used (Fig. 1):

“With Fresnel mirrors, we have observed interference when the path difference for blue rays near line F was 1737 waves. Through reflexion upon both surfaces of a thin plate, interference was noticed when the path difference went up to 3406 waves. With crystalline plates the phenomenon has been followed for remarkably high thick- nesses . . . ‘*

In the second Note, the not-so-thin isotropic plate thickness went up to 1 mm and path difference to “seven or eight thousand times the fundamental interval A”, thanks to the use of up to five prisms in series. (See Note 3.) Fourteen years later, Fizeau (by then no longer paired with Foucault), was to take up again the question of large path differences, but with a different source. His paper,“’ “Note sur la lumiere du sodium brulant dans lair”, was about a popular subject at the time; indeed, he refers to the “many and important results present in the work of M. M. Kirchhoff and Bunsen”. Fizeau is here mostly concerned with his own discovery of an inuerteci pair of D lines from a piece of burning sodium; but he mentions almost incidentally that from an ordinary sodium flame he has obtained:

i‘ . . . the phenomenon of Newton rings, observed with that very simple light, by normal reflexion on the two surfaces of plates that are relatively very thick for such phenomena, since the thickness went up to 10.058 mm, corresponding to a ring of order 52193.”

A detailed account is promised “in my next work”. Thanks to the fat pile of scribbled laboratory notes preserved at the Academic, we can follow Fizeau’s progress: he was building the first variable-path interferometer for wavelength measurement by fringe counting (Fig. 2) surely the ancestor of all present so-called “Fourier interferometers” (a particularly absurd contraction). The results were presented a few months later, somewhat hidden away within a paper on interference measurement of refractive index thermal variation for various solids. lh) The small instrument built by Duboscq is described, together with the now familiar sight of Newton’s rings (here observed as “Fizeau fringes” between a plane and spherical glass surface) expanding or contracting when the separation is changed. Fizeau notices and explains correctly the successive coincidences between the fringes from the two D lines, up to the 52nd period of the phenomenon, i.e. 50,000 fringes and a plate separation of 15 mm. The red line of lithium is also observed. and the wavelength given from the coincidences with the D lines. There is no attempt yet to deduce wavelengths from the known pitch of the screw. However, in,fine, the remarkable stability of fringes seen on a thick glass plate (at constant temperature) is ncticed; it is estimated that with the path difference being about 50,000 fringes and a 1/2&h of a fringe motion being easy to detect, the stability must be better than 10. ‘. Fizeau concludes by expressing the hope of important applications. From this remark. all of modern length metrology was to arise. fi PIERRE CONNES

Fig. 2. Laboratory note of Fizeau, dated 3 February 1862, illustrating the mechanical design of the first variable-path interferometer for wavelength measurement by fringe counting: “I have made an agreement with Mr Duboscq for constructing a small instrument suitable for measuring the wavelengths of light. .” The device shown is a differential screw with two nuts, one fixed (pitch 0.5 mm) and one mobile (pitch 0.5 x 20/21 mm). For sodium light Fizeau computes a count of 85 fringes/revolution. Also preserved is the draft of a letter to Duboscq (a well remembered instrument maker), discussing some points and ending with the plea: “. I rely on your promise of actively pushing the work on this small instrument which 1 find quite indispensable at present. .“. Results were to be given soon after. in Ref. (6). (Reproduced courtesy of Archives de 1’Acadimie des Sciences. Paris.)

As soon as Fizeau and Foucault had perfected their technique they applied it to the detection of IR radiation, and to measuring IR wavelengths. In the first paper,(‘) “Recherches sur les interfkrences des rayons calorifiques”, the source was the sun again, and the detector an alcohol thermometer with a 1 mm dia spherical bulb; a 1” temperature change induced 8 mm of column expansion, which was followed with a microscope; one micrometer division corresponded to l/400-. Early history of FTS 73

Fringes were observed, and followed into the IR, again from a pair of Fresnel mirrors, and next from crystal plates; Fresnel diffraction from a straight edge was also observed. Shortly before, Fizeau and Foucault had presented their first daguerreotypes of the solar spectrum (here they had been preceded by Draper) to the Societe Philomathique. In their paper,@) they discover that the red rays have an inhibiting effect on the mercury vapour condensation process. Furthermore: “ . . the inverse action begins noticeably beyond the visible spectrum, which is affected up to the orange rays region; it is most intense near the A line (i.e. near 7600 A); the part extending outside the visible spectrum is about as wide as the red, and, like the rest of the spectrum, is marked by lines.” Hence, members of the Societe had been offered the very first IR photographic spectrum, albeit in negative form. There was more to come. At the end of the same year, 1847, Fizeau (this time, alone) discussed before them the “Longueurs d’onde des rayons calorifiques.“(4) He had been able to use the channelled spectrum technique, but only with a crystal plate cut parallel to the axis between polarizers. We are not told why, nor which crystal was used. The influence of double-refraction dispersion is described and taken care of by extrapolation of results collected in the visible part of the spectrum. Fizeau ends by giving wavelengths: “ . for the principal points of the obscure region: those presenting a maximum, a minimum or a line in millionths of a millimeter . . . We find for the invisible heat rays 1011, 1196, 1320, 1445; this last corresponds to a remarkable line; for a more distant point, 1745; then, for the limit of sensible heat, when the spectrum is produced by a flint prism, 1940.”

0.2 5.- I, I, I, I,, , I,, , , , I, ( , I , , , , , , I ,

0.2 o-

Solar Irradiation Curve Outside Atmosphere

Solor Irradiation Curve ot Sea Level -7 0.1 5- oa -Curve far Blackbady at 59OO’K “: I E

I^a, O-

0.0 s-

,3- ta 0 0.2 0.4 0.6 0.6 I.0 1.2 1.4 1.6 1.6 2.0 2.2 3.2

LIMIT Fig. 3. A modern solar energy curve [reproduced from Handbook of Geophysics and Space Environments (1965), courtesy of McGraw-Hill, New York] compared with the wavelengths of the successive maxima and minima measured by Fizeau, using his channelled spectrum interference technique. 14 PIERRECONNES

As Fig. 3 shows, Fizeau was indeed scanning a recognizable solar spectrum; let us not forget that all the time he had to follow an alcohol column with a microscope and that a channelled spectrum was superimposed. With his resolution, the 1.I 5 and 1.4 pm water vapour bands could not be distinguished from Fraunhofer lines and his practical limit, 1.94 pm, was due to a combination of decreasing solar energy and prism dispersion. Even so, the first quantitative IR spectrum had been measured by an interference technique! Hence, Fizeau and Foucault initiated the long quest for extension of the IR range that was to be concluded by Nichols and Tear in 1923; at the same time they laid the groundwork for all future FTS. One may well ask the question: Why no IR wavelengths had ever been measured before with Fraunhofer gratings, just like visible wavelengths? The difficulties, although not clearly fo~ulated and set into print at the time, must have been the same that were to plague later experimenters: overlapping of orders is highly confusing if suitable filter materials have not been found and studied. Also the poor efficiency of gratings already gave a definite advantage to the interferometric method. However, the prism specfroscope played an essential role in the Fizeau-Foucault experiments. Indeed the physical display of bright and dark interference bands in a spectrum, either on a screen or within an eyepiece, must have been an unavoidable intermediary before the later formulation of FTS. Anyway, it remains one of the prettiest shows in optics. Today, that channelled spectrum is still there, plainly visible in the mathematical formula we use for depicting our interferograms; however (discounting the astute but somewhat myopic gropings of Rubens). an even century was to elapse before we grasped that neither slit nor prism had ever been needed to start with.

II. ALBERT ABRAHAM MICHELSON (1851-1931)

There is no question of recounting here the fulI story of Michelson, nor even of that Protean type of inteferometer that now bears his name. The initial goal, starting in 1881, had been the solution of the ether drag puzzle. However, 6 years later we find Michelson (now ensconced at the Cleveland Case School of Applied Science) turning his energies in a wholly new direction. In the Philosophical Magazine of 1887, immediately after the full account by Michelson and Morley of their famous experiment, is found a short note by Michelson, “On a method of making the wavelength of sodium light the actual and practical standard of length “,(‘) in which the essentials of the fringe-counting technique are given. After recalling that “Fizeau was unable to observe interference when the difference of path amounted to 50,000 wavelengths” (see Note 4). Michelson presents his own results:

“‘With metallic sodium in an exhausted tube provided with aluminum electrodes, it was found possible to increase this number to more than 200,000. . . Among other substances tried in the preliminary experiments were thallium, lithium and hydrogen. All of these gave interference up to fifty to one hundred thousand wavelengths, and could therefore all be used as checks on the determination with sodium. It may be noted that in case of the red hydrogen-line, the interference phenomena disappeared at about 15,000 wavelengths, and again at about 45,000 wavelengths; so that the red hydrogen-line must be a double line with the components about one sixtieth as distant as the sodium-lines.”

We see how, from a goal that had been metrology, Michelson was being drawn into s~ec~r~.~cop~ by the very nature of things. The two subjects are conceptually worlds apart, but closely connected by the trivialities of technology: if some atomic line is to be used as length standard, it must be both sharp and symmetrical, hence we have to plot the profile, find satellites etc. The mere idea of absolute wavelength determination by fringe counting was obviously straightforward enough, once Michelson had his interferometer in hand; truly, it had been Fizeau’s idea. However, for full line-profile analysis, primarily a wholly distinct, novel and conceptually far more complex technique had to be evolved. Fortunately for the historian, several Michelson papers in close succession enable us to follow the path he took. In his 1890 “Measurement by light waves”,‘*’ he calls attention to three closely-related forms of the two-beam interferometer, still called at this stage a “refractometer” (a vestige of the ether drag epoch): the tool may be used as a microscope, telescope or spectroscope. The first of the three proposais was not to be implemented, for lack of suitable star-like microscopic objects. Full demonstration of the second in the astronomical field was still 30 years away, for essentially practical reasons: only the Mount Wilson 250 cm reflector would prove rigid enough to carry the device. By contrast, spectroscopy (which remained Michelson’s immediate concern anyway), was to benefit immediately. Nevertheless, the description of the essence of the method was to be given ~!ith~~t mtwthing spectroscopy in the next two papers: “On the application of interference methods to astronomical measurements”(g) and “Visibility of interference fringes at the focus of a telescope”.“O’ Michelson, making use of the Rayleigh treatment of diffraction, introduces visibility as a measurable quantity from which source intensity distribution may be inferred. The now classical cases of a slit, a uniform or non-uniform disk, or two disks as sources are treated. Some details of the instrument needed to observe the fringes are given; but the aztempt was still in the distant future. The next tw5 papers, dated 1X91 and 1892, “On the a~~l~caiion of interference methods to spectroscopic measurernents”~“,‘*’ obviously play the centrai role in our attempted historical account. They are also classical, and sufficiently well known nat to require detaiIed analysis [a full treatment, using modernized notations, is found for instance in Ref. (37)]. We shall merely try to follow the flow of ideas. The main point of interest is that Michelson proceeded from the concept of a spatial visibility curve ta that of a spectral one: “In the present paper it is proposed to show that the same principle holds in spectroscopic observations and even with greater force, In fact, a spectral line might be viewed by a telescope converted into a refractometer, and the “line” could be studied in precisely the same way as a nebufous star. Such a method, however would multiply the necessary im~rf~ctions of lenses and prisms or gratings. Fortunately both spectroscope and telescope msy be dispensed with, by substi- tuting the refractometer in any form which allows a considerable and steady alteration in the diEerence in path between the two interfering streams of light from any approximately homogenous source.““‘) In other words, that very same tool_ the interference ‘“refractometer” that had already been proposed for the wavelength determination of the meter, also proved suitable for spectroscopic analysis. However, this discovery was at the time never more than a happy coincidence, a fact which must be stressed if one wants to understand both the brilliance and the limitations of ~~cbelso~~s interference spectroscopy achievements. The modern form of FTS is readily seen today as an accomplished technique for spectral analysis n& accurate wavelength d~terrn~na~io~. For Michelson and all his readers, the two problems remained conceptually separate, because separate indeed had been the ways by which both had entered the scene; and no point was ever seen in searching for the complete a.nd unique solution we are now familiar with.

Let us now comment on the main points. A brand new technique had been introduced which has “extraordinary advantages.. . in consequence of the almost complete freedom from errors arising from defects in optical or mechanical parts.” This is indeed fundamental, and the ensuing advantage over ~on~ent~o~a~ spectroscopic devices should, irs all fairness, bear Michelson’s name today+ But he never even ~~~rn~~d tk facquinot and Feligett advantages that his own inter- fervmeter exhibits-whew properly used. fndeed the question could hardly arise at all, considering the limitations of the technique he was using: his only detector was the eye, and the best he could do was estimafe friprge rsi;ribility, not record fringe inrensixy. He did it in an elegant manner, calibrating his own eye with fringes of known visibility, but this essential limitation explains why his whole treatment of interferometer data, geared to the sole use of the eye, was to remain incomplete. In particular, he had no concern at all with what today is the physically important parameter, i.e. the light flux (or power} available at interferometer output: what he dealt with was merely illumination. Hence there could be no point at all in speculating about the increase in Nux compared with gratings. 76 PIERKE CONXES

Next, we have an explicit statement showing that Michelson not only never considered the possibility of multiplexing, but found his method comparatively s10w:‘~”

“*The examination of spectral lines by means of the interferometer, while in some respects ideally perfect, is still objectionable for several reasons. In particular, it requires a very long time to make a set of observations, and we can examine only one line at a time.”

How did Michefson treat the problem ma~hematically~ The very first expression he gives for the intensity as a function of path difference D from a spectrum $(n) between wave numbers rri and f22is: ,,? @(tt) cos%c Dndn, i “I i.e. exactly the one we use today for our interferograms. However, he immediately introduces the simplifying assumption of a very narrow spectral range (he never, even much later, considered the general case), and proceeds to compute fringe visibility, introducing in the process two quantities he calls C and S: these are actually the cosine and sine Fourier transforms of his narrow spectral function. But he is not aware of treading familiar ground: the name of Fourier is written nowhere. and the subsequent treatment is wholly correct onfy for futly symmetrical lines or groups of lines. He is dimly aware of the difficulty: i, . . . in many cases it is difficult or impossible to decide between two or more distributions of lines which give very nearly the same visibility curve; and when there are many lines in the source, the combination of intensities and arrangements of these from which a type may be selected is enormously great. Indeed, even when the number of lines is greater than three. . . the resulting visibility-curve becomes so complex that it is very difficult to analyse”.“”

Almost immediately Lord Rayleigh pointed out the deficiency in the analysis.‘131 Introducing right from the start “Fourier’s theorem”, he shows that reconstructing an asymmetricai spectrum requires separate measurements of C and S, and adds: “However, the visibility curve by itseff gives, not both C and S, but only t’+ S’; so that we must conclude that in general an indefinite variety of structures is consistent with a visibility curve given in all its parts.”

The difficulty was fundamental: no mere increase in accuracy could solve it. Indeed Michelson had”--somewhat imprecisely-indicated how a solution could be found in principle:“” “In the case of an unsymmetrical source, it is possible to determine the position of the brighter portion by gradually increasing the difference in path from zero. If the fringes are thereby displaced in the positive sense from the position calculated from the mean wavelength, then the brighter edge lies toward the violet.” Indeed, but no practical means are even suggested for measuring Jiinge ~~sj~~~~,an entirely different kettle of fish from fringe viddity. As Lord Rayfeigh was to put it: “I suppose that a complete determination of C and S, though theoretically possible, would be an extremely difficult task.” The discussion of Rubens method (to be given a little later) will prove that the “extreme difhculty,” that neither Michelson nor Rayleigh were ever to solve, was not due to available technology, or at least not primarily so. We have to take as an historical fact that a conceptual stumbling block lay on the road between those deceptively close-looking neighbours: reconstruction of a small group of lines from the visibility curve, and that of a whole spectrum from the interferogram. The full solution was to be found only 50 years Iater; however we must note, in all fairness to Michelson that it indeed lay in “gradually increasing the path difference from zero.” Fig. 4. Michelson’s own interferometer. In 1892 he took it to the Bureau International des Poids et Mcsures, Sivres, France; a letter from him to the Director, Dr Benoit, is still preserved, where he announces his arrival at Le Havre with the crates. The instrument here is as depicted in the account of the famous measurement of the meter!‘4’ It is stlli in use at BTPM today with only minor modifications (in particular, inclusion in a vacuum tank), but of caurse, using phatomultipl~ers. ‘i-_._ Cd 6438 -.._---. I

--:‘------I. 1 I . I .

,~~~ mm”’ *--

- - *- * - ‘- - *- I-----&_ 200 mm

Tl 5350 i ~~~ 4358

x-*-k -p-y_ _. - L-~--y-.__‘.__ .__ -,=~r_

Fig. 5. Visibility curves and reconstructed structures from Michelson’s fundamental paper.“” The solid line visibility curve is measured. and the dotted one computed from the supposed structure. In this way, Michelson discovered and studied two essential spectroscopic phenomena: hyperfnc structure. and temperature-pressure-limited line widths. Early history of FTS 79

I Fabry and PBrot (1899) G

S Michelson (1982) Gr .f

Gale and Lemon (1910)

Dufour (1951) f u(?“%g Lla307.493

Blaise and Chantrel (1957) Gerstenkorn et al. (1982) Fig. 6. The saga of the mercury green line through the spectroscopic ages. Michelson (f892): same as in Fig. 5. Fabry and P&or (1899): first visual results from multiple-beam interferometer.~19) Gale and Lemon ( 191#):‘2*) an early review of available results (see Note 7). M: Michelson, from the visibility curve. FP: latest Fabry-Perot data. VB, J. G, S: results from various authors using the MicheIson echelon and Lummer plates. In all of these cases there is some ambiguity due to overlapping of orders. GR: results by the authors using the latest Michelson gratings. Dufaur (1951~: I321first recording by scanning photoelectric Fabry-Perot etalon; for the first time we have correct intensities. Blake and Chantrel (1957):‘36’ best results from double FabryyPCrot, with a liquid-hydrogen-cooled hollow cathode source. Gerstenkorn et al. (1982):‘43’ a recent FTS spectrum, but merely from a room temperature source (purpose of the study was not analysis of hyperfine structure). At last the result is totally free from overlapping of orders.

The immediate practical consequence was that only the very simplest structures plotted by Michelson were approximately correct. Even with just a pair of lines, he could not tell on which side the strongest of the two lay. The most complex structure he attempted to unravel was that of the green mercury line, in which he thought he’d detected four peaks, one with a complex shape.

Fig. 7. The difficult mercury green line case for three different source pressures as given in Ref. (12). No reconstructed structure is provided; the figure illustrates well how complex were the visibility curves Michelson had to deal with in the worst cases. 80 PIERRECONNES

Figure 6 follows the full saga of the mercury green line from those heroic times to the present day and shows how fact was-slowly--disentangled from fancy. Still Michelson did his best to use properly that visibility curve; at the beginning, the lines were found by pure guesswork followed by trial and error; in his paper”” two curves, one measured and one computed from the supposed structure are always given (Fig. 4). Clearly the process became laborious and the residual differences markedly increased, for the most complex structures. He soon felt the need for a more systematic approach and in 1898 described his “New Harmonic Analyser”,(‘h) a magnificent fully mechanical device which represents a distinct improvement over Lord Kelvin’s perhaps better known “Tide predictor”. It could generate the sum of up to 80 Fourier terms, and produce both sine and cosine transforms; in modern terms, it was able to reconstruct an 80-element spectral range. Michelson was pleased with the performance: “. . it would be feasible to increase the number of elements to several hundreds or even a thousand with a proportional increase in the accuracy of the integration”. Indeed the many tests reproduced in the paper show that accuracy was fairly good. Almost immediately an opportunity arose for demonstrating the instrument’s usefulness:

‘“A little over a year ago the scientific world was startled by the announcement that Professor Zeeman had discovered a new effect of magnetism on light. It occurred to me at once to try this experiment by the interference method, which is particularly adapted to the examination of just such cases as this, in which the effect to be observed is beyond the range of the spectroscopic rnethod.“(~~~

Indeed, where Zeeman had at first seen only a b~uu~i~nin~ of the lines, Michelson was at once able to shown that there was a splitting. He published two papers in quick succession. For the lirst,(‘s) in 1897, the analyser was not yet available; but it came in very handy for the second”” in 1898. Here we have the best and most elaborate results ever given by fringe visibility. Michelson appears to have even attempted, in this particular case, to derive the complete, correct solution (for a narrow band spectrum), using Lord Rayleigh’s lesson to the full:“”

“In genera1 the phase curve is troublesome to observe on account of the difficulty in securing a sufficiently homogeneous comparison source, but in the present instance this is furnished by the non-magnetized radiations. Usually, however, the assumption was made that the spectrum was symmetrical, and in only a few cases was the solution verified by the complete analysis.”

Indeed, no results from phase analysis were ever given. Still, as a consequence of that fortunate symmetry in the phenomenon, his Zeeman patterns (unlike his hyperfme structures) look all right to the modern eye (Fig. 8). “Fifty or more visibility curves” were actually taken, and the harmonic analyser fully proved its worth:“”

“It was for the solution of such problems that the “harmonic analyzer” was devised. The curve, V = f(k) is ‘fed’ to the machine, which then draws the curve y = Cp(x), the whole operation taking but a few minutes.”

Michelson appears to have kept using the analyser for a few more years, but he did not make the results available. In 1902 he writes:‘““’

“The curve of the green radiation of mercury. . . is really so complicated that the character of the source is still a little in doubt. The machine has not quite enough elements to resolve it satisfactorily, having but eighty when it ought to have eight hundred. The curve looks almost as though it were the exceptional result of this particular series of measurements, and we might imagine that another series of measurements would give quite a different curve. But I have made over one hundred such measurements, and, each time obtained practically the same results, even to the minutest details of the second secondary waves.” (See Fig, 7.)

And, the reconstructed line structure given in the accompanying figure is merely an exact reproduction of the 1892 one. Subsequent history of the harmonic analyser is unclear, apart from the fact that it is now on show at the Washington Smithsonian Museum, and welt worth a visit. Early history of FTS 81

Fig. 8. Three different types of Zeeman patterns given by Michelson in 1898. “‘) Abore: the visibility curves with the plane of polarization perpendicular (A) or parallel (B) to the field. B&W: the Zeeman patterns, as reconstructed by the analyser.

A last remark: in the very same year (1898) that Michelson first described his analyser, Sir Arthur Schuster introduced a numerical method of spectrum analysis, the computation of periodograms, which is crudely equivalent to that of Fourier transforms. They were actually applied to physical problems, first to meteorological phenomena, and then to the solar cycle.“‘) Neither Michelson nor Rayleigh ever considered the possibility of treating visibility curves in this way. Clearly, the computational labour involved to achieve similar results to the analyser would be substantial: we see today that at the very least, 80* = 6400 multiplications would have been required. (See Note 5.) A summary of Michelson’s work will be better presented after reviewing a totally unrelated effort that started a few years later.

III. HEINRICH RUBENS (1865-1922)

We consider now an altogether different spectroscopic problem, and a different set of scientists. In 1910 Heinrich Rubens was Professor of Experimental Physics at the University of Berlin, and Director of the Konigliches Physikaliches Institut. His main line had been the exploration of Maxwellian-Hertzian waves, particularly in the IR. Indeed extension of the IR range to longer wavelengths was a current problem; a large gap was still open between “heat rays” and radio waves. Thermocouples had become convenient detectors; the problems were to find sources and transparent materials, in particular suitable ones for making prisms and filters. Gratings seemed to behave rather poorly, overlapping of orders was confusing. Rubens had dealt with both the theoretical and experimental aspect of these questions; for instance, he had built improved galvanometers, bolometers and thermopiles; in collaboration with the American physicist E. F. Nichols he had discovered and christened the important phenomenon of “reststrahlen”, which gave the first means of producing “homogeneous radiation”, i.e. crudely monochromatic beams, the 82 PIEKKE CONNES

Fig. 9. The Michelson-Stratton “New Harmonic Analyscr” (courtesy of the Smithsonian Institution. Washington, D.C.). For the principle of operation see Ref. (20). accurate study of which seems to have been his immediate concern around 1910. No trace is left of the exact path by which he was led to consider the use of interference fringes for the problem; as will be seen, the connexion with Michelson’s technique appeared to him most tenuous, and was almost certainly not at the origin of his work. And since the Fizeau-Foucault times, nobody seems to have found a practical use for those channelled spectrum IR fringes (see Note 6). The first paper in the series is “Measurements in the extreme infrared spectrum”;“” it is written in collaboration with H. Hollnagel, who then disappears from the scene. We might say that it is Early history of FTS 83

Fig. 10. Above: the Fabry-Ptrot (1899) variable-path interferometer. “V Brlo~: the Rubens-Hollnagel (1910) device,@‘) which is merely a lower mechanical accuracy version of the above. However, it was tested and adjusted (and later stepped) by using sodium fringes.

the first paper on Fourier spectroscopy, but without Fourier. In the first experiments the source was a “Welsbach burner without the glass chimney”, the detector “a sensitive microradiometer”, and reflections on various crystals were investigated. We are mostly concerned with the tool: it was actually what we would call today a low-finesse Fabry-Perot interferometer, with two unsilvered quartz plates; reflectivity was merely that provided by Fresnel reflection, i.e. 14% from the 2.2 index of quartz in the FIR. Even the mechanical design was analogous to that of the variable-path Fabry-Perot interferometer described in 1899 (Fig. 10). (19)The transmitted intensity was computed from the Airy formula, but no reference was made to the work of Fabry and Perot. Indeed, as far as spectroscopy is concerned, the implemented method is so different from that which multiple reflections giving high-finesse fringes have made familiar, that nobody at the time noticed the above similarities: the sharp Fabry-Perot fringes were invariably used (until 1947) by viewing or photographing the ring system, while Rubens and Hollnagel recorded what we would call a true interferogram, i.e. output intensity vs increasing path difference with their “microradiometer”. No theoretical analysis at all is provided. The authors merely state (but not before page 9 of their article) that:

“The interference curves furnish not only the necessary data for the wavelength computation but also give a probable idea of the homogeneity of such rays. (Footnote): Our problem is one similar to that which Prof. A. A. Michelson had to deal with in drawing conclusions regarding the energy distribution of spectral lines from the visibility curves of the interference fringes obtained by great differences in optical path.”

Reference is made to Michelson’s paper,(12) and that is all. Despite this acknowledgement, the analyses provided by Michelson and Rayleigh are in fact totally ignored, and the name of Fourier appears nowhere. Thanks to the capital wisdom won from three-quarters of a century hindsight, we see easily that Rubens was simply not aware of two essential points: first, not only the Michelson-Rayleigh mathematical treatment but also the Michelson computer were available and PIEKKE CONNFS Fig. 11. Two typical Rubens-+lollnagel results.“” Left: Reststrahlen from rocksalt. Right: from sylvine. Top: recorded interferograms, showing equidistant measured samples; abscissae are micrometer divisions. The recordings do not quite start at zero path difference. Bottom: corresponding spectra built-up by a mostly trial-and-error process, which nevertheless correctly locates the two maxima. MzXife: synthetic interferograms computed from these spectra. X6 PIERRECONNES would have solved the spectral reconstruction problem completely. Second, this excellent experi- menter was fully performing that “extremely difficult task” by doing the two things Michelson had never done: recording true intensities and phases, and doing it by “gradually increasing the path difference”. This lack of awareness is in part explainable: Michelson’s technique was by then 20 years old and no longer used. Incidentally, he never realized either that the Michelson form of interferometer-no harder to build from the two quartz platesPwould have been somewhat better for his purpose. The spectra that Rubens was interested in were at first very simple, never showing more than two peaks. This means that the interferograms could be well approximated by adding two damped sine waves. In the reconstruction process there were only six unknowns: two amplitudes. two frequencies and two damping coefficients. Guesswork plus trial and error proved good enough (Fig. 11). However, the two co-workers seem to have evolved a great deal of practical skill in performing the operation. In particular, they were able to reconstruct unambiguously the relative positions of two peaks with unequal intensities. Their full demonstration is somewhat laborious (not using the language of Fourier transforms) and a bit lengthy for reproduction, but they provide an explanatory diagram (Fig. 12) and write:

“To illustrate this, two curves are shown. They are obtained by the superposition of two cosine curves whose periods are in the ratios of 7/6 and 7/8 respectively; the curve 0: represents the instance of the principal band accompanied by a long-waved radiation, while curve fi indicates the other case . The weaker (line) being the longer waved, the maxima and minima are very markedly crowded together in that portion of the interference curve where the interference is most feeble . ” (and vice versa).

Hence the whole issue of correct spectral reconstruction (truly, a key one) was properly understood, but Rubens never noticed, or at least neoer wrote doctn, that at this point he had achieved an essential improvement over the entire Michelson technique, which made his own far more widely applicable. Still, the analysis remained primitive. Resolving power was not even considered: when increasing path difference “observations were made as long as maxima and minima of sufficiently distinct character were apparent”. Slightly later,“4’ it was noticed that “the interference curves obtained with the quartz mercury lampomitting the radiation filter-showed a very irregular character”. In other words, these were too complex to be synthesized by pure guesswork, and no effort was made to analyse them, while---unquestionablythe Michelson computer would have been fully adequate. Which is a pity, because the attempted problems soon became more varied: transmission or reflection of many substances were studied, but this was always done at only a few reststrahlen wavelengths. Here is a typical result: Rubens and Hollnagel study the absorption of water vapour in a heated windowless brass tube 40 cm long; they can only express their findings in clumsy language:

“While the reststrahlen of rock salt and sylvine are almost completely absorbed by such a layer of vapour, a considerable percentage of the reststrahlen of potassium bromide and potassium iodide is transmitted. This indicates that the absorption of water vapour has, to be sure, a considerable value in the region of 80 to 160 P. though not so great as in the domain between 50 and 70 p. The same holds good for the liquid state.““‘)

However, on the way a truly fundamental advantage had been noticed, at least with the corner of the eye:

“The radiant beam must be diaphragmed as little as possible. In obviating such a diminution of energy, the interference method has a decided advantage over all spectrometric methods where the use of a slit is unavoidable. A second advantage consists in the evasion of the diffraction grating which is so uneconomical of energy. In previous wavelength measurements with the grating, it was not possible to separate the two bands because of the immense width of the slit which had to bc used in consequence of the limited energy.““” Early history of FTS x7

Fig. 12. Illustration of how Rubens and Hollnagel were able to remove the ambiguity that always plagued Michelson (see text). Curve CI: the stronger simulated line was lower wavelength. Curve /I: the inverse case. From Ref. (21).

Unquestionably, Rubens was fully aware of one of his interferometer’s main assets. But here again the total lack of systematic thought applied to the experiment robbed the result of general significance, and it went completely unnoticed. He had not asked the question: Where shall we stop the path difference? (i.e. that of resolving power); nor did he ponder about: What is the maximum permissible size for that diaphragm?, merely noticing that in his optical system as it stood “the extreme rays form an angle of 2.5” with the central ray. Since the cosine of this angle differs from unity by only 0.001 the error introduced by overlooking such a correction is negligible.” Why was it 2.5”, neither more or less? We are not told. The problem of luminosity had actually been solved; but the question had not even been asked. And the Rubens discovery was utterly forgotten when, in 1947, Jacquinot reopened the subject-in a definitive manner.“” Rubens’ technique went on being used in the FIR for about 12 years. He next applied it, in collaboration with R. W. Wood, to the “very long heat waves” obtained by the famous focal isolation method they jointly described, and indeed extended the measured IR range to 150 pm. As far as the interferometer was concerned, one noticeable improvement was made; could it have been a contribution by Wood?

“It was found that the readings of the graduated wheel of the interferometer did not give very reliable indications . . . The distance between the plates was accordingly determined in every case by observing the interference fringes formed by reflecting the light of a sodium flame. The deflexion of the microradiometer was taken and the plates slowly separated by turning the screw until 20 more fringes had crossed the mark.“(23)

Then the operation was repeated for each interferogram sample (as we would say); some modern interferometers are operated in just this way, using fast-counting and servo-control techniques of course. The reduction of data was still wholly by trial and error; a probable spectral profile was first assumed, and then:

“This curve we may now divide in elementary vertical strips, each one of which represents nearly homogeneous radiation. We now draw the inteference curves (sine curves) of the various strips and the superposition of all these curves should give us a curve identical with the curve obtained with the interferometer, if our energy curve has been correctly figured.”

The operation was rather more elaborate than in the reststrahlen case, since a distinctly asymmetric profile was found (see Fig. 13); the central result of the Rubens-Wood paper was critically dependent on correct spectral reconstruction, the conclusion being:

“It is thus apparent that we have experimental evidence of the presence of heat waves certainly 150 and probably 200 p in length.” PIEKKE CONNES

Fig. 13. Rubens and Wood (1912) interferograms and reconstructed spectra; I”’- Welsbach burner, and focal isolation method with two quartz lenses. Abscissae are number of sodium fringes from approximately zero path difference and wavelengths in microns, respectively. Upper interferogram and Curve u have been obtained with 2-mm thick quartz plates; the two other curves correspond to 7.3 mm plates. We see easily today that the high path difference cut-off was arbitrary and rather too small. No apodization being used. the authors may have been somewhat overestimating the intensity in the critical 200 pm region. It would be an interesting exercise to reproduce the analysis using modern techniques.

Wood was at the time Rowland’s successor as Professor of Experimental Physics at Johns Hopkins and indeed one of the most gifted and universal experimentalists ever engaged in optics. His 1905 Physical Optics was already a classic. It is interesting to see how he treats the matter in the next edition.“” The Michelson visibility technique appears in the chapter on “Interference spectroscopes”:

“The method has not been used to any great extent by other observers, partly from the great difficulty of estimating “visibilities” of the fringes, and partly from the difficulty in interpreting the results. Michelson’s results were due to his great skill in this respect, which resulted from long experience and familiarity with his instrument.”

As to the “interferometer investigation of long heat-waves”, it is found within the chapter on the theory of dispersion (which is revealing in itself). Both the residual rays and the focal isolation experiment are given fair coverage, but the analysis of the interferometric technique itself ranks but a few brief lines, being mostly dismissed with the comment: Early history of FTS x9

“ . . . an intensity curve was obtained precisely similar in every respect to the visibility curves obtained by Michelson with his instrument.”

This is of course an overstatement, and again shows that the essential difference (and improvement) introduced by Rubens was not perceived. Next Rubens and von Baeyer (24)found still longer waves in a quartz mercury lamp radiation, up to 300 pm, and the final tests were reported by Nichols and Tear in 1923.(27) It has remained a classic experiment. Ultrashort radio waves were produced with “Hertzian oscillator”, measured with an interferometer and detected with a thermal radiometer, hence establishing continuity between radio and IR techniques. Here we are only concerned with the recording of the “interference curves” as a function of path difference; this is the part of the experiment that seems to have attracted the least attention. The method is essentially that of Rubens and Hollnagel. Michelson is no longer mentioned. The interferometer is a “Boltzman” type, simply consisting of two plane parallel mirrors side by side (thus the ancestor of our present day lamellar gratings). No transparent material is needed, but the beam has to be accurately parallel (which is no drawback from a coherent oscillator). The recorded curves were again analysed as the sums of at the most three damped sine waves. Clearly they were not considered capable of giving any more refined information, and “as a further means for analysing the waveform and contents of very short wave oscillators, a reflecting echelon grating was constructed”. The principal aim of all these efforts had been unifying radio and IR waves rather than spectroscopy. The task had been completed and the gap was stopped.

The gap had been stopped and the interferometric method remained a stop-gap: a rather desperate device usable at best in that nature-forsaken spectral range where at first nothing else seemed to work. As prism and grating technology improved somewhat and provided direct results without guesswork, fringe recording was simply discarded. Exactly the same fate had befallen- even faster-Michelson’s fringe visibility. The reason was also improvement of gratings, particularly by Michelson himself, but mainly the “Nouvelle methode de spectroscopic interferentielle”“9) presented by Fabry and Perot in 1899 (Fig. 6). The authors could rightly state that “some of the preceding conclusions differ from those given by M. Michelson as a result of his beautiful studies of fringe visibility. But we must not forget that exact knowledge of the visibility curve is not sufficient to determine completely light composition . . . ” Furthermore the Fabry and Perot technique could easily be used with a photographic plate. Thus fringe visibility, like the harmonic analyser, became a museum piece, piously referred to in all handbooks but wholly obsolete. The connexion with the Rubens fringe recording had been seen but vaguely, and not properly understood. The most puzzling part of the whole story is that Lord Rayleigh missed it altogether-he who had so immediately reacted to the first Michelson paper in 1892. In the very same Philosophical Magazine which had just published the Rubens-Hollnagel-Wood-Baeyer results, one finds in 1912 an important Rayleigh paper on “Remarks concerning Fourier’s theorem as applied to physical problems”,‘26’ where he introduces the concept of weighting a curve before applying the Fourier transformation (which later led to apodization). The obsolescent visibility technique is mentioned; there is nothing about the new fringe recording method. As to Michelson himself, also in 1912, he published in Nature a review on “Recent progress in spectroscopic methods ” (25)Fringe visibility, Fabry-Perot interferometers, the Lummer plate, the echelon and the latest gratings are discussed; there is not a word about Rubens. Let me play, for just a few lines, the risky game of rewriting history to suit my fancy. I merely inject into the Rubens-Hollnagel 1910 paper (2’) the following additional footnote, right after the pair-of-peaks demonstration:

“We notice that precisely in the same case, Prof. A. A. Michelson’s analysis only produced ambiguous results. In his discussion of the problem, Lord Rayleigh stressed that complete spectral reconstruction would be extremely difficult; this is in fact just what we have been doing.” 90 PIERRECONNES

I submit as a foregone conclusion that even such a minor onslaught on a pair of scientific worthies (both Nobel Prize winners of a few years before) could not have failed to catch the eye of some readers (see Note 7). Then what? Once aware of the matter, Rayleigh would have instantly applied “Fourier’s theorem” to the method just described; as to Michelson, he could feed the published interference curves into his machine right away, “the whole operation taking but a few minutes”. The entire FTS show could have been on, 40 years earlier. Would it indeed have happened? That will be the theme of my conclusion. Let us now return to factual history. As things were in the real world, Michelson (waxing somewhat nostalgic at the close of his life), could only write in 1927 that:“9’

“While it may be admitted that the analysis of spectral lines by the method of visibility curves is somewhat indirect and not entirely certain, it has nevertheless proved of considerable value, especially in cases where the effects to be observed are beyond the power of the spectroscope. At the time of its inception the resolving power of the instruments available was far too small for many of the problems which have yielded to the new method, such as the resolution of fine structure, the effect of temperature, of pressure, and of the magnetic field. While other methods have for many purposes superseded this method of light-wave analysis, there are still applications for which the process is the most powerful means at our disposal.”

By “applications” Michelson meant metrology, for which his interferometer was to remain in use. And the judgement of his successors was exactly the same. In the 1951 monograph on “Modern interferometers” by Candler,‘33) 42 pages are devoted to the Michelson echelons, and less than 2 to fringe visibility, stressing the basic ambiguities, and concluding that:

“This limitation is so serious that the method could never be used today, having been replaced by the high resolving power instruments, the FP etalon, the two echelons and the Lummer plate. History records however that Michelson with this method obtained a whole series of remarkable results.”

But History never stops. That is why it is so difficult to catch. In the same year Peter Fellgett submitted his Thesis.

IV. CONCLUSION

At the close of the Durham Conference, Fellgett and Jacquinot described in their own words the actual When and How of the heroic and lonely FTS beginnings. There will be no attempt here to improve upon the account of the technique’s true fathers. But our modest historical study would not be complete if we did not try to understand Why the appearance upon the scientific scene of their proposals was possible, and ultimately successful (these being two different questions), at that particular time and not before. In brief, why did the flowering of FTS take place in those two successive and unconnected bursts, with such a complete withering in between? Are we able to pinpoint at least one essential element lacking in the Michelson-Rayleigh-Rubens era and available in the Fellgett-Jacquinot one? Was it merely improved mathematics? Clearly not. Rayleigh’s understanding and formulation of the problem were fully adequate, and the only element lacking was a suitable input. He never saw (any more than Michelson did) the need for applying his equations to the broad spectral range case, and never realized either that, in between his 1892 formulation of the problem and his 1912 review, the recording of fringe amplitude and phase had become a practical proposition. Was it general advance in optical technology? Even in 1983, we had better be modest about our latest and fanciest interferometers; while a few excrescences have been added (some of which took quite a while to grow), they are essentially the old Michelson or Boltzman types. All could have been developed at the beginning of the century, given a few years, and used anywhere in the IR; of course, with thermopiles, galvanometers and successive samples jotted down by hand. But this is precisely how the very first Fellgett spectra were produced. What about the laser revolution? Every single Fourier spectrometer today uses a laser as reference for checking path difference. Nevertheless, the role lasers assume here does not compare Early history of FTS ‘)I in any way with the essential one they play for instance in holography. Most of the key FTS demonstrations had been achieved around 1965, using the good old 198Hg green line as a reference. In the last years of the pre-laser era, a simple experiment was also made, showing that adequately sharp artificial lines could be generated merely using an FP etalon as a filter.‘39’ Then, was it was the computer revolution that did the trick? Here, a fair and balanced answer is more difficult. We have seen that in 1898 Michelson had perfected a suitable computing device that might easily have fathered a full line of descendants. Indeed, Michelson himself was ready for just anything: “Among the methods which appeared most promising were additions of fluid pressures, elastic and other forces, and electric currents.” Actually, at the onset of the second period (roughly from 1950 to 1965) such devices were still much talked about. At the 1957 Bellevue meeting, Strong and Vanasse presented a lo-channel analog computer,(3x’ and in the ensuing discussion Fellgett himself confessed to having wasted much time trying to develop analog systems. Several more attempts, some quite elaborate, were to be made later. Most probably, if it had not been for the overwhelming digital computer invasion, these devices would have been carried much further, and have achieved a limited degree of success. Alternatively, one may argue that without digital computers, the Fourier spectroscopy idea might well have been left on the shelf as one more laboratory oddity, most instructive for students to play with, but of no practical value. Indisputably the discovery of the Fast Fourier Transform algorithm in 1965 has been the major step, while faster machines did the rest. As a consequence it became possible to compute spectra with up to several millions of samples, an enormous increase in the multiplexing factor beyond anything that might have been dreamed of around 1950. Altogether, an honest answer must be that the general-purpose digital computer, while largely responsible for the practical success of Fourier spectroscopy in recent years, had nothing to do with the discovery (or rediscovery if you prefer) of the technique in the 1950s: it came (or at least became accessible), only later. In the two fundamental papers,‘34.35’ the only means of performing Fourier transforms were the harmonic analyser, and the “Lipson-Beevers”““) cardboard strips; but the latter, being digital tools, clearly pointed out the way to the future. However, one essential change had taken place in physical laboratories between 1910 and 1950: electronic amplifiers had become commonplace, and here we have the answer to our central question: it lies in the conceptual revolution brought in by these devices. In the Rubens-Hollnagel experiments, the accuracy-limiting factor had been, rather naively, pressure changes and draughts acting on the thermopile:(21)

“Under favourable conditions, especially in clear weather and no wind, and a constant room temperature the error in a single observation rarely exceeded 0.2 mm (galvanometer deflexion) . . When the wind was high, the zero point of the instrument varied most irregularly, the variations often attaining several millimeters . . To obviate this, a vacuum tight bell jar with a quartz window was constructed. ”

NOW, these conditions were not conducive to any form of Fourier spectroscopy taking root. As long as such elementary tricks seemed able to improve results, any devious method which provided spectra only through either guesswork or laborious analysis could not possibly be seen as the definitive answer. However, by 1950, spectroscopists had completely run out of simple tricks; their amplifiers were daily displaying on all plots various forms of fundamental noises, which simply could not be rubbed out. Hence two of them eventually arose who not only reinvented the whole technique from scratch, but also had the courage-in the midst of general unbelief-to state unequivocally that it was worth the trouble and more-even though no better means of computation than wave analysers and cardboard strips were yet available. Altogether, the second Fourier epoch is simply a long delayed outcome of the quantum revolution.

NOTES

(1) Here is a clear illustration of the point. In 1966 R. R. Ernst and W. A. Anderson, then at Varian Associates in Palo Alto, Calif., proposed the “Application of Fourier spectroscopy to magnetic resonance.“(40) The paper makes no reference to the previous work of any optician: indeed, at the time, optical Fourier spectroscopy was still relatively unknown outside a restricted field of specialists. Questioned on the subject, Ernst has amiably answered that at the inception of the concept, Anderson and himself were not yet conscious of the work of Fellgett and Jacquinot, but that awareness soon came: in a second paper, also published in 1966,(4” we find references to Fellgett & Co, and read that: “This method can be called Fourier transform spectroscopy. It has a strong similarity to the interferometric methods used in infrared spectroscopy. The main difference is that in rf spectroscopy coherent radiation sources can be applied which allow the distinction between absorption and dispersion mode. Whereas in infrared spectroscopy merely power absorption is measured. The corresponding method in rf spectroscopy would involve a broad band noise irradiation.” The conceptual simifarity between the optical and NMR situations is indeed a close one, and even more so when two parallel slightly later developments are taken into account: on the one side, we now have *‘magnetic resonance with stochastic excitation”,‘“” and on the other “dispersive Fourier transform spectroscopy” (well discussed at Durham), in which both absorption and dispersion of a sample are accessible. Although the two technologies are wholly different (the magnetic spectrometer bears no resemblance to an interferometer), the basic reason for success has been the same in both fields: in both the optical and the NMR case a considerable improvement in SNR for a given recording time is realized compared to the older scanning instruments; in both cases we may write with Fellgett,(‘41 that when Fourier transformation is applied u posteriori to the broad-band recorded signal ““then the measurement of each (spectral) element depends on the contribution made by this element throughout the whole time 01 observations”. Or in other words, the multiplex advantage fully applies in both fields. The practical success of NMR Fourier spectroscopy has since been considerable; indeed it came far more rapidly than in the optical case, because while the poor opticians had a laborious struggle to develop a new breed of o~ticffl spectrometers, the mu,gneric ones could be used through Fourier transformation with relatively few nlodifications. But the main reason is that the computing problems which loomed so forlnidably around 1950 had become trivial by 1966. Altogether the technique is now firmly entrenched. Readily granting the starting point to have been independent, one must nevertheless conclude that this whole development illustrates extremely well the considerable power of the new principle first introduced into methodology by opticians. (2) The two fundamental papers are Refs (34) and (35). Both in fact were almost inaccessible at the time. and have remained obscure. May we make a plea for their republication by some optical journal’? (3) These two notes are in fact abstracted from two more detailed memoirs submitted to the Academic des Sciences, and reported upon on 19 June 1848 by Arago, Regnault and Babinet, who recommend their publication in the M&mires des Smants Etrungers. However, the 1848 Revolution was then in full swing, with late June being one of the bloodiest and sorriest periods in French history. Arago was a member 01 the provisional government, and President of the Executive Commission to boot. The FizeauFoucault papers were Forgotten about, and remained in the Archives de I’Academie as manuscripts. Of these nation-shaking events, a discreet trace is left on the yellowing paper of Fizeau’s next references. in the Pro&s Verbam de fu So&G Phff~~~zutfqufJ.I”’- The 1847 volume is marked on its front page, like all its predecessors, with a Biblioth~que Royale stamp, encircling the crown of the Roi Citoyen Louis Philippe. At the end of the same volume, the bewildered librarian, earnestly trying to keep abreast of troubled times. just managed to stamp Bibliotheque Nationale, with the now familiar republican monogram RF. Alas! There was no second try; on the next volume our Bibliotheque has become Imperiale, and adorned with a suitably fierce looking eagle. Was there some delay in publication? The Seconde Republique was overthrown only in 18.51. Even so, in that very volume is found one of the most fundamental papers (also, one of the shortest) in the history of both optics and astronomy: the Fizeau anticipation of a linr shift due to source-observer motion. (4) A minor slip of the Michelsonian pen. We should read “. distinctly more than 50.000 fringes.” (5) Schuster had given shortly before a detailed paper on “Interference phenomena” [Phil. Mug. 37, 509 (1894)] and their implications for the nature of light, which shows him to be well aware of Rayleigh and Michelson. And when presenting the numerical analysis method, (“’ he again briefiy discusses spectroscopic resolving power and double lines. But he never suggests that Michelson’s visibility curves could be treated by his own method, Here also, we are able today to see a missed connexion: ali the devices required for successful FTS had been discovered; they were never brought together. (6)A few years before, John Koch had measured refraction indices of gases and liquids at reststrahlen wavelengths using a Jamin interferometer with rocksalt plates [Ann/n Pt~~s. 17, 658 (1905)]. This is a zero-path-difference device, and no spectroscopy was attempted. But the paper, quoted by Rubens. probably made him aware of interference methods in the IR. (7) Here is one specific illustration of how closely missed was the connexion between Michelson and Rubens. In the very same year 1910, Gale and Lemon present their review of all studies of the mercury lines’“’ (see Fig. 6). Their own work had been performed in Michelson’s Chicago private laboratory, with one of his best gratings, and he was to comment favourably on the results in his 1912 Nuture paper.“” Concerning the green line, Gale and Lemon reprint the old 1892 visibility curve and reconstructed spectral curve (which again illustrates the lack of results from the harmonic analyser). However. they have done some reanalysis of their own, helped by all the new data recorded since 1892. In the course of the discussion. they remind the reader that only “the visibility curve and the phase curve together give a definite solution”. and that Early history of FTS 93 in principle “by comparing the fringes with those of a line which has no satellites, like the red line 6438 of cadmium, the so called phase curve may be plotted”. Actually, they have not tried it, and the remark does not go beyond those of Michelson almost 20 years before, as they stress themselves. But they add a more technical comment; when there are two lines of unequal intensities:” the fringes at a minimum will lag behind a perfectly definite fractional part of a fringe if the satellite is on the violet side of the main line, and they will be a definite fractional part of a fringe ahead at a minimum if the satellite is on the red side.” Which is a marginally different solution of the problem that Rubens-Hollnagel were discussing at exactly the same time, and soltling experimentally (see Figs 11 and 12). Within a year Rubens and Wood were to measure that phase, using the sodium lines as reference-but also using a totally different language: they never suid so. The “miss” was a narrow one indeed.

REFERENCES

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