CHAPTER 3—PART 2

MOVING IN THE INNER CIRCLE

Opportunism and Eclecticism (continued)

The Sun, the Moon, and Shooting Stars

In England, Cassiopeia is high in the sky in the early evening in the fall and Huggins returned to g Cassiopeiae from time to time, to check on the contrast and character of the alleged bright lines.  But he was soon distracted from this effort by a passing interest in solar phenomena, in particular, the so-called red flames, or prominences, observed around the disk of the moon during a total solar eclipse.  A notebook entry on 10 November 1866 tells us that he "tried a new method of endeavouring to see the red-flames," a method he believed would make the solar prominences visible without an eclipse to darken the sun's bright surface.  Huggins reasoned that if the flames were gaseous, their red color would be composed of "one or two refrangibilities only."⁴⁸  Rather than isolate these individual wavelengths prismatically, Huggins turned to a rather awkward mechanical method involving several pieces of differently colored glass cemented together with Canada balsam to filter out all but the desired light.  The search for a method of viewing solar atmospheric phenomena without an eclipse was one which Huggins returned to throughout his career and which played a significant role in his later disputes with J. Norman Lockyer.  We shall return to a more detailed discussion of Huggins' struggles with the solar prominences in chapter 6.  Suffice it to say here that in spite of his interest in prismatic analysis of the light emitted by celestial bodies, Huggins apparently did not recognize that method's potential as a practical means by which the colorful solar prominences might be rendered visible.

But solar prominences were not the only items of astronomical interest to capture Huggins' attention in the fall of 1866.  At the annual opening meeting of the RAS in November, RAS President Charles Pritchard called the attention of the members to the forthcoming shower of meteors which was anticipated on the 13th or 14th of the month.  Pritchard warned the assembly, "If any man went to bed on either of those nights, he was not worthy to be called an astronomer."⁴⁹

This meteor shower was a much anticipated event -- the first one to have been predicted in advance.  Professor Hubert A. Newton, of Yale College, had examined historic accounts of notable displays of shooting stars and announced the night of 13-14 November 1866 as the probable night for a repeat of 1833's spectacular meteor display.  Huggins prepared himself by viewing the spectra of sudden flashes of flaming metallic substances produced by fireworks displays in September and October at the Crystal Palace, not far from his home.  He used an instrument he called a meteor-spectroscope, a small direct-vision spectroscope which he described in a brief paper submitted to the Royal Society in December 1867 for publication in the Proceedings (see Figure 19).⁵⁰  The hand-held instrument had three contiguous prisms, one of flint glass which was inverted and sandwiched between two of crown glass.  The prism train was held in place in a tube attached to a small viewing telescope with a field of view of about 7°.  The records of his fireworks observations indicate that he had no difficulty spotting transient events and felt confident that he could detect spectral characteristics in the light produced.⁵¹

Figure 19.  Schematic drawing of William Huggins' direct-vision spectroscope [from Proc. Roy. Soc., 16 (1868):  241].

Huggins spent the evening of 13 November examining the star, g Cassiopeiae with Miller, while in the early morning hours of 14 November he made an effort to view the meteors between 1:45 and 3:15 a.m.  He reported that he saw many small meteors during the first hour of his vigil, but very few afterwards.  Only one bright meteor appeared, but it was behind a cloud.  "Saw one or two faint ones through prism, but nothing satisfactory.  The display at this time, a very poor one."⁵²

Huggins' comments are remarkable given his advance preparation and the fact that this particular display of meteors was noted by other observers as being especially fine.  It came just as Professor Newton had predicted and did not disappoint most of those who reported their observations.  Accounts of observations made under excellent weather conditions filled almost half a page in the London Times the next day.⁵³  The astronomer, Robert Ball, recalled in later years being encouraged to look for the meteor shower during his two year tenure with Lord Rosse at Parsonstown:

I suddenly heard a shout.  Looking up, I was just in time to see a brilliant streak of light overhead....  Shortly afterwards another appeared....  But presently they began to come in twos and threes.... From a height of sixty feet above the ground I saw a spectacle which, even after an interval of forty years, was the grandest I ever remember having seen....  Each of them was bright enough and sufficiently conspicuous to arrest attention.  But when they came in dozens, in scores, in uncounted hundreds, and finally in myriads, the scene was unspeakably sublime.⁵⁴

According to Agnes Clerke:

The display, although ... inferior to that of 1833, was of extraordinary impressiveness.  Dense crowds of meteors, equal in lustre to the brightest stars, and some rivalling Venus at her best, darted from east to west across the sky with enormous apparent velocities....  Nearly all left behind them trains of emerald green or clear blue light, which occasionally lasted many minutes, before they shrivelled, and curled up out of sight.  The maximum rush occurred a little after one o'clock on the morning of November 14, when attempts to count were overpowered by frequency.  But during a previous interval of seven minutes five seconds, four observers at Mr. Bishop's observatory at Twickenham reckoned 514, and during an hour 1120.⁵⁵

Comments made by John Browning at the RAS meeting of 11 January 1867 concerning his own successful spectroscopic observations of the meteors in this particular shower generated considerable discussion among those present.  If Huggins participated in the discussion, his comments were not recorded.⁵⁶

In his Proceedings article on the hand-held spectroscope, Huggins wrote only "Unfortunately, I was prevented from making the use of the instrument which I had intended at the display of meteors in November 1866."⁵⁷  Clearly, Huggins was concerned about how his colleagues would construe his failure to observe many meteors, let alone their spectra, in spite of his advanced preparation.  I would argue that his need to maintain his reputation as a careful and capable observer pressed him to state that he had been prevented from using the instrument on this particular occasion.

Another thing which lured Huggins' interest away from the problem of variable stars was the exciting announcement in October 1866, by the director of the Athens Observatory, J. F. Julius Schmidt, that Linné, a small lunar crater, had somehow vanished.⁵⁸  This news broke at a time when interest in the study of lunar features was increasing among British astronomers.⁵⁹  The report of this alleged change in a lunar crater stimulated a great deal of speculation.  Some saw it as evidence of recent volcanic activity on the moon, while others thought the crater may have been erased by a lunar atmospheric disturbance.  "A change," Agnes Clerke wrote, "always seems to the inquisitive intellect of man like a breach in the defences of Nature's secrets, through which it may hope to make its way to the citadel."⁶⁰

Figure 20.  The lunar crater Linné [from Astr. Reg. 6 (1868):  212].

Linné is located near the western edge of Mare Serenitatis and had been noted simply as a round white spot with no mention of any crater-like features by the German astronomer Johann Hieronymous Schröter as early as 1788 (see Figure 20).⁶¹  In the 1820s, Wilhelm Lohrmann and Johann Heinrich Mädler described Linné as a deep crater with a diameter of some five to six miles, a size which made it the third largest crater in an otherwise smooth and barren plain.  When in 1830, the astronomer Mädler teamed up with Wilhelm Beer, a Berlin banker, to produce their renowned lunar map, crater Linné was clearly depicted.  In the early 1840s, Schmidt himself confirmed these observations.  In 1866, however, it seemed to him that the crater had suddenly and inexplicably vanished.  Schmidt concluded that a real and significant change had recently taken place on the lunar surface. Schmidt communicated this observation by letter to the avid English lunar observer, W. R. Birt, who immediately set to the task of corroborating Schmidt's finding. Birt alerted British observers through notices in the Astronomical Register and the Monthly Notices.⁶²

Huggins first examined Linné in December 1866 and monitored it sporadically through December 1873.  Judging by his notebook entries, Huggins had shown no interest in lunar surface features before 1866.  But nearly two years previous, he had searched for evidence of an atmosphere on the moon by observing through a spectroscope the extinction of the light from a star as it was occulted by the moon.  The negative results of this effort were interpreted by him as probable, though not conclusive, evidence against a lunar atmosphere.⁶³  Renewed speculation that changes in lunar features might be caused by the weathering effects of an atmosphere drew him to examine Linné.

In his notebook entries on Linné, Huggins referred to the region ascribed to the crater as a "white hazy patch" and "less defined" than other areas on the moon's surface.⁶⁴  On 8 May 1867, he suggested that the crater Hercules also presented what he called a "twilight" appearance.  Huggins claimed the twilight effect was absent in other more sharply defined craters.  But he did not view this as evidence of a lunar atmosphere.  By the time he wrote his first paper describing his observations of the crater's appearance, he had learned of Schröter's earlier depiction of the feature as a round white spot.  Huggins attributed Linné's "cloudy appearance" to a "peculiar, partly reflective property of the material of which Linné consists."⁶⁵

In January 1874, Huggins submitted a summary of six years of observations of Linné⁶⁶ including selected extracts from his notebook records of the appearance of the crater under different degrees of illumination.⁶⁷  It was Huggins' conclusion that changes in the crater were, in fact, illusions caused by variations in the direction of the light hitting the moon's surface in that region.⁶⁸

Thermometric Research

Contemporaneous with Huggins' observations of the crater Linné, a new and completely different type of observation captured his attention, namely measurement of heat reaching the earth from the moon and brighter stars.  Between 14 February and 3 June 1867, Huggins recorded observations on 18 separate occasions, 13 of which included some kind of thermometric work.⁶⁹  He made no public announcement of these efforts until February 1869 when he described what he had done both in his yearly Observatory Report in the Monthly Notices and in a brief paper submitted to the Proceedings.⁷⁰

In his Proceedings paper, Huggins explained that he undertook this difficult work because he believed reliable quantitative measures of the heat of stars could be used to complement spectral data in the important task of determining the "condition of matter from which the light was emitted in different stars."⁷¹  He did not found this conjecture on any physical theory, however.  In addition, Huggins did not mention in either his private notebooks or his published accounts why he decided to measure the heat of stars.  Nor does he divulge how he learned about the instrumentation required for these measurements.

Huggins' thermometric research has been ignored by his biographers and by historians of astronomy.  Lawrence Parsons, the fourth Earl of Rosse, and Edward James Stone, the First Assistant at Greenwich are the individuals normally associated with thermometric observations of celestial bodies during this period. Both of these men, however, began their work long after Huggins had given it up and were ignorant of his earlier efforts.⁷²

Ronald Brashear has compiled a detailed chronology of early attempts to measure the quantity of radiant heat that reaches the earth from celestial bodies.⁷³ Brashear draws attention to a number of individuals who developed ingenious methods of adapting the thermopile to the telescope for such purposes in the decades preceding Huggins' stellar heat measures.⁷⁴  Thus, there were precedents for this project and published reports from which Huggins could have drawn motivation and technical assistance.  But if his previous performance is any clue, Huggins did not derive his research questions from the existing literature.  That Huggins elected to work on the measurement of the heat of the moon and stars at this particular time, a task which involved the acquisition and mastery of an entirely new kind of instrumentation and investigative method presents something of a puzzle.

One clue may be found in the minutes of the RAS meeting on 10 January 1867, just one month before Huggins' first thermometric observation was recorded. At that meeting, J. Park Harrison presented a paper on the radiation of heat from the moon.  Harrison sought to show, through analysis of long term records of terrestrial temperatures kept at the world's major observatories, that these temperatures were directly related to lunar phase because sunlight reflected from the moon's surface had the capacity to evaporate cloud cover on the earth.  What Harrison was arguing was not new.  Nearly twenty years earlier, John Herschel had presented nearly identical views in the first edition of his classic Outlines of Astronomy.⁷⁵  Still, the presentation generated considerable discussion.⁷⁶

The fluidity of disciplinary boundaries at this time was exposed as Warren De La Rue expressed doubt that Harrison's work fell within the purview of the RAS.  He suggested that perhaps the Meteorological Society might be better suited to handle such discussion.  RAS President Charles Pritchard indulgently responded, "I think we have the right to keep the Moon here."  De La Rue then pressed Harrison on his research methods:  "Could not Mr. Harrison measure the heat from the Moon by a thermo-electric apparatus?"  But Harrison was convinced that the heat was "used up in the atmosphere" leaving little or nothing to measure.  The subsequent discussion was lively if inconclusive, drawing on the authority of Greenwich and the absent John Herschel for confirmation of Harrison's results.⁷⁷  In the next month's number of the Astronomical Register, a report from Joseph Baxendell refuted Harrison's claims and the subject was never raised again.⁷⁸

William Huggins had no interest in accounting for terrestrial temperature fluctuations, but it is intriguing to speculate on the indirect impact Harrison's presentation to the RAS may have had on him at that time.  Thermometric work involved a greater degree of instrumental intervention than was commonly used in astronomical work.  No human sense directly received the information being examined and measured.  Instead, a thermopile was placed at the focus of a telescope aimed at a particular celestial body.  The apparatus was arranged so that, ideally, heat of that body would trigger a differential expansion in two kinds of metal in the receiving end of the thermopile.  The instrument then converted the slight temperature difference into an electrical impulse, the strength of which could be inferred by the degree of deflection observed in the attached galvanometer's needle.

Huggins may have been encouraged to try to measure the heat of the moon and stars from an interest in the instrumentation and the gadgetry rather than any theoretical concerns.  Since Huggins is silent on the matter of what motivated him to begin his thermometric investigations, we cannot know for sure, but if instrumentation was the lure, then John P. Gassiot may have served as Huggins' expert link to past efforts in this unusual research area.  The elderly Gassiot (1797-1877), was an eminent electrical researcher and maker of thermopiles who lived near Huggins in Clapham.⁷⁹  Although Huggins states that he had his own thermopile constructed by a Mr. Becker of Messrs. Elliott Brothers (see Figures 21a and 21b), perhaps Huggins obtained his tacit knowledge on how to employ such an instrument with a telescope from Gassiot.⁸⁰

Figure 21a.  Notebook sketch of William Huggins' thermopile (from entry 27 May 1867, Notebook 2).

Figure 21b.  Published drawing of William Huggins' thermopile [from Proc. Roy. Soc. 17 (1869):  310].

In any case, Huggins worked hard to cajole consistent results from his apparatus.  He drew diagrams of his equipment, gauged the accuracy of his measures on the basis of the consistency of the data he collected, suggested possible sources of error and described modifications which he felt would reduce those errors.  He even provided advice to Thomas Romney Robinson on techniques of carrying out such research in early 1869.⁸¹  In the end, however, his disappointment over the unreliability of the results, coupled with the difficulty in converting deflections of the galvanometer's needle into an equivalent quantity of heat persuaded him to abandon thermometrics in favor of other projects.

Motion in the Line of Sight

By the 1860s, astronomers had long accepted that stars move in relation to one another, but since stars are mere points of light to an earthbound observer, it was only possible to determine a star's apparent motion across the field of view, its "proper motion."⁸²  Normal visual cues for judging an object's motion towards or away from us, its "radial velocity," are missing in the case of stars.

In April 1868, Huggins submitted a paper to the Royal Society describing his recent research on the relative motion of the earth and stars along the line of sight. In this paper, he compared the positions of stellar spectral lines with those produced by a known terrestrial light source.  Huggins professed to having been concerned about this issue from the very beginning of his work on stellar spectra, but claimed he was prevented from pursuing this line of research by a lack of precision in his instrumentation.  If that were the case, there is no indication in any of his notebook entries that he entertained such an interest, nor that he had made any such effort.

In his introductory remarks to the paper, Huggins reminded the readers of his Philosophical Transactions paper on the motion of stars towards or away from the earth, that some 25 years earlier, in 1841, the Austrian physicist, Christian Doppler, had derived a mathematical principle which stated that the pitch heard emanating from a moving sound source was dependent on the source's direction and rate of motion relative to the listener.  Doppler himself had suggested that the same principle should apply to light emitted by stars and, in fact, hypothesized that the color of stars might be attributed to such stellar motion along the line of sight.⁸³ Doppler's prediction concerning the pitch of a sound emitted by a moving source was confirmed by Buijs Ballot a few years later.⁸⁴  At the time, there was no comparable way to establish that the same principle could be applied to luminous bodies, although the controversy surrounding Doppler's claims encouraged his followers to attempt this feat.⁸⁵

Huggins noted the results of others' attempts to measure stars' motion in the line of sight, including recent papers by W. Klinkerfues⁸⁶ and Father Secchi.⁸⁷  But he criticized their observational methods, and, in the case of Secchi, the faulty theoretical foundation for interpreting the observations he had made.⁸⁸  In contrast, Huggins boasted a superior array of instrumentation which, he argued, permitted him to make the fine distinctions required in the positions of the spectral lines found in celestial sources compared to stationary terrestrial sources.  While Huggins continued to use his 8-inch Alvan Clark refractor, he acquired a new micrometer and a new and more dispersive train of prisms.  He also created an improved method for calibrating the telescopic spectrum with that of the comparison terrestrial spectrum.  Such instrumental changes, in Huggins' view, made the detection of any slight difference in position easier and the measurement of the difference less prone to error.  In describing these improvements, Huggins displayed a sophisticated appreciation for the causes of observer or instrumental error.  He also demonstrated a creative knack for developing an experimental design to eliminate what he perceived as potential sources of those errors.  These factors helped Huggins convince other astronomers of the research potential of this innovative spectroscopic application.

On 12 June 1867, Huggins received a lengthy and technical letter from the physicist James Clerk Maxwell (1831-1879).  Huggins may have become acquainted with Maxwell during his earlier collaborative efforts with Miller, as Maxwell was, until 1865, professor of natural philosophy and astronomy at King's College, London where Miller taught chemistry.⁸⁹  Certainly, the two would have had occasion to meet through the Royal Society.  The June 1867 letter was a reply to Huggins' request for Maxwell's views on the effect of the relative motion of a luminous body on the appearance of the radiation it emits as seen by a viewer on earth.  An excerpt of this now-famous letter was included by Huggins in his Philosophical Transactions article to put some British authoritative weight behind the theoretical foundation of his research.⁹⁰

Maxwell described two experiments based on the application of Doppler's principle to luminous bodies.  The second experiment, which Maxwell had in fact performed, does not seem to have interested Huggins.  It involved an attempt to determine the motion of the earth through the luminiferous ether by measuring the small changes in the index of refraction in a prism as light was made to travel through it in different directions relative to that of the ether flow.

The first experiment, which Maxwell had not attempted, intrigued Huggins and formed the basis of his research on the motion of stars in the line of sight. Maxwell suggested that if an individual line in a star's spectrum could be positively identified with one produced by a known terrestrial element, then observing the two spectra simultaneously would reveal any lack of coincidence in the lines' positions. This difference, analogous to Doppler's change in pitch in the case of sound, could then be used to ascertain the relative motion of the star and the earth.  Using a simple hypothetical example of a stationary star being observed from the earth in orbit around the sun, Maxwell calculated it would be necessary to employ instrumentation capable of detecting a shift on the order of one-tenth of the distance separating the sodium D lines, or about 0.6Å in modern terms.⁹¹  While the precision of Huggins' old micrometer may have been adequate for measuring distances between binary stars -- each whole revolution of the micrometer screw head registered a shift of only a little more than 17 arcseconds -- it was insufficient to detect a shift of only 0.6Å.

On 10 May 1867, Huggins purchased a new double-image micrometer from William Dawes.  He immediately tested its ability to measure the angular distance between the close binary pair g Leonis which at that time was separated by approximately 3 arcseconds.  A few weeks later he began determining the value of the vernier divisions on this new micrometer.⁹²  Huggins claimed that an interval measured by two-hundredths of a division of the micrometer screw head was equivalent to a wavelength difference of 0.046 millionths of a millimeter (or a little less than half an angstrom in modern terms).⁹³  Based on information Huggins had supplied earlier on the precision of his old wire micrometer, we can infer that each division of that instrument allowed him to register a displacement equivalent to about 2Å.⁹⁴  By comparison, an equivalent turn of the screw head on his new micrometer was able to register a shift of less than one-quarter angstrom, an interval small enough to note the minute shifts anticipated by the relative motion of stars and earth.

Armed with his new micrometer and with information about the specific position of emission lines in stationary terrestrial gases, Huggins attempted to observe the shift of stellar spectral lines.  On 25 June 1867, less than two weeks after receiving Maxwell's letter, he aimed his telescope armed with a new arrangement of "two dense 60° prisms ... Hoffman's [sic] & Ross's dense prism on a Bootes [Arcturus]."⁹⁵  As mentioned earlier, Arcturus's large proper motion had been noted by Edmund Halley in 1718.⁹⁶  Huggins may have reasoned that a star with a large proper motion would show a more readily noticeable motion along the line of sight as well.  He recorded in his notebook, "Atmosphere unfavourable.  The lines seen, but not with the steady distinction necessary for determining whether they have any motion towards or away from the earth."⁹⁷

It is unclear what his observational objective was the following night.  He selected the beautiful binary pair, E Bootes, and took "one measure on each side of zero ... with wire micrometer."  The blue/orange contrast of the components of this pair suggests a test of Doppler's contention that a star's color might be associated with its radial velocity, but Huggins only recorded data useful for determining the angular separation of the two stars.⁹⁸  Huggins did not record any additional observations relevant to stellar radial velocity until February of the next year even though he had obtained a new and more highly dispersive spectroscope in late November 1867, equipped with two dense Hofmann's compound prisms, two 60° prisms of Guinand's glass made by Simms, and one 45° flint prism made by Browning of Chance's glass (see Figure 22).

Figure 22.  Schematic drawing of high-dispersion spectroscope showing arrangement of two direct-vision spectroscopes in combination with three simple prisms [from Phil. Trans. 158 (1868):  536].

The highly dispersive compound prisms he used were similar in construction to hand-held direct vision spectroscopes.  They each consisted of a train of five prisms, including two very dense flint prisms placed alternately among the other three and cemented together with Canada balsam thus reducing light loss at surface boundaries.  These compound prisms made by Hofmann were arranged by Huggins along with the Browning and Simms prisms in such a way as to avoid the necessity of placing the prisms in a nearly complete circle, an awkward arrangement for telescopic observation.  Huggins boasted that this improved instrumental design enhanced the observer's convenience even more by making it possible to completely remove one of the two compound prisms in order to observe faint objects.⁹⁹

If Huggins began immediately to make observations of stars for the purpose of measuring their radial velocities, he did not record any such work in any surviving notebook.  Nevertheless, he did record observations of a number of familiar objects over the next four months to gauge the new spectroscope's performance against earlier observations with his old instrument.  He seems to have needed this time to become used to manipulating his new instrument.¹⁰⁰

On 11 February 1868, Huggins directed the new spectroscope with both compound prisms at the star Sirius.  He compared the Fraunhofer F line in Sirius to that of the hydrogen spark.  (Refer to the excerpts and accompanying diagrams from Huggins' notebook entries reproduced in Figures 23a and 23b.)  The F line is the bright blue-green line in the hydrogen spectrum at 4861Å. 

Figure 23a.  Excerpts from notebook entries made in February 1868 including sketches of observed position of hydrogen F line in spectrum of Sirius compared with that of terrestrial hydrogen sample (from William Huggins, Notebook 2).

Figure 23b.  Excerpt from notebook entries made in March 1868 including sketches of observed position of hydrogen F line in spectrum of Sirius compared with that of terrestrial hydrogen sample (from William Huggins, Notebook 2).

Huggins wrote:

Compared this with the line of H in tube with Se.  Appeared to me very slightly more refrangible than line of H....  Afterwards compared the red line.  It is defined in H.  With a narrow slit could not see line in Sirius distinctly enough, but suspected that it was also very slightly more refrangible."¹⁰¹

There is nothing here to indicate that Huggins interpreted this observation as evidence that Sirius and the earth were moving toward one another or that he ascribed any physical interpretation to the observation.

Nearly two weeks later, on 24 February, Huggins made a second comparison of the F line in Sirius with that produced by a hydrogen spark:

The bright line [of] H was seen thin and sharp.  It was not coincident with the centre of the dark line [in Sirius] but just grazed the more refrangible edge.  It was therefore more refrangible than the dark line by half the thickness of the dark line.  The thickness of dark line I estimated to be about equal to interval of sodium lines.  This would make the line of H more refrangible by rather less than half of width of sod. [sodium] say .4.¹⁰²

Here he has begun to write in terms of visual comparisons suggested to him in Maxwell's June 1867 letter.  Also, Huggins' ascription of a numerical value to this otherwise rough-sounding visual comparison implies he intended to calculate the relative velocity of Sirius in the line of sight.  The proportional difference Huggins claimed to have observed in the positions of the two lines was four times larger than that Maxwell suggested would be observed due simply to the motion of the earth in its orbit.  Huggins does not mention the direct conflict of this observation with that made on 11 February, in which he observed the F line of Sirius to be more refrangible than that of the comparison hydrogen spectrum.  The next night, other stars were examined in a similar fashion, but Huggins could not see their spectral lines steadily enough to let him make comparisons.¹⁰³

At this point, Huggins was still employing a means of making comparisons between his celestial subject and a terrestrial spark which he and Miller had used in their early work on stellar spectra.  This had been adequate for noting gross similarities in groupings of spectral lines and judging them to be indicative of the presence of identical elements.  Shifts in position of a particular group of a star's spectral lines relative to its counterpart in the comparison terrestrial spark were not as important then, because there was no interest in the difference.  Now that many lines had been positively identified with a particular element, that difference became the focus of attention and careful alignment of the telescopic spectrum with the comparison terrestrial spectrum became critical.

Huggins' observations convinced him to develop a new arrangement for throwing the comparison spark into the telescope.  In his paper, he stated that the unreliability of his earlier measures using the old method had forced him to reject "observations of many nights."¹⁰⁴  The notebook record contains only a few measures, however, raising serious questions about his method of selection.¹⁰⁵ Huggins completed the task of designing a new comparison arrangement on 5 March which included the placement of two small silvered pieces of glass arranged so as to produce two comparison spectra which appeared to the observer both above and below the celestial spectrum in question.¹⁰⁶

After some instrumental alignment adjustments, Huggins turned to the comparison of the F line in Sirius with that of hydrogen, but now he complained that he could not be sure of the proper placement of the hydrogen spectrum: "Made the observation at least 20 times but without absolute certainty.  Upon the whole, my impression was that of coincidence, but if there was any difference line of H on more refrangible side."¹⁰⁷  A few days later, he observed again using the new apparatus but this time with only one of the compound prisms.  He recorded, "I am almost certain after a great number of trials that the line of H is a very little more refrangible than line in Sirius.  Certainly not more than 1/2 its breadth."¹⁰⁸ His confidence in these observations was soon shaken, however, as evidenced in his next notebook entry recorded just two days later:

Certainly the line of H appeared on the less refr. side of the line in Sirius.  The opposite of the very satisfactory observation with tube before object glass.  Arranged tube before obj. glass, but clouds prevented any further observation.¹⁰⁹

By 18 March, Huggins had become convinced that it was difficult to be sure of the exact calibration of his celestial and terrestrial spectra.  In the past, he had contented himself that the two instruments were properly aligned by placing a luminous object directly in front of the object glass of the telescope to serve as a substitute star and then adjusted the arrangement of the apparatus for viewing the terrestrial spectrum until it coincided with that produced by the surrogate star.  He soon discovered that slight changes in position of the light source serving as his stellar stand-in resulted in dramatic differences in the location of its spectrum, thus throwing into question the accuracy of any calibrations that had been considered already made.  He wrote:

Vacuum tube before the Obj. glass.  Atmosphere not very good. When best seen the line of vacuum tube always more refrangible than the line in Sirius.  Once or twice as the star was going out [of the field of view] I was not quite so sure of this, & I had almost the impression that they were more nearly coincident.  I am not sure of this, but certainly when line best seen, then ... the line of H appeared, as above, more refrangible.  Still it will be well, while the slit remains as it is, to see with some other stars, whether or not the motion of the star sensibly alters position of line.

Afterwards with same app. both compound prisms being inserted, compared line of Na in induction spark between pt. [platinum] wire & Mg with lines in alcohol lamp placed before the end of the adapter.... I found it was possible by moving the lamp to alter position of lines from lamp relatively to those of spark by a quantity about equal to abt. half the distance of the separation of lines of Na.

I then placed a diaphragm of paper in the end of the adapter [holding the spectroscopic instruments in place] with a small hole about 1/4 dia. in centre.  I then held the lamp before the hole, it was no longer possible to alter position of lines of Na.  I found however that these lines did not coincide with lines of spark, but were about 1/3 or 1/4 of distance of lines of Na more refrangible.  This state of things accounts perfectly for my recorded observations of Sirius.

On 30 March, Huggins returned to the comparison of Sirius with hydrogen.  This time he "strongly suspected" that the hydrogen line was indeed more refrangible than that of Sirius:

Taken in connection with the satisfactory result of Feby 24, it may be considered as certainly confirmatory of the observations on that night. I made numerous comparisons during an hour, almost always the same result."¹¹⁰

By 4 April, Sirius was too low in the sky to continue the comparisons. Huggins compiled his stellar observations into a paper which was submitted to the Royal Society on 23 April.  He was satisfied that he had successfully resolved the instrumental problems he had faced, and stressed the care he had taken to insure the reliability of his measures, as well as underscored the soundness of the theoretical foundation for his interpretation of those measures.  The tone of the paper is therefore one of confidence and spirited adventure.  All obstacles named are overcome either by clever manipulation of instrumentation or enviable patience. Observations considered unworthy can be justly disregarded because sufficient numbers of them were made in the first place to allow for judicious weeding.

The question of which observations "count" as supporting a particular interpretation is an important one.  Huggins does not provide the reader with any clues to the total number observations he actually made and, of those, how many were discarded and why.  Nevertheless, his method of presenting the data and his interpretation of it conveyed a feeling of extreme judiciousness and care.

Figure 24.  Published diagram showing position of hydrogen F line in spectrum of Sirius compared with that of terrestrial hydrogen sample [from Phil. Trans. 158 (1868):  Plate XXXIII].

The Astronomer Royal, George Airy, served as a referee for this paper.  In his report of 18 July 1868, Airy complimented Huggins' detailed descriptions of his apparatus and his exposition of the observational difficulties which had led to adjustments to this arrangement.  However, Airy questioned the legitimacy of assuming that the line "at or near F observed in Sirius" was, in fact, due to hydrogen.  It seemed "illogical" to Airy that one could argue on the one hand that the line was due to hydrogen because it coincided with the line in hydrogen seen in terrestrial comparison spectra, and then to argue on the other hand that its lack of coincidence indicated that the star was in motion.  In addition, he argued that Sirius was a difficult star to use as a test case because of the fact that it is always located close to the horizon where atmospheric distortion will certainly plague the observer. He urged observations of some more northerly stars.  Still, he deemed the paper a "very important one," and recommended it be published in the Transactions.¹¹¹

The physical theory supporting Huggins' line-of-sight measures was a challenge to the best of contemporary astronomers.  Huggins was soon called upon to elucidate further the basic premise of Maxwell's mathematical method of determining a star's radial velocity to RAS President, Charles Pritchard.  In a rather hastily written letter of 6 July 1868, Huggins recounted the major points of Maxwell's letter, including the mathematical formulae for making the necessary computations.  He also included a brief discussion of Doppler's principle.¹¹²  A few years later, Airy himself wrote to George Stokes:

In some of the German pamphlets now afloat, on optical subjects, there is repeated allusion to das Doppelsche Princip, or some such term, conveying the idea that a man named Doppel has introduced some optical principle.  It has something to do with change in the velocity of light but I see no clear description of it -- can you help me?¹¹³

Stokes replied by citing information about Doppler's work contained in Huggins' 1868 paper, the very paper Airy had himself refereed!

As we have seen in the last chapter, Airy had conducted a brief foray into the spectroscopic study of stars in 1863, about the same time Huggins launched his own research efforts in that area.  But such observations did not fit into Airy's program of systematic and repetitive observations of celestial bodies to record changes in their positions.  Celestial mechanics was not concerned with the chemical composition of stars.  Nevertheless, spectroscopic study of the sun found its way into the Greenwich routine beginning in 1872¹¹⁴ and Airy eventually began to inquire about the utility of spectroscopy as a way of corroborating other kinds of observations being planned, such as the transit of Venus.¹¹⁵

It is important to note here, although it will be discussed in more detail in chapter 5, that George Airy's newfound interest in astronomical physics was not simply a response to the success of Huggins' spectroscopic work.  It was a visceral reaction to attacks by a vocal and energetic group of British scientists on the perceived inability of Greenwich, as it was currently structured, to do what was necessary to maintain and even enhance Britain's stature in physical science.¹¹⁶  Led by Lieut.-Colonel Alexander Strange (1818-1876), an expert designer of scientific instruments and advocate of State support for science, this group wished to establish a number of national physical laboratories which would assume responsibility for prosecuting a research program which included the new astronomical problems and methods.  Airy viewed this move as a moral trespass -- a usurpation of his job as Astronomer Royal.¹¹⁷  He worked hard and quickly to demonstrate that Greenwich was capable of incorporating spectroscopy into its daily routine.¹¹⁸  As a result of Airy's efforts, Greenwich published for the first time in 1876, results of line of sight measurements conducted by E. W. Maunder and W. H. M. Christie.¹¹⁹  Thus it was that routine measurement of stellar radial velocities became an element in the research agenda of the mathematical astronomers, although, as A. J. Meadows has pointed out in his volume on the history of the Greenwich Observatory, spectroscopic study of any sort was given low priority until after the turn of the century.¹²⁰

In conceiving the method by which Doppler's principle could be applied to astronomical research, contriving instrumentation to meet the exacting specifications that the measurements required, and introducing simple adjustments to improve the precision of those instruments, William Huggins gave astronomy an elegant and reliable research tool of broad utility.  Even so, I would argue that his greatest contribution to the introduction of this new method into astronomical research lay in his successful persuasion of his contemporaries that he had, in fact, accomplished what he claimed, in spite of the overwhelming mensurational and interpretive difficulties the method entailed.  In the end, however, it must be said that Huggins had unwittingly introduced a method the ready implementation of which was beyond the ken, resources and ability of many of his fellow amateurs, thus effectively closing doors to them in favor of the mathematical astronomers.

NOTES
[click on footnote number to return to text]

• Preface

• Introduction

• Chapter 1—

• Chapter 2—

• Part 1—

The Astronomical Agenda:  1830-1870

• Part 2—

"A sudden impulse..."

• Part 3—

Reception of Spectrum Analysis Applied to the Stars

• Chapter 3—

• Part 1—

Cultivating Advantageous Alliances; Opportunism and Eclecticism

• Part 2—

Opportunism and Eclecticism (continued)

• Part 3—

Achieving "A mark of approval and confidence"

• Chapter 4—

• Part 1—

The Solitary Observer

• Part 2—

Celestial Photography

• Chapter 5—

• Chapter 6—

• Part 1—

The Red Flames

• Part 2—

The Eclipse Expedition to Oran

• Part 3—

Photographing the Corona Without an Eclipse

• Part 4—

The Bakerian Lecture

• Conclusion

• Note on Sources

• Published Papers of William Huggins

• Bibliography

• Vita