Maxwell was present in the audience on May 26, 1864, when William Huggins and William Allen Miller presented their paper, “On the Spectra of Some of the Fixed Stars,” before the Royal Society. The discussion afterward turned to star colors. Huggins and Miller proposed that a star’s color derives from a combination of chemical and physical conditions in its incandescent envelope, and is not imposed by external factors, such as the Doppler effect. They noted that in their limited sample, a star’s color often relates to the distribution of its absorption lines: A reddish star like Betelgeuse features a profusion of lines in the green and the blue, whereas whitish Sirius displays a rather uniform obscuration of its colors. Of course, further observations were needed before Doppler’s explanation of star colors could be ruled out.
At this point, Maxwell announced to his colleagues that there already exists a way, in principle, to test the action of the Doppler phenomenon on the colors of stars. With that, he introduced Fizeau’s proposition that “if the colours were really tinged in consequence of the motion of the star or our earth, the lines in the spectrum of the star would not be coincident with the bands of the metal observed on the earth, which gives rise to them.” The shift of the Fraunhofer lines, Maxwell calculated, would be exceedingly small, but potentially measureable with a very high dispersion spectroscope.
There was only one person in the audience that evening with the expertise to run Maxwell’s gauntlet: William Huggins. Over the next three years, while pursuing other projects, Huggins refined his telescope’s clock drive and acquired a higher-dispersion spectroscope and a more precise micrometer. Despite the improved apparatus, he understood the operational challenge of discerning slight wavelength shifts of stellar spectral lines. Even as starlight is enfeebled by the dispersive action of the spectroscope’s train of prisms, the entry slit must be reduced to the merest sliver, to render the lines as narrow as possible. But a razor-thin slit virtually extinguishes the continuous spectrum against which the dark lines are seen; at some point, the visual contrast between the lines and their backdrop becomes all but imperceptible.
Also, if individual line-shifts are to be gauged, it becomes imperative that the stellar and comparison spectra be properly aligned with respect to each other in the field of view. In Huggins’s study of the chemical compositions of stars, precise spectral alignment was important, but not pivotal to the realization of the project. Each chemical substance generates a unique pattern of spectral lines; thus, Huggins could rely on a redundancy of line coincidences to support a given identification of a stellar element with a terrestrial element. Measuring a radial velocity inverts the experimental logic: instead of adopting a line coincidence as the sign of an elemental matchup, Huggins would have to assume that the nominal line coincidence has been disrupted by the star’s motion; any wavelength disparity between the stellar and terrestrial lines is attributed to the Doppler effect.
Huggins made a series of radial velocity attempts between June 1867 and March 1868, acquiring his new thirteen-prism spectroscope midway through. He focused intensively on bright Sirius, comparing the position of its F line of hydrogen to one generated in an electrified discharge tube in the observatory. The observations were dogged by continual misalignment of the stellar and terrestrial spectra, requiring Huggins to develop a completely new means of diverting light from the discharge tube into the spectroscope.
On April 23, 1868, Huggins submitted his report to the Royal Society, concluding that the F line in the spectrum of Sirius is redshifted compared to its terrestrial counterpart: Sirius is speeding away from Earth at 29.4 miles per second. Huggins’s result stood in stark contrast to that of Italian spectroscopic expert Angelo Secchi, who simultaneously reported no measurable line shift. In retrospect, the significance of Huggins’s paper lies not in its quantitative result, which is indeed way off the mark (Sirius moves at six miles per second toward Earth), but in its symbolic place in the rise of astrophysical observation. It is equal parts instructional and aspirational. Huggins delineates the theoretical and mathematical foundations of stellar radial velocity measurement, quoting liberally from his correspondence with Maxwell.
The need for such review is evident from contemporary accounts of astronomers’ lack of grounding in Doppler’s and Fizeau’s theories. Both Royal Astronomical Society president Charles Pritchard and Astronomer Royal George Airy sought assistance with these novel concepts. Evidently, Airy forgot Huggins’s report, for he later wrote Cambridge mathematician George Stokes seeking an explanation of “das Doppelsche Princip,” a term he kept encountering in German research papers. “It has something to do with change in the velocity of light but I see no clear description of it—can you help me?”
In his 1868 report, Huggins goes on to detail the procedure to measure stellar radial velocities. He documents his Herculean efforts to overcome a host of instrumental difficulties and justifies the exclusion of certain measurements that did not conform to the final answer. And, although he asserts the validity of his radial velocity of Sirius, there is a sense that he knows it matters little whether it is right or wrong. He sees the larger ramification for the future, that in pushing the limits of technology to their utmost, he has begun to define “a new method of research, which, transcending the wildest dreams of an earlier time, enables the astronomer to measure off directly in terrestrial units the invisible motions in the line of sight of the heavenly bodies.”
In June 1872, now using a fifteen-inch Grubb refractor lent by the Royal Society, Huggins published a follow-up study of stellar radial velocities. Despite his own optimistic outlook on the results, the uncertainties of measurement proved to be nearly as large as the reported velocities themselves; bluntly put, the numbers were scientifically worthless. The upshot was clear: Spectroscopic determination of a star’s radial motion cannot be accomplished by visual means. The key features of the stellar spectrum are simply too faint and too cramped—and the human eye too subjective—to yield trustworthy measurements through the eyepiece of a small telescope. Yet no sooner had this visual door slammed shut than the photographic door opened, if only a crack.
Just two months later, across the Atlantic and up the Hudson, Henry Draper recorded the first photograph of a star’s spectrum with his new twenty-eight-inch reflector. Within the dime’s-width spectral band of Vega, a million times more remote than the Sun, appeared several of Fraunhofer’s absorption lines. Of course, there was no hope of gauging their Doppler shift on such a compressed scale, nor boosting the exposure times allowed by the crude wet-collodion process. Nevertheless, the future of stellar radial velocity measurement lay inchoate in that dusky image. Even as the visual study of stellar spectra exited the stage, its successor readied in the wings. The 1870s brought the essential chemical leap in photography—the gelatin dry-plate process—that would spark the auspicious merger of photographic and spectroscopic practice in astronomy.
Chapter 19
BURN THIS NOTE
I want to tell Huggins how much you have done—for strictly speaking between ourselves I think he is afraid of you now.
—U.S. Naval Observatory astronomer Edward S. Holden to Henry Draper, August 2, 1876
IN 1875, WITH HIS COLLEAGUE William Allen Miller dead now five years, William Huggins married Margaret Lindsay Murray, an Irish solicitor’s daughter and an astronomy enthusiast since childhood. Twenty-four years his junior, Margaret swept into her husband’s solitary household like an invigorating breeze. Serving initially as Tulse Hill’s “scientific housemaid,” as she wryly put it, she rose quickly to become William’s full-fledged research collaborator. Within a year, Margaret’s handwriting appears in the observatory notebooks, sometimes in a telltale first-person reference. Entries grow more detailed than before, and reveal her involvement in every facet of the work. She was the inexhaustible engine who kept Tulse Hill’s demanding night work on track. Only many years later would she be listed as coauthor with William of their various research papers. For now, she accepted h
er unsung role, impelled by her own inquisitiveness to take up a vocation otherwise closed to women.
In her recollections, Margaret opens a window onto the ritualistic tedium of astronomical observing, as well as the intellectual fire that drives one to endure this hard and sometimes hazardous line of work. She provides a glimpse of her ascension into the nuts-and-bolts domain of Victorian-era astronomy: “I had to teach myself what to do by degrees: at first I had my difficulties, but now my eyes are trained and are very sensitive. Also my hands respond very quickly and delicately to any sudden necessity. I can go and stand well at good heights on ladders and twist about well. (Astronomers need universal joints and vertebrae of India rubber.) . . . As I observe, I direct William as to what I need and he moves me bodily on my ladder, so that I am not disturbed more than is necessary.”
Margaret Lindsay Huggins.
The Tulse Hill research program, being astrophysical in nature, included a never-ending roll of daytime tasks to support the nighttime observations. In the household laboratory, for instance, the work is varied. “It may be photographic,” Margaret relates, “in which case I should help in arranging instruments, keeping the light right, and so on, if we are working on the sun. If working electrically, I should work the batteries, fix electrodes, and be generally handy. I may take a turn mixing up chemicals, pounding, weighing, dissolving, boiling—in short be a jack of all trades. When needful I dust and wash up the laboratories, for no housemaid is allowed into those sacred precincts.” Until William hired someone for the onerous task, Margaret took the cleaning of the laboratory’s steam engine in stride: “One is interesting with a lump of engineer’s waste in one hand and some nasty oily stuff in a can in the other.”
In 1874, a year before Margaret’s arrival at Tulse Hill, William Huggins had written to Henry Draper about the prospects for stellar spectrum photography. Having succeeded in 1872 where Huggins had failed, Draper was sanguine about his own efforts, but cautioned others: “I am very glad to learn that you think of continuing your former experiments in applying photography to stellar spectra. I have made some new trials in that direction with my silvered glass reflector of 28 inches aperture and find that I can get the great bands of Vega readily and even the spectrum of alpha Aquilae. It is a very difficult subject and requires so favorable a series of circumstances that a number of observers might work at it a long time before fine results were achieved.”
For a veteran visual observer like William Huggins, the photographic art was a substantial deviation from past practice. By its chemical operation, it lacks the immediacy of peering into the eyepiece and seeing whether anything is there, whether proper focus has been achieved, whether magnification should be modified. In telescopic photography, the astronomer is repurposed as an auxiliary device, to direct and stabilize the light stream into the camera, flip the shutter, swap plates, and pray that the accruing image, when developed, is worth the investment of time put into its making.
If Huggins balked at pursuing Draper’s “very difficult subject,” his partnership with Margaret appears to have banished any doubts. With evident determination, Huggins set about to transform his spectroscopic study of stars from visual to photographic, exploiting his wife’s longtime experience with the camera. (Photography was a rare, but growing, avocation of Victorian-era women.) The fifteen-inch Grubb refractor had been dismounted for an eighteen-inch reflector, also lent by the Royal Society, that was more suited to photography. Margaret reports that she “was occupied on all favorable days in testing and adjusting this photographic apparatus upon the solar spectrum: at the same time testing different photographic methods with a view to finding . . . the most sensitive and . . . the quickest method for star spectra.” Both wet-plate and dry-plate technologies were tried, with the latter winning out by summer’s end.
The photographic plates were one and a half inches long, with the spectrum itself crammed into a mere half-inch. Even so, the image definition sufficed to permit precise measurement of line positions under a microscope. Because of the lack of red-sensitivity of the photographic emulsion, only the violet and ultraviolet parts of the spectrum were recorded. The yellow D lines of sodium, for example, were literally out of the picture. (Such longer-wavelength spectral lines continued to be studied by eye.) Even with the telescope’s improved clock drive, manual correction was necessary to capture the delicate features of the spectrum. The beam of starlight cast onto the spectroscope slit had to be monitored for the entire exposure—typically fifteen to thirty minutes—and the appropriate corrective jogs applied to the telescope.
Throughout 1876, William and Margaret Huggins tested and tinkered equally, adjusting their apparatus for optimal alignment, stability, sensitivity, and ease of use. Nevertheless, a wealth of instrumental obstacles and dearth of limpid nights conspired to impose a glacial pace of progress. By year’s end, William Huggins was eager to assure the Royal Society that its telescope was being put to effective use. Although his results were preliminary at best, Huggins rushed a report on the spectra of Sirius and Vega to the Royal Society on December 6, 1876. Right up front, he hoists the priority flag, suggesting that the new project is a resumption of his failed photographic efforts with Miller in 1863. His concern over priority proved to be well founded.
Photographic spectrum of the star Vega (Alpha Lyrae) by William Huggins, September 1, 1876.
Just four months before their report to the Royal Society, Mr. and Mrs. Huggins received a visit from astronomer Edward S. Holden, sent by the United States government to assess the state of astronomical instrumentation in England. (Holden would later be appointed director of California’s Lick Observatory.) “Huggins is very pleasant & everything about him is thorough,” Holden informed his friend, Henry Draper, in New York, “his Obsy & working places are part of his house & every bit of apparatus in them works like a charm—smoothly and easily. He has a wife now, & she is devoted to him & science & altogether they seem to have work in them.” To Holden, the high-tech equipment at Tulse Hill was of the sort in Henry Draper’s observatory at Hastings-on-Hudson.
Since Draper’s breakthrough picture of the spectrum of Vega in 1872, he and his wife Anna had practiced their photographic spectroscopy on a variety of celestial objects. In December 1876, coincident with Huggins’s report to the Royal Society, Draper filed a progress note with the American Journal of Science and Arts. He confirms that, starting in October 1875, wet-collodion plates of Vega’s spectrum were obtained through both his twenty-eight-inch reflector and twelve-inch refractor. A number of spectroscopes were tried, but none produced an image markedly better than Draper’s original. Draper’s 1876 report only hints at the overwhelming tide of frustration that rises from his observatory notes. “The research is difficult and consumes time,” he complains, “because long exposures are necessary to impress the sensitive plate, and the atmosphere is rarely in the best condition. The image of a star or planet must be kept motionless for from ten to twenty minutes, and hence the driving clock of the telescope is severely taxed.”
In the midst of his spectroscopic travails, Henry’s father John counseled him from the sidelines: “You can either make star spectra, or examine different regions of the sun’s surface, and have a paper ready for the Astronomical Society before Christmas. To do this you ought to come out every day on the 4 o’clock train, have dinner on your table at 5, have the carriage at your door at 6, get to Hastings at 6½ and work till 9½, then go home. Permit nothing whatever to interfere with this program and you will accomplish a great deal.” With the publication of his report and with no evident route toward success, Henry Draper suspended his stellar spectrum studies. For the next two years, he was consumed by his controversial research on the alleged presence of oxygen in the Sun. Meanwhile, William and Margaret Huggins proceeded full-bore, gathering exposures of stellar spectra and hoping to get their results into print.
During the summer of 1879, while in England for a meeting of the Royal Astronomical Society, Henry and
Anna Draper rode out to Lambeth to visit William and Margaret Huggins. After pleasantries, they spoke about their respective attempts to photograph the spectra of stars, as well as about the faster gelatin dry plates now commercially available in London. Touring the observatory, Draper asked where he might acquire some of Huggins’s specialized prisms and lenses. Huggins arranged with his optical supplier that Draper return to the United States with the materials in hand.
“I was willing to show Draper everything,” Huggins confided to his influential American colleague Charles A. Young on January 31, 1883, shortly after Draper’s death. “He was greatly surprised at my spectra, and I told him the main points I had made out, and that my paper was in the course of preparation. I then offered to show him my special apparatus and arrangements; he immediately said ‘I should like to see everything but I have given up star spectra and I do not intend to do any more, you need not hesitate to show me your apparatus. . . . I believed him to be a man of honour.’” Having conveyed his distress, Huggins instructed Young to “burn this note.”
What sparked Huggins’s ire was Draper’s hurried resumption and publication of his stellar spectroscopy research. Draper presented his paper, “On Photographing the Spectra of the Stars and Planets,” to the National Academy of Sciences on October 28, and Huggins his paper, “On the Photographic Spectra of Stars,” to the Royal Society on December 11, 1879. Huggins firmly believed that Draper’s uncannily rapid progress could only have come from his adoption of equipment and methods perfected at Tulse Hill. Margaret Huggins mailed a copy of her husband’s 1883 letter to Edward Holden, a friend of both theirs and Draper’s. “You cannot imagine the pain this Draper matter has caused us,” she adds. “I was bitterly angry that my gentle noble-breasted husband should have been so used. . . . Who ever heard to any purpose of Dr. Drapers star results before his visit to Tulse Hill?”
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