Starlight Detectives Page 13
It would be a full four years after Draper’s 1872 milestone before anyone else recorded a stellar spectrum. Having succeeded in the photographic challenge of the decade, Draper contemplated the future of cosmic imaging—and it looked bleak. The obstacle was neither optical nor mechanical, but chemical. The wet-collodion process, which had triumphed over the daguerreotype, had by the late 1870s taken nighttime photographic astronomy as far as it could. Its relatively low light sensitivity and hard limit on time exposures drew an effective curtain around Earth, barring detection of objects in deep space. And it was there, in the remote shadows, where opportunities for discovery lay.
For Henry Draper—indeed for the entire astronomical enterprise—the future arrived with surprising swiftness. During the summer of 1879, Draper toured the observatory of his nominal rival, English spectroscopist William Huggins, who alerted him to a new dry-plate photographic process. Not only did the dry plates permit longer-duration exposures than wet plates, they were more light sensitive. And they were being produced commercially in London. When Draper boarded the ship back to America, inside his luggage were several boxes of dry plates, and inside his mind were his next photographic projects. First, he would redouble his efforts to record faint stellar spectra. Then, he would try to capture the face of a celestial object that was, by its very nature, even dimmer: a galactic cloud—a nebula. With the approaching dry-plate revolution, the curtain on deep space was about to be lifted.
Chapter 9
FROM CLOSET TO COSMOS
Collodion—slow old fogey!—your palmy days have been, You must give place in the future to the plates of Gelatine.
—From the rhyme “Gelatine,” British Journal Photographic Almanac, 1881
COMPARE THE LUNAR PHOTOGRAPHS of John Draper, Warren De La Rue, Lewis Rutherfurd, and Henry Draper, and the progress over time is clear, like the iconic cartoon of human ascension, from knuckle-dragging brute to quick-witted hunter. The aesthetic and scientific standards for celestial photography rose steadily between 1840 and 1880; the exceptional image of one decade became the norm of the next. To effectively render a lunar crater or resolve an ovate stellar image into a discrete pair of stars, inch-wide negatives had to be examined under a magnifier or enlarged and printed. As often as not, enlargement would reveal the graininess of the photographic emulsion, the celestial target itself camouflaged among a riot of grayish specks.
Much of the advancement in celestial imaging during this period was technological: the introduction of the wet-collodion plate and the simultaneous improvement of telescope drives. But there was also a human element that pressed the effort forward. The cadre of amateur innovators—practical men all—attacked the obstacles as engineers would: coolly analyzing problems; developing solutions on the fly; spending inordinate amounts of time (and money) on the minutest details; and, at least for the most passionate, never giving up.
Visual astronomers praised their photographic colleagues for their celestial curios, even as they judged the images to be proof of the human eye’s superiority over the camera. To date, no camera-equipped telescope had discovered anything of consequence in the cosmos. The images merely depicted what had been seen before through the eyepiece. The major advantage of the photograph—its objectivity—was trumped by its failure to surpass the resolving power of the eye, not to mention its inconvenience. Acceptance of the camera as a vital telescopic accessory awaited a dramatic improvement—in fact, a revolution—in image quality, photochemical sensitivity, and ease of use. Until then, it would remain a sideshow to the visual exploration of the heavens.
To the photographic astronomer, the wet-collodion plate was a technological dead end; the limit on exposure time left vast numbers of faint cosmic objects beyond the reach of the camera. Landscape photographers were equally desirous of a better process, unencumbered by the rolling darkroom. The wet-collodion plate had barely arrived before practitioners tried to convert it to a dry process. The experiments were more an alchemical lottery than a scientific investigation, featuring a host of pantry additives: licorice, raspberry syrup, beer, tea, coffee, honey, albumen, grape sugar, gin, and gum—so-called preservatives that promised to eliminate the need to wet the collodion plate prior to exposure. However, the gain in convenience offered by the various dry-collodion recipes was countered by a decrease in photosensitivity, a tradeoff no self-respecting celestial photographer would accept. It was hard enough, even in the 1870s, to get a telescope to accurately track a celestial object; to double or even triple the required exposure time merely replaced a chemical problem with a mechanical one.
The key breakthrough came in the form of a brief report by physician Richard Leach Maddox in the September 8, 1871, issue of the British Journal of Photography. Maddox was, by that time, almost two decades and several medals into his noxious hobby of wet-collodion photomicroscopy. This he pursued in an unventilated closet in his home at Woolston, near Southampton, in southeastern England. Long sickened by the fumes of the wet collodion, Maddox had conjured various dry versions, tossing in lichen, linseed oil, quince seed, pulverized rice, and tapioca. The loss in photosensitivity drove him to consider noncollodion alternatives, one of which was gelatin: a flexible, transparent, odorless substance derived from rendered animal tissues. Gelatin had been proposed as a silver-salt adhesive more than twenty years earlier. Maddox softened the gelatin in water, then added a couple of drops of aqua regia (nitric and hydrochloric acid). The subsequent addition of cadmium bromide and silver nitrate created a uniform suspension of light-sensitive silver bromide crystals. The milky emulsion was poured onto a glass plate and remained light sensitive, even when dry.
Maddox admitted that his formula was extremely slow, but, deferring to his medical practice, he hoped that his report might spark others to sort out the deficiencies. Experimenters soon found that the presence of the aqua regia retarded the action of the emulsion. So did the excess silver nitrate suspended in the gelatin; Maddox’s recipe used more than was necessary to maximize the production of light-sensitive silver bromide. The gelatin itself came under chemical scrutiny. Not only was it found that gelatin contains sulfur compounds that enhance the sensitivity of silver bromide, but the substance also protects any unexposed grains of silver bromide from the action of the developer, generating improved contrast over wet-collodion pictures. Gelatin was more than just a passive matrix for the photosensitive compounds; it was a true chemical advance over collodion.
Primitive gelatino-bromide dry plates appeared on the commercial market in small numbers starting in 1873; by decade’s end, some twenty brands of precoated plates were available. No longer was a traveling darkroom required; the new dry plates remained sensitive for months, and they could be developed at leisure. Touring the Middle East in 1882, Philadelphia photographer Edward L. Wilson carried his plates for “twenty-two thousand miles on steamer, on donkey-back, on camel-back, and across the Atlantic and the Mediterranean, through the hills of Arabia, in Egypt and other hot countries of the East, and developed eight months afterwards, again in Philadelphia.” Echoing the elation of his fellow photographers, Wilson declared, “Blessed be the dry plate!”
A further breakthrough came with the discovery that “ripening” the emulsion for several days at low heat greatly enhances its sensitivity. By the late 1870s, exposure times had plummeted. Photographers could freeze-frame fast-moving objects: the image of water droplets falling from moistened flowers created a sensation at the South London Photographic Society. The hoary thirty-minute daguerreotype had evolved into the dry-plate snapshot. It didn’t take long for the rising drumbeat of commercial photographers to fire up their astronomical brethren. Photographic technology had caught up to the demands of the observatory.
During the summer of 1880, no doubt influenced by his friend Lewis Rutherfurd’s example, Henry Draper traded in his twelve-inch visual refractor for an eleven-inch photographic refractor from the boutique optical house of Alvan Clark and Sons in Massachusetts. Like Rutherfurd’s
thirteen-inch refractor, Draper’s eleven-inch could be used either as a conventional optical telescope for direct viewing by eye or—with the attachment of a correcting glass—as an outsize telephoto lens for a camera. Like its predecessor, the new instrument was bolted parallel to the existing twenty-eight-inch reflector; an object sighted in one would appear simultaneously in the other. Draper’s rustic observatory at Hastings, coupled with its city-based laboratory, had become one of the best-equipped celestial research facilities in the world.
Under a moonless sky on the night of September 30, 1880, Draper swung his twin telescopes toward the Orion Nebula, a luminous patch below the trio of stars that define the mythical hunter’s belt. Although bright enough to be discerned by the naked eye and a majestic sight in a telescope, the diaphanous billow had so far eluded the photographic plate—a circumstance Draper hoped to remedy that evening.
Henry Draper’s common-mounted reflector and refractor telescopes, 1880.
Ancient skywatchers surely saw the Orion Nebula; yet the oldest extant report of its existence dates to 1611, shortly after the telescope’s introduction. The noted Dutch observer Christiaan Huygens published a crude sketch of the nebula in a 1659 book about Saturn. Other depictions followed, from an international roster of astronomical heavyweights: Charles Messier in France, J. L. Schröter in Germany, John Herschel in England, William Bond in the United States, Otto Struve in Russia. The disparity among these hand renderings stoked speculation that the Orion Nebula is coalescing, dispersing, or otherwise evolving.
George Bond at Harvard took up its visual mapping with a particular fervor during the 1850s, after Struve criticized his father’s work on Orion. (Although evangelical about photography, George Bond knew that the nebula’s diffuse light was insufficient to activate the wet-collodion plates of the time.) Bond’s assistant, Asaph Hall, who would later discover the moons of Mars, recalled these marathon sessions with Harvard’s Great Refractor: “[H]ow cold my feet were when he was making his winter observations on Orion. I sat in the small alcove of the great dome behind a black curtain, and noted on the chronometer, the transits of stars when Professor Bond called them out. . . . Sometimes I was called to the telescope to examine a very faint star, or some configuration of the nebula. Professor Bond had one of the keenest eyes I have ever met with.”
To chemically render the nebula’s subtle swells and filaments, a time-exposure of unprecedented length would be required, during which the image had to remain utterly still and in focus. Draper decided to attach his camera, not to his light-efficient twenty-eight-inch reflector, but to the more rigid eleven-inch refractor. The roughly six-fold sacrifice in light-grasp he would offset by lengthening the exposure, confident that his latest hand-built clock drive would faithfully track the nebula as it drifted across the sky.
Draper exposed one of the new gelatino-bromide dry plates from England for fifty-one minutes, yielding a negative that showed clear signs of Orion’s ghostly cloud. Convinced that lithographic reproduction of the picture in journals would elide critical details, he sent around photographic prints of the image mounted on six-by-eight-inch cards. On each was the caption: FIRST PHOTOGRAPH OF THE NEBULA IN ORION, TAKEN BY PROFESSOR HENRY DRAPER, M.D. Because stars on the plate were greatly overexposed in capturing the elusive nebula itself, Draper inset a five-minute shot of the cloud’s familiar central star quartet, known as the Trapezium. An appended note to his friend Edward S. Holden, then at the U.S. Naval Observatory, conveys Draper’s joy in his success: “The exposure of the Orion Nebula required was fifty minutes; what do you think of that as a test of my driving clock?”
Henry Draper’s fifty-one-minute dry-plate exposure of the Orion Nebula, September 30, 1880.
The response to the Orion picture split generally along national lines: American astronomers were delighted, eager to shed their second-rate status in the global scientific community; English astronomers were unmoved, declaring that Draper’s photograph showed less detail than contemporary drawings. The modern eye tends toward the latter judgment: the image smacks more of proof-of-concept than definitive portrait. It conveys only the presence of nebulosity, with no sense of its structure or extent. Evidently, Draper saw it the same way. On March 11, 1881, he made a 104-minute exposure of the Orion Nebula, with noticeable improvement. He tried again just over a year later, on March 14, 1882, when the nebula stood uncommonly vivid against the black drape of the night sky. The thermometer hovered at twenty-seven degrees Fahrenheit; fifteen-mile-an-hour gusts buffeted the dome. Draper upped the magnification of the eleven-inch refractor to 180, attached the camera, engaged the clock drive, then shuddered in the icy gloom while Orion’s feeble glow dripped onto the plate for 137 minutes.
Draper’s 1882 photograph, although gauzy from a modern perspective, depicts the dark lanes and the winged structure of the nebula so familiar to observers. The picture was hurriedly published as a photolithograph appended to Edward Holden’s Monograph of the Central Parts of the Nebula of Orion, a commentary on two centuries of visual observations and drawings of the famous celestial cloud. Holden himself had just completed what would become the last great visual study of the Orion Nebula, using the Naval Observatory’s twenty-six-inch refractor from 1874 to 1880. Of the monograph’s many images, Holden selects the one compiled over a span of years by Harvard’s George Bond as “the best representation of a single celestial object which we have by the old methods.” Yet his final aesthetic and scientific verdict is clear: “Dr. Draper’s negative was made in 137 minutes, and for nearly every purpose is incomparably better than the other.”
Henry Draper’s 137-minute dry-plate exposure of the Orion Nebula, March 14, 1882.
In fact, later spectroscopic studies revealed that the eye and the camera had altogether different perspectives on the Orion Nebula. The great cloud is a tenuous assemblage of atoms that, when energized by embedded stars, emit specific colors: primarily a pair of green wavelengths arising from oxygen, plus red, blue, and violet wavelengths from hydrogen. (By contrast, our denser Sun releases a more or less continuous rainbow of light, in the manner of an incandescent bulb.) The human eye is most sensitive to oxygen’s green emissions, whereas cameras of the 1880s responded more to the blue and violet of hydrogen. Any difference in the distribution of these two elements would present contrasting views to the visual and the photographic astronomer. No wonder skilled artists rendered details in the Orion Nebula that were not seen in photographs.
If Henry Draper’s 1882 photograph did not settle the longstanding matter of whether the nebula had changed over time, it did herald the imminent end of reliance on subjective drawings of astronomical objects. In an 1886 report on celestial photography, Harvard researcher Edward Pickering alerts astronomers to the presence on Draper’s plate of a particular faint star, only barely visible to the eye through a comparable telescope. Pickering concludes, “The photographic plate . . . had now become as efficient an instrument of research as the eye itself.”
During the summer of 1882, Henry Draper resigned his medical professorship to devote his full attention to astronomy. Although the Orion Nebula continued to beckon, Draper planned to widen his photography of the spectra of stars. Already he had recorded the spectral-line patterns of the brightest luminaries: Vega, Arcturus, Altair, and Capella. And in his laboratory, he had photographed the analogous features for common chemical elements and the Sun. Identification and cross-comparison of the various line patterns, he knew, were key to the establishment of a star’s elemental makeup. In principle, astronomers could analyze starlight to the same effect as a chemist analyzing a sample of the star’s atoms in a laboratory. The practical hurdles were daunting; except for the Sun, the hair’s-breadth gaps in a star’s spectrum were barely visible through the eyepiece. However, a time-exposure photograph would enhance these elusive patterns, render them countable, identifiable, measurable. With the new dry-plate technology, Draper considered the marriage of the camera and the spectroscope to be absolutely vit
al to the advancement of science.
Only by photographing the spectra of hundreds of stars could Draper assess the variation in chemical composition of the stellar species. To carry out such a project would require years, if not decades, along with significant improvements to the observatory at Hastings. His first priority was to design an even better clock drive: during a two-hour exposure, a star’s image could deviate no more than 1/300 of an inch at the telescope’s focus, lest it miss the narrow entrance slit of the spectroscope. In April 1882, Draper reported to the National Academy of Sciences that he had succeeded in photographing the spectrum of a star of tenth magnitude. “It is only a short time since it was considered a feat to get the image of a ninth magnitude star, and now the light of a star [some two-and-a-half times dimmer] may be photographed even when dispersed into a spectrum.” At the same time, he dreamed of the heightened images he would capture of the Orion Nebula in the coming months. “I think we are by no means at the end of what can be done,” he confided to Edward Holden. “If I can stand 6 hours exposure in midwinter another step forward will result.”
In September 1882, Draper accompanied a pair of army acquaintances, Generals Randolph Marcy and William Whipple, on a two-month riding expedition through Wyoming and Montana. A snowstorm overswept the trio during their return leg, and they were forced to camp overnight without shelter above the tree line. The ordeal sapped the strength of the normally vigorous Draper, who arrived back in New York wan and exhausted.