PhotoHistory

THE BRITISH JOURNAL OF PHOTOGRAPHY
No. 1392. VOL. XXXIV. JANUARY 7, 1887

STELLAR PHOTOGRAPHY

Photography has of late years become so completely the handmaid of Science, that we cannot be surprised at any of its most recent developments. The microscopist is no longer dependent on the pencil of the draftsman for accurate representations of the objects under his lens, and the passing eclipse is secured for future study by the photographer who has become so important an assistant on that critical occasion. The astronomer sweeps the heavens with aids at his command which the Herschels never dreamed of possessing; and Dr. Huggins only lately secured a plate of the sun’s corona under circumstances which bid fair to render a natural eclipse no longer necessary for the student of solar physics. A close examination of the moon’s photographs have revealed hills and valleys and dismal craters which the observer did not at first note through the telescope, until, as Professor Newcomb lately declared, the surface of that planet is better mapped than many sections of the United States.

But it has been left to M. Struve to confirm by means of actual observation a fact which, for several months, had rested on the authority of the photographic plate alone. In November last the Brothers Henry succeeded in obtaining a photograph of the Pleiades. In itself this was a sufficiently notable feat, for though Rutherfurd and other astronomers have succeeded in making photographic charts of the heavens and one of the earliest applications of the art was by Warren de la Rue taking an exceedingly sharp impression of the same group — the plates hitherto obtained were faulty in many respects. But what was most remarkable in the Henry plate was not the actual picture of the “Six Sisters,” but the fact that an object hitherto unnoticed by the thousands of eager eyes who had scrutinized the constellation appeared in it. This was not Sterope, the “invisible one,” whom the ancients fabled to have hid herself in shame, because she alone had married a mortal, and who is supposed to have existed in the form of a seventh star, which has gradually disappeared. What met the astonished gaze of the photographers was a large spiral nebula, apparently proceeding from the star Maia. Twice, at the interval of three weeks, the constellation was photographed, and with a like result, the nebula invariably reappearing of the same form and in the same position. On examining a plate taken at the Harvard Observatory ten days before the one referred to, this identical nebula was noticed, but, in spite of the exertion of the Paris astronomers, assisted by their largest instruments, no sign of the nebula revealed by the lens could be detected. But M. Struve, of Pultova, having got his gigantic telescope into position, is at last able to report that he has actually seen what hitherto had to be taken on the credit of the photographic plates on which the Pleiades told their own story.

The interest of this revelation is twofold. It proves, in the first place, what a wealth of worlds still lies for the explorer of the heavens to discover, in spite of the vigils of all the watchers, from Job to Janssen. Scarcely a year elapses during which some new planetoid is not announced, and comets are so frequently coming into view that ignorant folk, who do not appreciate the scientific importance of these wanderers in space, are beginning to grow blase with the news. Not long ago a new star appeared in the nebula of Andromeda, and now a fresh cluster of unknown and, for the time being, undetermined luminaries is revealed in the Pleiades.

Several views of this region have already been figured. But it is strange that, with all the study devoted to the famous cluster, it has been left for the photographic camera to tell what the most powerful telescopes had, up to the date of M. Struve’s confirmation of the Henry photograph, failed to reveal. Yet each of the shining specks that make up the flocculent curl, which looks like a curdling fluid, is most probably a little star, imperceptible to the naked eye, simply because the rays of light proceeding from each of them are so blended with those from the stars in their close proximity, through the influence of irradiation, as to produce an effect which can only be compared to a luminous mass. What is the exact nature of the new nebula is less important than the fact that it exists. Many nebulae are variable. The large irregular nebula which surrounds the star Eta Argus has been thought to be of thin nature, and the great nebula in Orion is under a similar suspicion regarding its constancy of form. Others have vanished since the period when they were first noticed, while, on the other hand, there are nebulae which have appeared in places where no nebulosity had been noticed, though the space had been carefully scanned by observers who could scarcely have missed it. It is just possible that the new one may be of that nature, though the chances are against it. But what is of even greater interest is the circumstance under which it has been discovered. Now that photography is proved to be invaluable, not only as a recorder but as an observer, the field which it opens up seems boundless. A star millions of miles away takes its own portrait, though no human eye, aided or unaided, has been able to see it. The spectroscope has enabled us to analyze the constituents of the sun and the other celestial bodies; and it is every day becoming more and more likely that before many years elapse we shall know almost as much about the chemistry of the fixed stars as we do at present regarding the envelopes of the sun, and its glowing atmosphere.

When photography first began to aid astronomy, the art was hampered by difficulties which no longer exist. Modern dry plates, which are not only cleanly but rapid, have obviated many of the mechanical obstacles which the old processes put in the way of the observer, so that the physicist has an entirely new tool at his disposal. Dr. Draper photographed the moon just forty-six years ago. But the first stellar photographs ever taken were those of Alpha Lyra; by the elder Bond, at the Harvard Observatory, in 1850. In 1857 his son carried similar investigations much further. At first, however, as the Brothers Pickering showed, in a paper read before a recent meeting of the American Academy of Sciences, they were unable to obtain clear images of stars of the second magnitude, while now it is possible to print those of the fourteenth. In other words, we can, owing mainly to the improvements in dry plates, transfer to paper an image produced by objects only a hundred-thousandth part as bright as formerly. By employing a new instrument, in which a photographic lens of eight inches aperture and forty-four inches focus is mounted equatorially and moved by clockwork, photographs of several different regions may be taken upon the same plate, and the stars distinguished by varying the exposures. By these means a new map of the sky may be secured, showing the position of the stars at any given moment, and thus securing for the astronomers of coming generations a record the importance of which it is impossible to exaggerate. Indeed, the only limit to the further extension of stellar photography is the sensitiveness of the dry plate, and as this limit is plainly not yet reached, Professor Pickering, of Harvard, who is actively engaged in these researches, is confident that even better results may be expected. All this is very different from the day when Warren de la Rue was essaying the portraiture of the Pleiades, or by the aid of an equatorial mechanism for keeping the image of the moving planet for several minutes in the center of the field of view of the telescope which moved along with it succeeded in photographing Jupiter with his parallel zones. He even fixed on the collodion the rough surface of the planet Mars, and the wondrous image of the “belted ball” of Saturn. The moon is now almost as well photographed as a popular actress, and, it may be added, with a great deal more accuracy, for there is no “touching up” the negatives of Luna. However, as the pioneers in this field of research could not beg the moon “not to move, please,” it was necessary for the telescope, at the focus in which the image was to be fixed, to move at exactly the same rate as the object photographed, so that mechanical accuracy was hard to obtain. These difficulties are now all but historical. The almost instantaneous gelatine plate, more sensitive than the retina, has made the astronomer’s task no longer so hard, while such results as those just obtained impart to his pursuit all the excitement of a scientific lottery.