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dion, silver bromide being exclusively might, it was to be feared, suffer neg used as the sensitive substance. The ad- lect through the predominant attractions vantages of the new process were quickly of its younger, more versatile, and brilliant perceived and improved. Gelatine is not, competitor; or its lofty standard of perlike collodion, a merely neutral vehicle. fection might become lowered through the It possesses a reducing power of its own influence of workers more zealous than which steps in as an effective auxiliary to precise, recruited from every imaginable that of light. Hence the extraordinary quarter, inventive, enthusiastic, indefat rapidity of the "gelatino-bromide " plates igable, but unused to the rigid requirenow universally employed. Chief among ments of mathematical accuracy. their recommendation to "astrographers are the faculties of keeping indefinitely, and gaining fivefold sensitiveness by drying. They can thus be prepared at leisure, exposed with constantly accumulating effect for an unlimited period, and developed when convenient.

Their singular adaptation to the exigencies of celestial research was first perceived by Dr. Huggins, who used "dry plates "in his experiments on photographing stellar spectra in 1876; and his advice and example were followed, a few years later, by Draper and Gould in America, by Common and Janssen in Europe. The change has proved of the highest moment

to science.

Both these perils have been happily averted. The prospect has suddenly cleared and brightened. The new astron omy has submitted to bear the yoke of the old. The old astronomy has adopted the new methods, and is even now anxiously fitting them to its own sublime purposes. It has enlarged its boundaries without departing one iota from its principles. By an effort which shows it to be still young and elastic, it has seized the key of the situation, and now stands hopeful and dominant before the world.

This union of the two astronomies has long been in remote preparation. Artists and experimenters innumerable have unconsciously urged it on. It has been promoted by improvements in the manufac ture of glass, in the shaping of lenses, in the grinding, polishing, and silvering of mirrors, by the growth of intimacy with the peculiarities of salts of silver, and by the growth of skill in their employment for the purposes of light-portraiture. The meeting last year at Paris of an International Astrophotographic Congress marked its accomplishment. This event will undoubtedly prove to be of the epoch-making

We have heard much lately of the power and promise of the "new astronomy," and celestial physics have indeed, in our day, entered upon a splendid career. Like" England's great chancellor," it "has taken all knowledge to be its province." No truth regarding the material universe is indifferent to it. It assimilates every variety of information. Scarcely an experiment can be performed in a laboratory without directly or indirectly promoting its interests. The labors of electricians, description. Future ages will look back meteorologists, geologists, mineralogists, chemists, are all made available. No science can be its rival, because each one is its colleague and ally. The results have been commensurate with this vast extension of resources. Knowledge, am- Stellar photography originated with a ple and assured, has been accumulated of daguerreotype of Vega (a Lyræ) taken at a kind which, previous to the middle of Harvard College July 17, 1850. The oval the present century, appeared to the pro- shape of an image of Castor obtained foundest thinkers forever unattainable. about the same time indicated its duplicUndreamt-of analogies between celestiality; but these impressions were very faint, and terrestrial phenomena have been disclosed. Above all, boundless prospects of future discovery have been thrown open, and the keenest stimulus to persistent effort has thus been supplied.

The new astronomy has accordingly found eager and numerous votaries in all its various branches. Yet its popularity seemed attended by a twofold danger. The majestic elder astronomy-the astronomy of Hipparchus, Bradley, and Bessel, of Newton, Leverrier, and Adams

to it as the beginning of great achievements. To have been concerned with it will in itself be counted as giving a title to fame. Circumstances concurred to bring it about just at the right moment.

and none at all could be derived from objects of inferior lustre, such as the polestar. Then the collodion process was introduced, and with its aid the younger Bond, in 1857, extended the depicting powers of the camera to stars of the sixth magnitude. Still more significantly, he demonstrated the applicability of photog raphy to the astronomy of double stars by executing upon prints of Mizar in the tail of the Great Bear a set of measures which proved superior in accuracy to those

of the ordinary visual kind. He also led | the way in photographing what are called "star-trails." When Vega, the clock being stopped, was allowed to "run" upon the plate by its own diurnal motion, its passage remained remarked by a fine line. The principle of "trails " has been turned variously to account in recent investigations.

Rutherford reached the limit, in this direction, of what was possible to be done with wet plates. In and after the year 1864 he secured photographs of a number of clusters, including stars down to the ninth magnitude, from one of which Dr. Gould deduced places for nearly fifty Pleiades, agreeing so closely with Bessel's, of a quarter of a century earlier, as to put beyond doubt the extreme minuteness of the relative motions of those stars. When it is added that quantities of of an inch were measurable on Rutherfurd's negatives, it becomes clear that the era of observations "of precision" by photo graphic means was fast approaching.

sively, as the rays continue to impinge upon it, all the orders of the stars, all the secrets of the sky, disclose themselves to its patient stare. It has thus become possible to photograph stars too faint to be seen with the same optical aid. Some of those sprinkled over the Orion nebula, in Mr. Common's beautiful picture of it, were probably beyond the reach of direct observation with the 36-inch mirror employed; and Dr. Draper at the time of his death in 1882 was making arrange ments for exposing plates during nearly six hours, by which he hoped to get noti fied of the existence of stars sunk in depths of space hopelessly inaccessible to telescopic vision.*

But the decisive impulse towards the greatest astronomical undertaking of this century came otherwise. The Royal Ob. servatory at the Cape of Good Hope was, in 1882, unfurnished with any photographic appliances. The activity reigning there was of a rigorously orthodox kind. The ample programme of work in course of With the introduction of dry plates it execution included nothing for which may be said to have arrived. They were Halley or Maskelyne would have been indeed indispensable, no less for charting unprepared. "Astrophysical " tendencies, than for exploring the skies. Photog- of whatever description, were absent from raphy is of service for these purposes just it. Nor did any such exist in the mind in proportion to the number of faint stars of the royal astronomer. Dr. Gill beit can register. But here length of expos-longed to the strict school of Bessel; ure is all-important; and long exposures in the use of the heliometer he was are impossible with plates subject to change by evaporation.

Impressions on the sensitive plate are cumulative as well as permanent. Those on the living retina are neither. The maximum effect of a luminous object on the human eye is produced in one-tenth of a second. Beyond that limit there is continual effacement and renewal. Were it not for this faculty of rapid obliteration, we should see, with the strangest results of visual confusion between time and space, not what we were actually looking at, but what had met our eyes some short time previously. A vast gain in penetrative power would, however, ensue upon a very moderate extension of the time during which the eye can collect impressions. By lengthening it to one second the brightness of visual images would be nearly decupled, and the whole heavens would appear, like the Milky Way, dimly luminous with minute stars.*

This retentive power is possessed, in an eminent degree, by a sensitive gelatine film. No limits have, so far, been set to the time of useful exposure. Succes

⚫ Janssen, Annuaire, p. 809. Paris: 1883.

Bessel's legitimate successor. His leading title to distinction at that time was a masterly determination of the sun's distance, for which the opposition of Mars in 1877 had given the opportunity; and he was engaged upon a set of measures for stellar parallax of unsurpassed excellence, and now of standard authority. His energetic administration was mainly directed towards promoting the interests of practical astronomy in the southern hemisphere; and he was far from suspecting that in the camera an instrument was at hand more rapidly effective for the purpose than the transit or the heliometer. He was not, however, slow to avail himself of it.

The splendid appearance, at the Cape, of the great comet of 1882 challenged photographic portrayal; and Dr. Gill employed for that end the apparatus, and profited by the experience, of Mr. Aldis, a local artist. An ordinary portrait-lens, of only two inches aperture and eleven focus, was attached to the stand of the observatory equatorial, the telescope itself serving as a guide to the small corrections

Rayet, Bulletin Astronomique, tome iv., p. 320.

needed of the clockwork following motion during exposures lasting from half an hour to two hours and twenty minutes. A series of pictures resulted, one of which was exhibited by Dr. Gill in the course of his lecture at the Royal Institution, cited, from its importance to our present subject, among our authorities. They were remarkable, not only for the strength and fidelity with which their principal subject was represented, but for the accessory wealth of stars they displayed. The entire background was thickly strewn with them. Forty or fifty, down to the ninth magnitude, shone across the interposed film of the comet's tail.

The sight of the Cape photographs set the whole astronomical world upon the business of stellar chartography. They emphasized the advantages to be derived from the use of lenses of short focus and wide field, giving small, bright images of tolerably extensive sky-landscapes. To Mr. Common they "came as a revelation of the power of photography" for starcharting purposes; and he proposed to Dr. Gould, then (in 1883) at Cordoba in South America, a joint photographic survey of the whole heavens, which it was not however found practicable just then to undertake. Investigations of relative stellar brightness by photographic means were almost simultaneously executed by Professor Pickering at Harvard and by Mr. Espin in Lancashire; and Mr. Roberts of Liverpool began, and has made considerable progress with, a detailed chart of northern stars.

But by far the most important of these preliminary enterprises was that of completing, in the southern hemisphere, the great northern star-census executed by Argelander at Bonn above a quarter of a century ago, and lately extended by Schönfeld to twenty degrees south of the equator. The Durchmusterung, comprising in its two sections nearly four hundred and fifty-eight thousand stars, may be described as the roll-call of the stellar army. Stars not entered in it have no official existence; should they fade and vanish, the fact cannot be attested; should they brighten into conspicuousness, we are obliged to regard them as "new" for lack of previous acquaintanceship. Whatever is known of the distribution of the stars in space is founded on this grand enumeration, which was besides an essential prelude to more refined measurements.

Mr. De la Rue showed experimentally in 1861 that such instruments were the most proper for mapping the stars. (Report Brit. Ass., 1861, p. 95.)

A corresponding enrolment of southern stars was one of the most pressing needs of astronomy; and it is now, by novel means, in course of being supplied by Dr. Gill. His photographic Durchmusterung will extend from the limit of Schönfeld's zones to the south pole, and will include all stars brighter and many fainter than the ninth magnitude. The requisite number of plates will probably have been secured in two or three years; while the catalogue derived from their measurement, through the disinterested labors of Professor Kapteyn of Groningen, may be completed in five or six. It will give the places (exact to one second of arc) and magnitudes of thirty per cent. more stars per square degree than are contained in the Bonn catalogue, and will furnish "working lists" for still more accurate determinations for about the epoch 1900.*

But we have not yet exhausted the results of the comet-pictures of 1882. Thirty-six years have elapsed since Chacornac began, at the Paris Observatory, the laborious task of charting ecliptical stars to the thirteenth magnitude. His object was the detection of asteroids, by obtaining an individual acquaintance with the small stars strewing their route in the sky; but he died in 1873, leaving the work only half finished. For its completion the resources of the newer astronomy had to be called into play.

His successors were MM. Paul and Prosper Henry, two brothers united by a rare community of tastes and endowments, inseparable in their labors, scarcely distinguishable by fame. In ten years they constructed sixteen additional maps out of a total of seventy-two; but they were arrested by encountering, where the ecliptic crosses the Milky Way, a throng of minute objects, totally unmanageable by the ordinary methods. The perplexity in which they found themselves was dissipated by a glance at the starry background of Dr. Gill's comet. They determined to have recourse to photography; their stars should henceforth register themselves. From that hour visual star-charting became a thing of the past.

The unmistakable success of some preliminary experiments earned for their scheme the warm approval of Admiral Mouchez, director of the Paris Observatory, the title of whose valuable little book heads this article; and the construction of the largest photographic telescope yet seen was officially sanctioned.

Auwers, Monthly Notices, vol. xlvii., p. 455

In

May, 1885, an instrument on a somewhat novel plan, the optical part by the MM. Henry, was mounted in the garden of Perrault's edifice. It consists of two telescopes, one adapted for chemical, the other for visual use, enclosed in a single rectangular tube. The photographic objective is of thirteen inches aperture and eleven feet focus, its curves being computed to enable it to take in a wide area of the sky without sensible deformation of the images. Their complete immobility in the field is secured by a skilful use of the guiding telescope. During the time of exposure the eye of the operator is never removed from it, and incipient deviations are checked by his hand.

The results of the employment of this apparatus by the MM. Henry were summed up by Admiral Mouchez before the Academy of Sciences, January 18, 1887.

At the Paris Observatory [he stated] we now easily obtain, with exposures of an hour, plates upon which thousands of stars down to the sixteenth magnitude are portrayed with the utmost nicety and distinctness over an area of six or seven square degrees. That is to say, the limit of visibility with our best telescopes under the sky of Paris is considerably overpassed, and we have even obtained many seventeenth magnitude stars doubtless never anywhere directly observed. The stellar images, varying in diameter proportionately to magnitude, afford useful data for photo

metric determinations.

be disclosed on the plates of the MM. Henry
could they be exposed under the pure skies of
the tropics, or at so favorable a station as the
Pic du Midi. Stars of the eighteenth magni-
tude would then not improbably emerge to
depths never before sounded.
view, showing a penetration of the heavens to
Such plates
would doubtless, at a little distance, like the
firmament itself in serene tropical nights,
assume a uniformly nebulous aspect.
hope then to apply photography not only to
the regular prosecution of celestial chartog-
raphy, but to researches on double stars, and
to explorations in search of unknown heavenly

bodies.

We

Specimens of the Paris photographs were soon in the hands of astonomers in all parts of the world. They were received with admiration not unmixed with incredulity. They seemed too absolutely perfect to be wholly genuine. Abundant evidence was however at hand to show that their extraordinary precision was really the fruit of unparalleled skill, and this conviction, once attained, was decisive of the future of astronomy.

On one of the plates, covering an area of about four square degrees in the constellation Cygnus, where 170 stars had previously been identified, some five thousand were clearly imprinted. Wolf's great map of the Pleiades, founded on laborious observations extending over several years, contains 671 stars; photographs taken in a few hours by the MM. Henry supplied Objects other than stars, invisible in our materials for charting 1,421 stars of the most powerful instruments, sometimes appear same group down to the sixteenth magnion the plates. Such is the Maia nebula in tude with an exactitude unattainable by the Pleiades, depicted like the tail of a bril- visual means. The significance of such liant little comet attached to the star, yet results was not to be mistaken. They heretofore undetected, notwithstanding the ex-pointed to a great task, the execution of ceptional amount of attention bestowed upon which was felt to be imperative so soon as the Pleiades group. Unknown bodies, in it had become possible; and Dr. Gill gave sufficiently rapid movement to become sensibly displaced in an hour-minor planets, expression to a universal sentiment when for instance, comets, the problematical trans- he proposed, June 4, 1886, an InternaNeptunian planet, or undiscovered satellites tional Congress for the purpose of organiz -may reveal their existence by imprinting ing a photographic survey on a grand the line of their route among the fixed stars, scale of the entire heavens. as Pallas has been observed to do.

Fifty-five delegates of fifteen different The distinct visibility, on a photograph sub-nationalities took part in the deliberations mitted to the Academy, of the interval of of the memorable assembly which met at 04 between the rings of Saturn, gives a Paris, April 16, 1887. They were conprospect of securing impressions of double stars at that apparent distance. cluded in nine days, and were as harmoniThe satellite of Neptune has been photographed in every ous as they were prompt. Enthusiasm for part of its orbit, even when it is only 8" from a great end secured unanimity as to the the planet.* means; differences of opinion vanished as if under the pressure of some supreme crisis. The upshot of the meetings was to set preparations on foot for the charting of over twenty millions of stars! So far have we got by the aid of photography.

With the consideration before us that stars below the sixteenth magnitude have thus been photographed amid the turbid atmosphere of Paris, it becomes difficult to imagine the prodigious quantity of new objects which would

•No visual observations of Neptune's satellite have ever been made at Paris.

* Mouchez, La Photographie Astronomique, p. 37.

The co-operation of ten or twelve observatories in both hemispheres can be reckoned upon, and the work will be executed upon an identical plan with instruments similar in every respect to that of the MM. Henry. About ten thousand plates (duplicated to avoid accidental errors), each exposed during a quarter of an hour, will record the positions of all the stars in the sky to the fourteenth magnitude the prescribed limit of faintness. This part of the undertaking can scarcely occupy less than five years. For the orientation of each plate, a single": "star-trail (necessarily running along a parallel of declination) will suffice. The absolute places of the imprinted stars will be deduced from accurate measurements of their situations relative to certain standard stars, of which a sufficient number will be found on every plate.

large errors attest its incompetence below the eleventh or twelfth magnitude. The sensitive plate, on the other hand, measuring light-intensity as it were by the clock, records its gradations between faint objects more precisely than between bright, because the corresponding intervals of time are larger. Stars of the first, second, and third magnitudes can all be photographed in a small fraction of a second; but stars of the thirteenth magnitude require five, of the fourteenth thirteen, of the sixteenth eighty minutes, before they become perceptible with the apparatus of the MM. Henry. Intermediate positions on the photometric scale can hence, it is obvious, be assigned much more easily and securely towards its lower end.

A star of any given order of lustre emits just two and a half times as much light as a star of the magnitude next below. One But there is to be a catalogue as well as of the sixteenth is accordingly a million a chart, and, in Dr. Gill's opinion, "the times fainter than one of the first magniwork which astronomers of future genera- tude, and under identical conditions takes tions will be most grateful for, and which a million times longer to get photographed. will most powerfully conduce to the prog- This is the proper and only definite criteress of astronomy, will not be the chart rion of the rank of such feebly luminous but the catalogue." Plates showing four-objects, visual estimates of which are little teenth-magnitude stars, however, are nec- better than guesswork.

essarily over-exposed for the brighter ones, It is true that color exercises a disturband are hence not available for the most ing influence owing to the predominant refined determinations. A set of short-sensitiveness of silver salts to the more exposure plates, reaching to the eleventh refrangible rays. Aldebaran, for instance, magnitude, are accordingly to be taken is reduced by the fiery tinge of its light with a view to cataloguing about one mil- to the fifth or sixth chemical rank; and lion and a half stars to serve as reference- small red stars are frequently missing points for the twenty millions crowded on from photographs which display crowds the chart plates. Such a catalogue (we of objects equally or less bright to the again quote Dr. Gill) " may be considered eye. Such discrepancies, however, have complete for the practical purposes of an interest of their own, and they do not astronomy, because the eleventh magni-impair the general correspondence betude is the faintest which can be measured tween visual and photographic evaluations with accuracy in the larger class of equato- of brightness. Nor, even when they differ, rials usually employed in working observatories."

The mass of stellar statistics thus collected will include data as to relative brightness. The magnitudes of stars can be derived from photographs either by comparing the size of their images on the same plate, or by measuring the time that elapses before they produce a sensible impression. Estimates founded on the circumstance that the diameters of the photographic discs of stars bear a strict ratio to their lustre have proved accurate (on an average) to one-fifth of a magnitude; and varying length of exposure affords the only fixed standard of brightness at present available for the minuter orders of stars. The photometric range of the eye is somewhat narrowly limited, and

is there any valid reason for preferring the former to the latter. Both serve as means to the same ends; and chemical determinations are in so far at least to be preferred that they are authentic over a wider range.

Accurate comparisons of stellar brilliance serve two chief purposes —an individual, so to speak, and a general. Taken separately, they are a direct test of variability; taken together, and on an average, they are a safe guide to distribution.

The great problem of the constitution of the sidereal universe is not one to be solved by a stroke of genius. The genera tions of men are but as hours for its study; each contributes its little quota of gathered facts, and more or less ineffec.

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