PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY.*

By R. RADAU.

Translated by AARON N. SKINNER, U. S. Naval Observatory.

To obtain the greatest result with the least effort, is not this the whole problem of modern industry, a problem which determines gradually the development of implements and machines? The engines which he invents permit man to infinitely multiply the efficiency of his organs, to extend their fitness, and to relieve them from the demands of exces sive efforts; they assist him, free him, more and more from the harsh servitude of material labor. Confining himself henceforth to overseeing the apparatus which labors for him, in proportion as he fatigues himself less, he produces more and at much better advantage. There is no comparison between the fabrication of a thousand needles from a manufactory, and the work of an artisan who undertakes fashioning them one by one by his own unaided efforts.

It is a progress of the same order which realizes to-day the definitive introduction of photography in astronomical observations: It is to deliver the astronomer from a labor, thankless, painful, irksome, and fatal for the eyes. When ten years ago I spoke in this journal of the great prospects of celestial photography † I dared scarcely to hope that routine and prejudice would be disarmed so speedily. Indeed the first attempts in astronomical photography go as far back as 1840, and during nearly half a century frequent attempts, unhappily always isolated, have shown that the difficulties of the problem have not been insoluble; but on account of tenacious prejudices, a blind adherence to the past proscribed the paraphernalia of photography from the sanctu aries or kept up the traditions of Cassini and of Bradley. It is in these later years that finally this spontaneous enthusiasm appeared, this grand movement which has found its expression in the "Astro-photographic Congress," convened in Paris in the month of April, 1887, and which promises to begin a work of the highest importance for future ages, the photographic execution of a general chart of the sky.

* From the Revue des Deux Mondes; April 1, 1889, vol. xc1, pp. 626–649. I.-E. Mouchez, "Astronomical Photography at the Paris Observatory and the chart of the sky," Paris, 1887. II.--Bulletin of the Permanent International Committee for the execution of the photographic chart of the sky, 1888, 1889.

↑ Revue des Deux Mondes of February 15, 1878.

I.

This application of photography is however so reasonable, its rôle was so clearly indicated and so well foreseen, that it seems an equal progress ought to have been obtained indeed from the first. There was one problem whose solution was perfectly circumscribed; it was truly no more than a question of time and money. The history of photography, since its origin, is like the logical development of a thought which is realized in a continuous manner before our eyes. The gropings by which we discover substances more and more sensitive, or the means of retaining, of fixing, more and more permanently, the fugitive traces of phenomena, all these ought to be, most certainly, hastened and matured more speedily in the fact of having a prize to gain; and here appear clearly the conditions always more tyrannical than expense in the scientific enterprises of our epoch.

Chemistry and the mechanical arts have singularly multiplied the resources of astronomers of the close of this century. Is there any occasion to recall the progress accomplished in the manufacture and grinding of optical glass, in the mounting of great telescopes, in silvered mirrors, in electric chronographs, in the spectroscope and in spectral analysis, whose entry on the scene, so brilliant and so unexpected, probably diverted for some time the attention of astronomers from the develop. ment of photographic processes? Unhappily this instrument, so powerful, this new apparatus which has extended the domain of observation, is very costly. In order to bring it into service, great efforts of eloquence are almost always necessary, because the scientific budget, as is well known, is one whose endowment is generally measured with the greatest parsimony. It is in such a situation as this that the assembly of a congress, with its solemn publicity, its persuasive programmes, and its imperious desires, offers always the best means of overcoming an opposition which is inspired by an ill-conceived economy.

The congress which held its sessions at the Paris Observatory, two years since, and which was called by Admiral Mouchez, under the auspices of the Academy of Sciences, had in view, primarily, the execution of a chart of the sky. It comprised fifty astronomers, who came from all parts of the globe, some already familiarized for a long time with the pratique of celestial photography.

It would be irksome to enumerate here even once, all the attempts which have been made, since Daguerre, to bring photography into the service of descriptive astronomy and the astronomy of precision. Recalling only that the most difficult part of the problem, the photographic re-production of stars, had been entered upon with some success in America by G. P. Bond (soon after the introduction of the collodion process permitted the shortening of the time of exposure), about 1857, the photography of stars to the sixth or seventh magnitude had been attained. These trials were repeated some years later in England by

Warren de La Rue, and later in America, with a success continually increasing, by Mr. Rutherfurd and by Mr. B. A. Gould, charged with the direction of the Cordoba Observatory under the fine sky of the Argentine Republic. Gould began his work in this line about 1875 and succeeded in gathering, in a few years, a collection of more than a thousand stellar photographs of the highest interest. After having tested, himself, the slowness of the wet collodion process, he was able at a later date to utilize bromide of silver gelatine dry plates, the invention of which marked a new phase in celestial photography.* It is necessary, finally, to mention here the attempts in stellar photography of Henry Draper, of Ainslie Common, and Isaac Roberts, who have studied the respective advantages of refractors and reflectors with silvered mirrors; of Pickering, who has constructed for the Harvard College observatory at Cambridge, United States of America, a photographic equatorial specially designed for the rapid execution of celestial charts on a moderate scale; of David Gill, the eminent director of the observatory of the Cape of Good Hope, who commenced in 1885 a photographic revision of the southern sky, comprising the stars to the ninth or tenth magnitudes, similar to the catalogue prepared by Argelander for the northern sky.

At the Paris Observatory like labors have been also pursued for some years with most marked success. Paul and Prosper Henry had undertaken, in 1871, to continue the "ecliptic chart" commenced by Chacornac who had been able to execute it only in part. This chart, extremely useful in searching for small planets, which was to contain all stars to the thirteenth and fourteenth magnitudes, extended along the ecliptic in a zone five degrees in breadth. Now at a certain point the Henry brothers found themselves arrested in this work by the manifest impossibility of constructing, by old processes, the sections of the charts where the swarming of the stars announces the approach of the Milky Way. It was then that they decided to resort to photography. They were, says Admiral Mouchez, admirably prepared to conquer these dif ficulties. "Following the traditions to-day too much forsaken, of great astronomers of former times who employed their own hands in the construction of their instruments, they devoted for a long time, in their modest work-shop of Montrouge, all the moments of liberty which were left them from their very active service at the Paris observatory, in the study of the grinding and polishing of optical glass. An extensive ac quaintance with the questions for solution, the harmony of fitness somewhat different and very happily associated in two brothers, an energetic will and a persevering labor, which no distraction ever chanced to trouble, could not fail in assuring for them a well-merited success. They became, in a few years, the most skillful artists of France, and their fame was no less great among foreigners." After having con

*Rayet: "Notes on the history of astronomical photography." (Bulletin astronomique t. IV, p. 318.)

structed, by way of trial, an objective of 0.16m, which gave very good results, the Henry brothers undertook to execute the optical part of a definitive apparatus of 0.33 in aperture, for which Mr. Gautier was to furnish the mechanical part. The new instrument was mounted at the observatory in 1885 and has not ceased since then to be in active use. The sensitiveness of the plates is such that the image of a star of the first magnitude is obtained in less than a hundredth of a second; that of a star of the sixth magnitude in a half second; for the tenth magnitude the length of exposure is about twenty seconds; for the fifteenth about thirty-three minutes; for the sixteenth one hour and twenty minutes is necessary.* The stars of the sixteenth magnitude! Here we are already far beyond the limits of visibility for the best telescopes under the sky of Paris! "Says Mouchez, even stars of the seventeenth magnitude have been certainly obtained, which without doubt have never before been seen." Finally the Paris plates have revealed the existence of nebulae hitherto unknown in the regions which had been often explored with the aid of the most powerful instruments; such is the nebula of Maïa, in the Pleiades, whose presence has been since directly verified.

After such success we understand that the director of the observatory has not hesitated in taking the initiative in an international agreement on the subject of the execution of the complete chart of the sky, by the means of photography. The possibility of this considerable work being to-day fully demonstrated, he has said to astronomers, we have assumed, for the science of the future, the trust of going about it without delay; whatever may be the value of the works in course of execution in the various observatories, they will never have, for the astronomers of future ages, an importance comparable with that of this general inventory which we shall be able to bequeath to them. It is besides, indispensable for us to concert together and distribute the labor and conclude upon a plan of work, in order to avoid the waste of force, the gaps, and the useless repetitions, and the result will be a work truly homogeneous. In regard to the expense which the enterprise will involve, it will be without doubt large enough necessarily, but very inconsiderable relatively to the importance of the result.

The astro-photographic congress convened in Paris, as we have said, in the month of April, 1887; sixteen nations were there represented. At the commencement certain technical questions were settled; the employment of reflecting telescopes, in spite of the advantages which they offer in some connections, has been rejected for the execution of the chart of the sky, and a unanimous vote recommended refracting telescopes; they will be constructed similar to the photographic tele

*

We take this information from the notice of Admiral Monchez, which dates from 1887, but these times of exposure are already much shortened by employing more sensitive plates, such as the American plates which Pickering uses, and which the Henrys have tested in their turn.

scope of the Paris observatory. In regard to the limit of the magnitudes of the stars to be photographed there was some difficulty in coming to an agreement. Taking into consideration the notable difference in the length of exposure necessary for bright stars and very faint stars, it was finally decided to make two classes of plates designed for two different uses.

For the double series of plates devoted to the picture of the sky, which is to comprise the stars to the fourteenth magnitude, the length of the exposure will be (in the climate of Paris at least) in the vicinity of twelve minutes.* For the supplementary series of plates comprising the stars to the eleventh magnitude only, and which must, on the one hand, secure an extreme precision in the micrometrical measurement of the stars of reference, and on the other hand, furnish the elements for a catalogue, the length of exposure will be much shorter (about thirtyfive to forty seconds). This catalogue will probably contain one and one-half million stars,-more than double the number that are certainly known to-day. In regard to the number of stars which will be found represented upon the chart properly so called, it may be esti mated to be from ten to fifteen millions. The two series of plates which will serve for constructing the chart will be arranged in such a way that the image of a star, situated in the corner of one plate of the tirst series, will be found as near as possible in the center of a plate of the second series; it is hoped that this will suffice for eliminating false stars and remove the inconvenience of unsensitized points which must exist on the plates.

In adopting for the chart an exposure of thirty minutes, it would be possible to reach the fifteenth magnitude and obtain a double or triple number of stars, perhaps thirty or forty millions, and possibly more. This was what several members of the congress desired, who could only with reluctance decide to curtail thus the common work of the astronomers of the nineteenth century. Mouchez, notably, has made the remark that the limit to which we are confined is very near that of the asteroids which are discovered every day; to obtain appreciable traces of these small stars, the exposure of twelve minutes runs the risk of being insufficient. Those who have combated the extension of the survey beyond the fourteenth magnitude have pleaded, in the first place, the length of time which the completion of the chart, under these conditions, would demand. In reply to them it may be said that in fixing at 14,000 + the total number of plates necessary for the execution of the chart, and in supposing that the work will be distributed among fifteen or twenty observatories each observatory will have only 1,000 plates to furnish; in counting twelve minutes to each plate, the work would easily be accomplished in one or two years; four years would suffice, in

*Perhaps also much less with the more sensitive plates.

+ In counting 6 square degrees to a plate, 7,000 will be needed to cover the sky, and 14,000 with the duplicates.