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THE grandest problems of astronomy have ever been problems of measurement. Descriptive observation may give us a picture of our solar system, and tell something of the apparent system of the stars. But purely descriptive astronomy knows nothing of absolute size and distance; it can furnish no predictions of the places of the heavenly bodies for the use of the navigator and the surveyor, no data by which to test and ultimately to justify the dynamical theories of the celestial motions. Such matters lie entirely within the scope of the astronomy of measurement.

Alike for their practical bearing and their grander interest these problems of measurement claim to-day by far the larger share of the astronomer's efforts. But they demand for their solution powerful and costly instruments; and, as refinement after refinement has been added to the telescopes to meet the demand for higher accuracy, the work has fallen more and more into the hands of the great public observatories. Modest instruments cannot improve our knowledge of the distance of the sun and stars, nor lay down the positions of the stars in those great clusters which may show in the future evidence of cosmical change. While there are a hundred such paths of investigation lying open to the fortunate possessors of powerful telescopes, institutions and individuals more humbly equipped are forced to confine their efforts to certain narrow fields of activity.

And so long as the measurements are made at the telescope itself, by visual observation, progress is comparatively slow. Yet it should be remarked that the mass of figures which an observer accumulates in a night's measuring at the telescope is frequently more than he can deal with single-handed on succeeding days. Before the complete result is obtained the measures must be corrected for many determined sources of error, and there are long calculations to be performed. But with a moderate amount of assistance in the more mechanical parts of the calculation it has generally been possible to keep pace with the observations, and the time spent in the after processes bears a just proportion to the time spent at the telescope.

These conditions have been profoundly modified by the recent application of photographic methods to astronomical measurement. Let us consider, for example, the old and the new methods of surveying a cluster of stars. Of old the measuring apparatus was applied to the telescope, and the astronomer laboriously measured the distance from star to star until the whole group was triangulated. Frequently it was a matter of assiduous labour on every fine night for many months, during the whole of which time the telescope was fully occupied. Nowadays the measuring apparatus is removed from the telescope, and a photographic plate is placed in its stead. In a few minutes every star has left its mark upon the sensitive film; the plate is removed for development, and the work of the telescope is finished.

By the application of photographic methods the output of the telescope can thus be increased a hundred-fold. The time spent in laborious measurement upon the stars themselves has been saved for the nonce, and in a single night the telescope can record the positions of field after field of stars. It is not producing the measures themselves, but the raw material for after-measurement. The photographs which are amassed in such profusion must each be placed under a microscope, and the distance measured from image to image of the stars. The work is not so exacting as direct measurement at the telescope, for it can be pursued in the comfort of the library and in despite of cloudy skies. But still it is a long process, and there remains afterwards an amount of calculation to be carried through at least comparable with that which was required in the old days of visual observation.

The position of an observatory equipped with a photographic telescope is therefore briefly this-that the power of the instrument to produce raw material is increased a hundred-fold, and the work to be done for every star after the telescope has dealt with it is perhaps doubled, because there is added to the calculation the work of measurement, which was formerly done at the telescope itself. But it is utterly impossible to multiply many fold the computing forces of the observatory, to enable them to deal with the vast output of a photographic telescope in continuous work. There is no longer a just proportion between the time spent in the computing room and the time spent at the telescope, and the only possible course is to limit the use of the instrument to a few nights in the year, or to employ it in producing pictures which are not intended for subsequent accurate measurement.

Now this difficulty, which is so embarrassing to the well-found but often under-manned observatories, is a golden opportunity for those who desire to do astronomical work of the highest refinement, but have only modest means. The photographic telescopes that are at work can produce far more material in the shape of plates than can

possibly be dealt with by the regular staffs of the observatories. They must call in help from without to aid in the labour of measurement and reduction of these photographic observations. And there should be no difficulty in securing this help when once it is realised how urgent is the call and how practicable the response. The sum which will buy the apparatus for measuring star photographs is small compared with the cost of equipping a very modest observatory. For a hundred pounds the college which desires to found a school of practical astronomy, or the amateur who is anxious to spend his leisure hours in work of true scientific value, can be placed on terms of equality with the most magnificent observatory in the world, in everything but the power of producing the star photographs on which to work. Nor need there be any fear that these would not be forthcoming. There are already available tens of thousands of photographs accumulated in the first pride of possession of a photographic telescope, ere it was realised that the work of utilisation could not keep pace with the powers of production. And there are many directors of observatories who have found themselves, to their keen regret, forced to limit the output of their splendid instruments, and who would rejoice over any increase in the power required to deal with the problems which could be attacked by photography, if only there were workers enough to carry on the work after the telescope had done its part.

There is nothing visionary in this estimate of the opportunities which photography has placed within the reach of would-be astronomers. Work of the highest value has already been accomplished precisely on these lines. Some years ago the Professor of Astronomy at the Dutch University of Groningen found himself with ample time for original work; but there was no observatory, and no money to build one. At the same time the Astronomer Royal at the Cape was completing a photographic survey of the southern sky, but the staff of his observatory was not large enough to enable him to measure the star pictures as they were obtained. The Dutch professor proposed to the astronomer at the Cape that the photographic plates as they were taken should be sent to Groningen, and that he should devote some years of his life to measuring the positions of the stars upon them, and preparing the catalogue of star-places which would result. The offer was accepted with enthusiasm; the plates were sent; and as a result there has recently appeared a catalogue of the positions of several hundred thousand southern stars, complementing the classic work which Argelander accomplished many years ago by direct observation for the northern sky. At first the professor worked at home. More recently he has obtained the use of a room in the physical laboratory of the university; a second measuring machine has been set up there; one or two students have joined in the work, and a beginning has been made of a wholesale determination of the distances of the stars, from plates which were taken

especially for them by the director of the Observatory of Helsingfors. Such are the beginnings of the astronomical laboratory of Groningen. To its eminent director belongs the credit of being the first to build up a school of practical astronomy and a true observatory of first-rate power in a university which possesses no observatory in the hitherto accepted sense of the term.

And at least two enthusiastic amateurs have set up for themselves private astronomical laboratories, if we may use this very convenient name to distinguish them from observatories of the familiar type. The mathematical master at an English public school is engaged in determining the positions of formations on the surface of the moon from photographs lent by the Paris Observatory. And there is a gardener in the North of England who spends his evenings measuring the positions of stars on plates taken at the Oxford University Observatory for the catalogue of the great photographic chart of the heavens.

It cannot be too strongly urged that the only hope of utilising the vastly increased powers which photography has given to astronomers lies in the multiplication of these astronomical laboratories. The work is pleasant; it can be pursued without any of the discomforts which attend working at a telescope in the cold and the dark; and it can be carried on regularly in despite of cloudy skies, which is a point whose importance can be realised only by those who have tried to work with a telescope during the past winter. There are many men who devote their leisure to astronomy and who have built for themselves small observatories. Not infrequently they have been keenly disappointed to realise that with their small opportunities they cannot add much to the sum of accurate knowledge, and they have grown tired of mere star-gazing. To such men photography has thrown open a field of boundless opportunity. If they are content to set up a measuring machine instead of a telescope, and to measure the star photographs which have been made at the great observatories, there is no limit to the aid which they can render to the progress of astronomy.

Let us take the case of a particular problem which is just now engaging the attention of astronomers all over the world. During the past winter the newly discovered planet Eros has come very near the earth. A great combined effort has been made to determine the distance of the planet. To this end thousands of photographs have been obtained recording the position of Eros among the surrounding stars. When they are completely measured and discussed we shall be in a position to deduce with great accuracy the distance of the planet, and that will lead to an improvement in our knowledge of the distance of the sun from the earth, a distance which is of paramount interest, because it is the unit in which all other astronomical distances are expressed. But there is the ever-present difficulty to

be surmounted. Far more photographs have been obtained than can be measured in a reasonable number of years by the astronomers who took them, even if they could afford to lay aside the photographic survey of the heavens on which they were before engaged, and to let their telescopes stand idle, producing no more results. There is fear that the new solution of the grandest problem in astronomy will be indefinitely delayed, and there can be no attack upon a hundred other problems which are pressing for solution, until it is realised by those who are in a position to help, that under the altered conditions introduced by the application of photography to astronomical measurement the necessary complement of each observatory equipped with a photographic telescope is a score of observatories of the new type, the astronomical laboratories.


VOL. XLIX-No. 291

3 H

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