THE PROGRESS OF ASTRONOMY.*
THOUGH the two books before us are of very unequal value, they may be mentioned together as affording a convenient measure of the advance made by astronomy in the last hundred years. Mr. Sime's readable account of Herschel's work, though only. a compilation, gives a fairly clear and comprehensive view of the state of astronomy about the beginning of the nineteenth century. By this time the story of Herschel's life, and of his devoted sister, should be familiar to every schoolboy. But in the excellent book which the Sa.vilian Professor has made out of the lectures which he delivered at the Royal Institution a year ago a novel and entrancing story is told. The progress of astronomy within the last quarter of a century is not very aeon- - • (1.) Modern Astronomy. By Herbert Hall Turner. 10 RA London: L. Constable unarneo. fee. net .]—(2.) Witt' in Her het and his Work. By James dime. M.A., F.B.S.B. Elfuburgh : T. en,a1 T. ('lark. [3e.:,
rately known to most of us. We have a general idea that remarkable discoveries have been made, from Professor Asaph Hall's vision of the tiny satellites of Mars in 1877 to Dr. Witt's discovery of our nearest neighbour among the outer planets, Eros, about two years ago. We know that instru- ments have been much improved, until the great Paris tele- scope is supposed to be able to show any life that might exist on the moon; while new ones have been invented, especially for the purpose of examining the physical conditions of the sun, on which we depend for all our life and work. We even learn from the occasional discourses of eminent astrono- mers that the inhabitants of this fifth-rate satellite of a minor star have conceived the hope of discovering the shape, extent, and origin of the universe in which they hold so in- considerable a place. But it would be affectation to suppose that the " man in the street " has any very clear conception of the almost epoch-making changes that have recently taken place in our observatories and computing-rooms. All these are clearly and charmingly set before the unlearned reader in Professor Turner's wholly admirable and concise little book. Its four sections deal respectively with modern instruments, methods, results, and mathematical astronomy. We can but glance at some of the chief points.
The history of no science presents a more curious revolution than that which has recently occurred in astronomy. " Before 1875 (the date must not be regarded too precisely), there was a vague feeling that the methods of astronomical work had reached something like finality : since that time there is scarcely one of them that has not been considerably altered or is not on the point of alteration ; and entirely new departures have been taken." The greatest alteration of all is one that was barely suspected in 1875, and whose possibility would have seemed the wildest and least scientific of dreams to Herschel. The hope of the observer in his day was limited to the improvement of telescopes : it never occurred to him that the human eye, that weak and very imperfect instrument, could itself be replaced by an untiring and automatic watcher. Yet the introduction of the photographic dry-plate achieved no less than this for the astronomer. The idea of using photography for recording the state of the heavens had been conceived long before the date mentioned by Professor Turner, but the diffi- culties of manipulation put it out of court. " The great and notable advances in astronomical method and discoveries by means of photography since 1875," writes Sir William Huggins, "are due almost entirely to the great advantages which the gelatine dry-plate possesses for use in the observa- tory." Messing about with collodion and its accessories was impossible for most observers, and the wet-plate could not be used for long exposures. Now there is practically no limit to the length of time during which a plate can be exposed to the sky, and it is no figure of speech to say that the camera has revealed the existence of stars and filmy nebulae which no human eye could see with any telescope that will ever be made. The im- pression on the sensitive plate is cumulative, and by leaving it exposed long enough it will record the invisible light that started perhaps a million years ago from stars so distant that the mind refuses to imagine the depth of space in which they exist. The utility of this particular observation is not clear; but some of the things that the camera has already revealed may wholly revolutionise our notions of the stellar universe. We would draw attention, for instance, to Professor Turner's reproduction of one of Dr. Max Wolf's beautiful photographs of the Pleiades, which shows the familiar stars deeply involved and almost buried in a seething whirl of nebulous structure that was never suspected till the camera showed it. Perhaps the whole sky will turn out to be covered with these "Regions of lucid matter taking form, Brushes of fire, hazy gleams," which Tennyson foresaw with the poet's eye more than fifty years ago. Here, as Professor Turner says, " we are getting information which we have only had time as yet to marvel at —not to interpret."
It would be difficult to over-estimate the importance of the "photographic eye," which is perhaps the most remarkable instrumental gain made by the astronomer since Galileo turned his optic tube on the moon ; the spectroscope is the only thing, at least, that can compete with it. The camera has furnished us with the power, not only of seeing the invisible, but of recording changes in the heavens with pre- cision. Professor Turner shows us the importance of its passionless and inhuman observation by comparing two photographs of the solar corona taken at one eclipse with two drawings made of it at another by different observers. The photographs show the same features in every respect, while the drawings could not be recognised for representations of the same thing; and there is no doubt that the camera.
tells the truth in this case. The international star- chart, the invaluable photographs taken regularly by Pro-
fessor Pickering (who was justly rewarded the other day with the gold medal of the Royal Astronomical Society), and the discovery of Eros by the trace which it left on a photo- graph, illustrate three separate branches of the camera's use in photography. We would gladly follow Professor Turner through his discourse on modern instruments and methods,.
but we have not the space, and we shall do our readers good service by persuading them to read his book for themselves.. We must just mention his sensible protest against recent popular estimates of the power of our latest telescopes,.
especially in regard to the possibility of seeing " signals from Mars." He justly argues that the best view we can get of Mars with the Lick or Yerkes telescope is about equal to the- naked-eye view of the moon. The likelihood of seeing any evidence of life may be imagined to be small.
The chapter on modern results gives a representative, though necessarily very incomplete, collection of the most
striking discoveries of the last twenty-five years. First among these is placed the discovery of the motion of the terrestrial pole, which the pertinacity of Mr. Chandler did so much to establish. The North Pole actually moves round an irregular orbit in a period of about fourteen months. Its motion never carries it more than a few yards from its mean position, but even this gives rise to apparent changes in the latitude of observatories which, though tiny, are quite perceptible.. Again, the distance of the sun has been ascertained of late with an exactitude long thought unattainable ; the error in the latest estimate of 92,874,000 miles is almost certainly less-- than 100,000 miles,—or less than the thickness of a stump in a cricket pitch of 22 yards. The discovery of the fifth satellite of Jupiter and of the satellites of Mars—which Swift anticipated by sheer force of imagination—is the• best testimony to the modern improvement in telescopes.
So is the confirmation by actual observation of the spectro- scopic suggestion that certain stars, such as Capella, were really double. This triumph fell to the new refractor at Greenwich.
Eros, whose importance is infinitely greater than that of any other minor planet, thanks to its nearness to the earth and the consequent aid that it will give to the measurement of planetary distances—like the ninth satellite of Saturn—was, as we have said, a photographic " catch." Much, too, has been definitely learnt about the probable constitution of the universe, where Herschel could do little more than guess ingeniously. Most interesting of all is the confirmation of the nebular theory—no longer a cloudy hypothesis—which Professor G. H. Darwin has given by his long series of classic investigations on the mathematical history of the solar system. Here, as in the reform of the lunar theory, we are getting into deep water, and we can only say that Professor Turner's last chapter explains both these high matters as- clearly as it can be done for the non-mathematical reader.
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