Astronomy

Astronomy

(From Gr. astron, star; nemein, to distribute).

A science of prehistoric antiquity, originating in the elementary needs of mankind. It is divided into two main branches, distinguished as astrometry and astrophysics; the former concerned with determining the places of the investigation of the heavenly bodies, the latter, with the investigation of their chemical and physical nature. But the division is of a quite recent date. The possibilities of antique science stopped short at fixing the apparent positions of the objects on the sphere. Nor was any attempt made to rationalize the observed facts until Greeks laboriously built up a speculative system, which was finally displaced by vast fabric of gravitational theory. Descriptive astronomy, meanwhile took its rise from the invention of the telescope, and the facilities thus afforded for the close scrutiny of the denizens of the sky; while practical astronomy gained continually in refinement with the improvement of optical and mechanical arts. At the present time, astrophysics may be said to have absorbed descriptive astronomy, and astrometry necessarily includes practical research. But mathematical astronomy, grounded on the law of gravitation keeps its place apart, though depending for the perfecting of its theories and the widening of its scope upon advances along the old, and explorations in new, directions.

PREHISTORIC ASTRONOMY

Formal systems of astronomical knowledge were early established by the Chinese, Indians, Egyptians, and BabyIonians. The Chinese were acquainted, probably in the third millenium B.C., with the cycle of nineteen years (rediscovered in 632 B.C. by Meton at Athens) by which, since it comprised just 235 lunations, the solar and lunar years were harmonized; they recorded cometary apparitions, observed eclipses, and employed effective measuring apparatus. European methods were introduced at Pekin by Jesuit missionaries in the seventeenth century. Indian astronomy contained few original elements. It assigned particular prominence to the lunar zodiac, called the nakshatras, or mansions of the moon, variously reckoned at twenty-seven or twenty-eight; and these, which were probably a loan from Chaldea, served mainly for superstitious purposes. In Egypt, on the other hand, considerable technical skill was attained and a constellational system of obscure derivation, came in use. The Babylonians alone, among the nations of the fore-time, succeeded in laying the foundations of a progressive science. Through the medium of the Greeks, they transmitted to the West their entire scheme of uranography, our familiar constellations having been substantially designed on the plain of Shinar about 2800 B.C. Here, too, at a remote epoch, the "Saros" became known. This is a cycle of eighteen years and ten or eleven days, which affords the means of predicting the recurrence of eclipses. The changing situations of the planets among the stars were, moreover, diligently recorded, and accurate acquaintance was secured with the movements of the sun and moon. The interpretation in 1889, by Fathers Epping and Strassmaier, of a collection of inscribed tablets preserved in the British Museum vividly illuminated the methods of official Babylonian astronomy in the second century B.C. They were perfectly effectual for the purpose chiefly in view, which was the preparation of yearly ephemerides announcing expected celestial events, and tracing in advance the paths of the heavenly bodies. Further analysis in 1899 by Father Kugler, S.J., of the tabulated data employed in computing the moon's place, disclosed the striking fact that the four lunar periods -- the synodic, sidereal, anomalistic, and draconitic months -- were substantially adopted by Hipparchus from his Chaldean predecessors.

GREEK ASTRONOMY

Astronomy, however, no sooner became a distinctively Greek science than it underwent a memorable transformation. Attempts began to be made to render the appearances of the sky intelligible. They were, indeed, greatly hampered by the assumption that movement in space must be conducted uniformly in circles, round an immobile earth; yet the problem was ostensibly solved by Appollonius of Perga (250-220 B.C.), and his solution, applied by Hipparchus to explain the movements of the sun and moon, was extended by Claudius Ptolemaeus (Ptolemy) to the planets. This was the celebrated theory of eccentrics and epicycles, which, by the ingenuity of its elaboration, held its own among civilized men during fourteen centuries. Hipparchus, the greatest of ancient astronomers, observed at Rhodes (146-126 B.C.), but is considered as belonging to the Alexandrian school. He invented trignometry, and constructed a catalogue of 1080 stars, incited, according to Pliny's statement, by a temporary stellar outburst in Scorpio (134 B.C.). Comparing, as work progressed, his own results with those obtained 150 years earlier by Timocharis and Aristyllus, he detected the slow retrogression among the stars of the point of intersection of the celestial equator with the ecliptic, which constitutes the phenomenon of the precession of the equinoxes. The circuit is completed in 25,800 years; hence the tropical year, by which the seasons are regulated, is shorter than the sidereal year by just twenty-one minutes, the equinox shifting backward to meet the sun by the annual amount of 50.25 inches. Greek astronomy was embodied in Ptolerny's "Almagest" (the name is of mixed Greek and Arabic derivation), composed at Alexandria about the middle of the second century A. D. It was based upon the geocentric principle. The starry spere, with its contents, was supposed to resolve, once in twenty-four hours, about the fixed terrestrial globe, while the sun and moon, and the five planets, besides sharing the common movement, described variously conditioned orbits round the same centre. The body of doctrine it inculcated made part of the universal stock of knowledge until the sixteenth century. The formidable task of demonstrating its falsity, and of replacing it with a system corresponding to the true relations of the world, was undertaken by the active and exemplary ecclesiastic, Nicholas Copernicus, Canon of Frauenburg (1473-1543). The treatise in which it was accomplished, entitled "De Revolutione Orbium Coelestium", saw the light only when its author lay dying; but a dedication to Pope Paul III bespoke the protection of the Holy See for the new and philosophically subversive views which it propounded. Denounced as impious by Luther and Melanchton, they were, in fact, favourably received at Rome until theological discredit was brought upon them by the wild speculations of Giorano Bruno (1548-1600), and the imprudent utterances of Galileo Galilei (1564-1642).

DESCRIPTIVE ASTRONOMY

Descriptive astronomy may be said to have originated with the invention of the telescope by Hans Lippershey in 1608. Its application to the scrutiny of the heavenly bodies, by Galileo and others, led at once to a crowd of striking discoveries. Jupiter's satellites, the phases of Venus, the mountains of the moon, the spots on the sun, Saturn's unique appendages, all descried with a little instrument resembling a uniocular opera-glass, formed, each in its way, a significant and surprising revelation; and the perception of the stellar composition of the Milky Way represented the first step in sidereal exploration. Johann Kepler (1571-1630) invented in 1611, and Father Scheiner of Ingolstadt (1575-1650) first employed, the modern refracting telescope; and the farther course of discovery corresponded closely to the development of its powers. Christian Huygens (1629-95) resolved, in 1656, the ansae of Saturn into a ring, divided into two by Giovanni Domenico Cassini (1625-1712) in 1675. Titan, the Iargest of Saturn's moons, was detected by Huygens in 1655, and four additional members of the family by 1684. The Andremeda nebula was brought to notice by Simon Marius in 1612, the Orion nebula by J.B. Cysatus, a Swiss Jesuit, in 1618; and some few variable and multiple stars were recognized.

THEORETICAL ASTRONOMY

The theoretical, however, far outweighed the practical achievements of the seventeenth century. Kepler published the first two of the "Three Laws" in 1609, the third in 1619. The import of these great generalizations is:

• that the planets describe ellipses of which the sun occupies one focus;
• that the straight line joining each planet with the sun (its radius vector) sweeps out equal areas in equal times;
• that the squares of the planetary periods are severally proportional to the cubes of their mean distances from the sun.

The geometrical plan of movement in the solar system was thus laid down with marvellous intuition. But it was reserved for Sir Isaac Newton (1643-1727) to expound its significance by showing that the same uniformly acting force regulates celestial revolulions, and compels heavy bodies to fall towards the earth's surface. The law of gravity, published in 1687 in "Philosophiae Naturalis Principia Mathamatica" is to the following effect: every particle of matter attracts every other with a force directly proportional to their masses, and inversely proportional to the squares of their distances apart. Its validlty was tested by comparing the amount of the moon's orbital deflection in a second with the orbital deflection in a second with the rate at which an apple (say) drops in an orchard. Allowance being made for the distance of the moon, the two velocities proved to tally perfectly, and the identity of terrestrial gravity with the force controlling the revolutions of the heavenly established. But this was only a beginning. The colossal work remained to be accomplished of calclulaIting the consequences of the law, in the minute details of its working, and of comparing them with the heavens. It was carried foreward first by Newton himself, and in the ensuing century, by Euler, Clairaut, d' Alembert, Lagrange, and Laplace. Urbain Leverrier (1811- 77) inherited from these men of genius a task never likely to be completed; and the intricacies of lunar theory have been shown, by the researches of John Cough Adams (1819-92), of Hansen and Delaunay, of Professors Hlll and Newcomb, and many more, to be fraught with issues of unexpected and varied interest.

DISCOVERIES IN THE SOLAR SYSTEM

The extraordinary improvement of reflecting telescopes by Sir William Herschel (1738-1822) opened a fresh epoch of discovery. His recognition of the planet Uranus (13 March 1781) as a non-stellar object of old to the solar system; two Uranian moons, Oberon and Titania, were detected by him 11 January 1787, and the innermost Saturnian pair, Enceladus and Mimas, 28 August and 17 September of the same year. Saturn was, in 1906, known to possess ten satellites. Hyperion was descried by W.C. Bond at the observatory of Harvard College 16 September, 1848, and Professor W.H. Pickering, of the same establishment, discovered by laborious photographic researches, Phoebe in 1898, and Themis in 1905. In point of fact, an indefinite number of satellites are agglomerated in the rings of Saturn. Their constitution by separately revolving, small bodies, theoretically demonstrated by J. Clerk Maxwell in 1857, was spectroscopically confirmed by the late Professor Keeler in 1895. The system includes a dusky inner member, detected by Bond, 15 November, 1850. The discovery of the planet Neptune, 23 Sepember, 1846, was a mathematical, not an observational feat. Leverrier and Adams independently divined the existence of a massive body, revolving outside Uranus, and exercising over its movements disturbances the analysis of which led to its capture. Its solitary moon was noted by William Lassell of Liverpool in October, 1846; and he added, in 1851, two inner satellites to the remarkable system Uranus. With the great Washington refractor, 26 inches in aperture, Professor Asaph Hall discerned, 16 and 17 August,1877, Deimos and Phobos, the swiftly circling moonlets Mars; the Lick 36-ich enabled Professor Barnard to perceive, 9 September, 1892, the evasive inner satellite of Jupiter; and two exterior attendants on the same planet were photographically detected by Professor Perrine in 1904-05. The distances of the planets are visibly regulated by a method. They increase by an ordered progression, announced by Titius of Wittenberg in 1772, and since designated as "Bode's Law". But their succession was quickly seen to be interrupted by a huge gap between the orbits of Mars and Jupiter; and the conjecture was hazarded that here a new planet might be found to revolve. It was verified by the discovery of an army of asteiods. Ceres, their leader, was captured at Palermo, 1 January, 1801, by Giuseppe Piazzi, a Theatine monk (1746-1826); Pallas, in 1802 by Olbers (1758-1840), and Juno and Vesta in 1804 and 1807, by Harding and Olbers respectively. The original quartette of minor planets began in 1845 to be reinforced with companions, the known number of which now approximates to 600, and may be indefinitely increased. Their discovery has been immensely facilitated by Professor Max Wolf's introduction, in 1891, of the photographic method of discriminating them from stars through the effects of their motion on sensitive plates.

The solar system, as at present known, consists of four interior planets, Mercury, Venus, the Earth, and Mars; four exterior; and relatively colossal planets, Jupiter, Saturn, Uranus, and Neptune, the diffuse crowd of pygmy globes called asteroids, or minor planets, and an outlying array of comets with their attentant meteor-systems. All the planets rotate on their axes, though in very different periods. That of Mercury was determined by Signor Schiaparelli of Milan in 1889 to be 88 days, the identical time of his revolution round the sun, and Venus was, in the following year, shown by him to be, in all likelihood, similarly conditioned, the common period of rotation and circulation being, in her case, 225 days. This irnplies that both planets keep the same hemisphere always turned towards the sun, as the moon does towards the earth; nor can we doubt that the friction of tidal waves was, on the three bodies, the agency by which the observed synchronisrn was brought about. All the planets travel round the sun from west to east or counter clock-wise and most of the satellites move in the same direction round their primaries. But there are exceptions. Phoebe, Saturn's remotest moon, circulates oppositely to the other members of the system; the four moon of Uranus are retrograde, their plane of movement being inclined at more than a right angle to the ecliptic; and the satellite of Neptune travels quite definitely backward. These anomalies are of profound import to the theories of planetary origin. The "canals" of Mars were recognized by Schiaparelli in August 1877, he caught sight of some of them duplicted two years later. Their photographic registration at the Lowell observatory in 1905 proves them to be no optical illusion, but their nature remains enigmatical.

COMETS AND METEORS

The predicted return of Halley's comet in 1759 afforded the first proof that bodies of the kind are permanently attached to the sun. They accompany its march through space, traversing, in either direction indifferently, highly eccentric orbits inclined ecliptic. They are accordingly subject to violent, even subversive disturbances from planets. Jupiter, in particular, sways the movements of a group of over thirty "captured" comets, which had their periods curtailed, and their primitive velocities reduced by his influence. Schiaparelli announced in 1866 that the August shooting-stars, or Perseids, pursue the same orbit with a bright comet visible in 1862; and equally striking accordances of movement between three other comets and the Leonid, Lyraid, and Andromede meteor-swarms were soon afterwards established by Leverrior and Weiss. The obvious inference is that meteors are the disintegration-products of their cometary fellow-travellers. A theory of comets' tails, based upon the varying efficacy of electrical repulsion upon chemically different kinds of matter, was announced by Theodor Brédikhine of Moscow in 1882, and gave a satisfactory account of the appearances it was invented to explain. Latterly, however, the authority of Arrhenius of Stockholm has lent vogue to a "light-pressure" hypothesis, according to which, cometary appendages are formed of particles driven from the sun by the mechanical stress of his radiations. But the singular and rapid changes photograpically disclosed as taking place in the tails of comets, remain unassociated with any known cause.

SIDEREAL ASTRONOMY

Sir William Herschel's discovery, in 1802, of binary stars, imperfectly anticipated by Father Christian Mayer in 1778, was one of far-reaching scope. It virtually proved the realm of gravity to include sidereal regions; and the relations it intimated have since proved to be much more widely prevalent than could have been imagined beforehand. Mutually circling stars exist in such profusion as probably to amount to one in three or four of those unaccompanied. They are of limitless variety, some of the systems by them being exceedingly close and rapid, while others describe, in millennial periods, vastly extended orbits. Many, too, comprise three or more members; and the multiple stars thus constituted merge, by progressive increments of complexity, into actual clusters, globular and irregular. The latter class exemplified by the Pleiades and Hyades, by the Beehive cluster in Cancer, just visible to the naked eye, and by the double cluster in Perseus which makes a splendid show with an opera-glass. Globular clusters are compressed "balls" of minute stars, of which more than one hundred have been catalogued. The scale on which these marvellous systems are constructed remains conjectual, since their distances from the earth are entirely unknown. Variable stars are met with in the utmost diversity. Some are temporary apparitions which spring up from invisibility often to an astonishing pitch of spendor, then sink back more slowly to quasi-extinction. Nova Persei, which blazed 22 February, 1901, and was photographically studied by Father Sidgreaves at Stonyhurst, is the most noteworthy recent instance of the phenomenon. Stars, the vicissitudes of which are comprised in cycles of seven to twenty months, or more, are called "long-period variables". About 400 had been recorded down to 1906. They not uncommonly attain, at maximum, to 1,000 times their minimum brightness. Mira, the "wonderful" star in the Whale, discovered by David Fabricius in 1596, is the examplar of the class. The fluctuations of "short-period variables" take place in a few days or hours, and with far more punctuality. A certain proportion of them are "eclipsing stars" (about 35 have so far been recognized as such), which owe their regularly recurring failures of light to the interposition of large satellites. Algol in Perseus, the variations of which were perceived by Montanari in 1669, is the best-known specimen. Hundreds of rapid variables have been recently detected among the components of glabular clusters; but their course of change is of a totally different nature from that of eclipsing stars. Edmund Halley (1656-1742), the second Astronomer Royal, announced in 1718 that the stars, far from being fixed, move onward, each on its own account, across the sky. He arrived at this conclusion by comparing modern with antique observations; and stellar "proper motions" now constitute a wide and expansive field of research. A preliminary attempt to regularize them was made by Herschel's determination, in 1783, of the sun's line of travel. His success depended upon the fact that the apparent displacements of the stars include a common element, transferred by perpective from the solar advance. Their individual, or "peculiar" movements, however, show no certain trace of method. A good many stars, too, have been ascertained to travel at rates probably uncontrollable by the gravitational power of the entire sidereral system. Arcturus, with its portentous velocity of 250 miles a second, is one of these "runaway" stars. The sun's pace of about 12 miles a second, seems, by comparison, extremely sedate; and it is probably only half the average stellar speed. The apex of the sun's way, or the towards which its movement at present tends, is located by the best recent investigations near the bright star Vega.

DISTANCES OF THE SUN AND STARS

The distances of the heavenly bodies can only be determined (speaking generally) by measuring their parallaxes, in other words, their apparent changes of position when seen from different points of view. That of the sun is simply the angle subtended at his distance by the earth's semi-diameter. Efforts were made with indifferent success to fix its value by the transits of Venus in the eighteenth and nineteenth centuries. The asteroids have proved more efficient auxiliaries and through the mediation of Iris, Sappho, and Victoria, in 1889-89, Sir David Gill assigned to the great unit of space a length of 92,800,000 miles, which the photographic measures of Eros, in 1900-01, bid fair to ratify. The stars, however, are so vastly remote that the only chance of detecting their perspective displacements is by observing them at intervals of six months, from opposite extremities of a base-line nearly 186,000 miles in extent. Thus, the annual parallax of a star means the angle under which the semi-diameter of the earth's orbit would be seen if viewed frorn its situation. This angle is in all cases, extremely minute, and in most cases, altogether evanescent; so that, from only about eighty stars (as at present known), the terrestrial orbit would appear to have sensible dimensions. Our nearest stellar neighbour is the splendid southern binary, Alpha Centauri; yet its distance is such that light needs four and one-third years to perform the journey thence. Thomas Henderson (1798-1844) announced his detection of its parallax in 1839, just after Bessel of Konigsberg (1784-1846) had obtained a similar, but smaller result for an insignificant double star designated 61 Cygni.

CELESTIAL PHOTOGRAPHY

The second half of the nineteenth century was signalized by a revolutionary change in the methods and purposes of astronomy. Experiments in lunar photography, begun in 1840 by J.W. Draper of New York, were continued in the fifties by W.C. Bond, Warren de la Rue, and Lewis M. Rutherfund. The first daguerreoype of the sun was secured at Paris in 1845, and traces of the solar corona appeared on a sensitized plate exposed at Konigsberg during the total eclipse of 28 July, 1851. But the epoch of effective solar photography opened with the Spanish eclipse of 18 July, 1860, when the pictures successively obtained by Father Angelo Secchi, S.J., and Waren de la Rue demonstrated the solar status of the crimson protuberances by rendering manifest the advance of the moon in front of them. At subsequent eclipses, the leading task of the camera has been the portrayal of the corona; and its importance was enhanced when A.C. Ranyard pointed out, in 1879, the correspondence of changes in its form with alterations of sunspots was published in 1851 by Schwabe of Dessau; and among the numerous associated phenomena of change, none are better ascertained than those affecting the shape of the silvery aureola seen to encompass the sun when the moon cuts off the glare of direct sunlight. At spot maxima the aureola spreads its beamy radiance round the disc. But at times of minimum, it consists mainly of two great wings, extended in the sun's equatorial plane. A multitude of photographs, taken during the eclipses of 1898, 1900, 1901 and 1905, attest with certainty the punctual recurrence of these unexplained vicissitudes. The fundamental condition for the progress of sidereal photography is the use of long exposures; since most of the objects to be delineated emit light so feebly that its chemical effects must accumulate before they become sensible. But long exposures were impracticable until Sir William Huggins, in 1876, adopted the dry-plate process; and this date, accordingly, marks the beginning of the wide-spreading serviceableness of the camera to astronomy. In nebular investigations above all, it far outranges the telescope. Halley described in 1716 six nebulae, which he held to be composed of a lucid medium collected from space. The Abbé Lacaille (1713-62) brought back with him from the Cape, in 1754, a list of forty-two such objects; and Charles Messier (1730-1817) enumerated in 1781, 103 nebulae and clusters. But this harvest was scanty indeed compared with the lavish yield of Herschel's explorations. Between 1786 and 1802 he communicated to the Royal Society catalogues of 2500 nebulae; he distinguished their special forms, classified them in order of brightness, and elaborated a theory of stellar development from nebulae, illustrated by selected instances of progressive condensation. The next considerable step towards a closer acquaintance with nebulae was made by Lord Rosse in 1845, when the prodigious light-grasp of his six-foot reflector afforded him the discovery of the great "Whirlpool" structure in Canes Venatici. It proved to be typical of the entire class of spiral nebulae, the large prevalence of which has been one of the revelations of photography. The superiority in nebula-portraiture of the chemical to the eye-and-hand method was strikingly manifested in a photograph of the Orion nebula taken b Dr. A. A. Common, 30 January, 1883. Its efficacy for discovery became evident through the disclosure, on plates exposed by Paul and Prosper Henry, and by Isaac Roberts in 1885- 86, of complex nebulous formations in the Pleides, almost wholly invisible optically. Professor Keeler (1857-1900) estimated at 120,000 the number of nebulae which the Crossley reflector of the Lick observatory would capable of recording in both hemispheres with an hour's exposure, while telescopically constructed catalogues include less than 10,000. But it is through the combination of photography with spectroscopy, constituting the spectrographic mode of research, that astrophysics has achieved its most signal triumphs.

ASTROPHYSICS

The fundamental principle of spectrum analysis, enunciated by Gustav Kirchhoff (1824-87), depends upon the equivalence of emission and absorption. This means that, if white light be transmitted through glowing vapours, they arrest just those minute sections of it with which they themselves shine. And if the source of the white light be hotter than the arresting vapour, there results a prismatic spectrum, interrupted by dark lines, distinctive of the chemical nature of the susbtance originating them. Now this is exactly the case of the sun and stars. The white radiance emanating from their photopheres is found, when dispersed into a spectrum, to be crossed by numerous dusky rays indicating absorption by gaseous strata, to the composition of which Kirchhoff's principle supplies the clue. Kirchhoff himself identified in 1861, as prominent solar constituents, sodium, iron magnesium, calcium, and chromium; by A.J. Angström (1814-74); helium by Sir Norman Lockyer in 1868; and about forty elementary substances are now known with approximate certainty to be common to the earth and sun. The chemistry of the stars is strictly analogous to that of the sun, although their spectra exhibit diversities symptomatic of a considerable variety in physical state. Father Angelo Secchi, S.J. (1818-78), based on these diversities in 1863-67 a classification of the stars into four orders, still regarded as fundamental and supplied by Dr. Vogel in 1874 with an evolutionary interpretation, according to which differences of spectral type are associated with various stages of progress from a tenuous and inchote towards a compact condition. Since 1879, when Sir William Huggins secured impressions of an extended range of ultra-violet white star light, stellar spectra have been mostly studied photographically, the results being, not only precise and permanent, but also more complete than those obtainable by visual means. The same eminent investigator discovered, in 1864, the bright-line spectra of certain classes of nebulae, by which they were known to be of gaseous composition, and recognized, as of carbonaceous origin, the typical coloured bands of the cometary spectrum, noted four years previously, though without specific identification, by G.B. Donati (1827-73) at Florence.

Doppler's principle, by which light alters in refrangibility through the end-on motion of its source, was first made effective for astronomical reseach by 1868. The criterion of velocilty, whether of recession or approach, is afforded by the shifting of spectral lines from their standard places; and the method was raised to a high grade of accuracy through Dr. Vogel's adaptation, in 1888, of photography to its requirements. It has since proved extraordinarily fruitful. Its employment enabled Dr. Vogel to demonstrate the reality of AIgol's eclipses, by showing that the star revolved round an obscure companion in the identical period of light-change; and the first discoveries of non-eclipsing spectroscopic binaries were made at Harvard College in 1889. These interesting systems cannot be sharply distinguished from telescopic double stars, which are, indeed, believed to have developed from them under the influence of tidal friction; their periods vary from a few hours to several months; and their components are often of such unequal luminosity that only one leaves any legible impression on the sensitive plate. Their known number amounted, in 1905, to 140; and it may be indefinitely augmented. It probably includes all short-period variables, even those that escape eclipses; though the connection between their duplicity and luminous variations remains unexplained. The photography in daylight of solar prominences was attempted by Professor Young of Princeton in 1870, and the subject was prosecuted by Dr. Braun, S. J., in 1872. No genuine success was, however, achieved until 1891, when Professor Hale of Chicago and M. Deslandres at Paris independently built up pictures of those objects out of the calcium-ray in their dispersed light, sifted through a double slit onto moving photographic plates. Professor Hale's invention of the "spectroheliograph" enables him, moreover, to delineate the sun's disc in any selected of its light, with the result of disclosing vast masses of calcium and hydrogen flocculi, piled up at various heights above the solar surface.

SIDEREAL CONSTRUCTION

The investigation of the structure of the sideral heavens was the leading object of William Herschel's career. The magnitude of the task, however, which he attempted singlehanded grows more apparent with every fresh attempt to grapple with it; and it now engages the combined efforts of many astronomers, using methods refined and comprehensive to a degree unimagined by Herschel. An immense stock of materials for the purpose will be provided by the international photographic survey, at present advancing towards completion at eighteen observatories in both hemispheres. About thirty million stars will, it is estimated, appear on the chart-plates; and those precisely catalogued are unlikely to fall short of four millions. The labour of discussing these multitudinous data must be severe, but will be animated by the hope of laying bare some hidden spring of the sidereal mechanism. The prospect is indeed remote that the whole of its intricacies will ever be penetrated by science. We only perceive that the stars form a collection of prodigious, but limited, extent, showing strongly concentrative tendencies towards the plane of the Milky Way. Nor can the nebulae be supposed to form a separate scheme. The closeness of their relations, physical and geometrical, with stars excludes that supposition. Stars and nebulae belong to the same system, if such the sidereal world may properly be called in the absence of any sufficient evidence of its being in a state of dynamical equilibrium. We cannot be sure that it has yet reached the definitive term appointed for it by its instability and evanescence help us to realize that the heavens are, in very truth, the changing vesture of Him whose "years cannot fail".

AGNES M. CLERKE