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 re- frangibility tlirough the end-on motion of its source, was first made effective for astronomical research by Huggins in 1868. The criterion of velocity, 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 accu- racy through Dr. Vogel's adaptation, in 1888, of photography to its requirements. It has since princd extraordinarily fruitful. Its employment en;il)Ieil Dr. Vogel to demonstrate the reality of .Algol'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 Har- vard College in 1889. These interesting systems cannot be sharply distinguished from telescopic doulile 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 in- clutlcs all sliort-period variables, even those that escape eclipses; though the connection between their duplicity and luminous variations remains unexplained, The pliotography 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. De.slandres at Paris inde- peniiently built up pictures of tliose objects out of the calcium-ray in their dispersed light, sifted through a double slit on to moving photographic plates. Professor Hale's invention of the " spectrohelio- graph" enables him, moreover, to delineate the sun's disc in any selected quality 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 Constrdction. — The investigation of the structure of the sidereal heavens was the leading object of William Herschel's career. The magnitude of the task, however, which he attempted single- handed grows more apparent with every fresh at- tempt to grapple witli 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 hemi- spheres. About thirty million stars will, it is esti- mated, appear on the chart-plates; and those pre- cisely catalogued are unlikely to fall short of four millions. The labour of discussing these multi- tudinous data must be severe, but will be animated by the hope of laying bare some hidden springs 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 nebulte be supposetl to form a separate scheme. The closeness of their relations, physical antl 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 Creator. Suggestive hints, on the contrary, of instability and evanescence help us to realize that the heavens are, in very truth, the changing vesture of Him whose "years cannot fail,"
Newcomb, Popular Astrovomy (London, 1883); Young, General Astronomy (Boston, 1898): Young, Manual of As- tronomy (Boston, 1902); B.\ll, The Story of the Heavens (London. 1900); Grant, History of Physical Astronomy (Lon- don, 1852); Clerke, Hist, of Astr. during the 19th Century (London, 1903); Berry, Hist, of Astronomy (London, 1898); Dreyer, Hist, of the Planetary Systems (London, 1906); Epping and Stras8maier, Astronomisches aus Babylon (Frei- biu-g, 1889); Kugler, Die babylonische Mondrechnung (Frei- burg, 1900): Tannery, Recherches sur t'hist. de I'astr. an- cienne (Paris, 1893^; Jensen, Kosmologie der Babytonifr (Strasburg, 1890); Young, The Sun (New York, 1897): Newcomb, The Stars (London, 1901); Clerke, The System of the Stars (London. 1905); Clerke, Problems in Astrophysics (London, 1903); Pickering, The Afoon (New York, 1903): Nasmyth and Carpenter, The Moon (London, 1903); Schei- NER, Die Speclralanalyse der Gestirne (Leipzig, 1890; tr. Bo.s- ton, 1894); Scheiner, Die Photographic der Gestirne (Leipzig. 1897); MOller, £>te Photometrie der Gestirne (Leipzig, 1897): Secchi, Le soleil (Paris, 1875-77); Moredx. Le problhne solaire (Paris, 1900); Turner, Modern .Astronomy (London. 1901); MouLToN, An Introduction to Astronomy (New York, 1906).
Agnes M. Clerke.
Astronomy in the Bible. — No systematic ob- servations of the heavenly bodies were made by the Jews. Astral worship was rife in Palestine, and they could hardly have attended closely to its objects without yielding to its seductions. Astronomy was, under these circumstances, inseparable from as- trolatry, and the anathemas of the prophets were not carelessly uttered. As the most glorious works of the Almighty, the celestial luminaries were indeed celebrated in the Scriptures in passages thrilling with rapture; but the appeal to them for practical pur- poses was reduced to a minimum. Even the regula- tion of times and seasons was largely empirical. The Jews used a lunar year. It began, for leligious pur- poses, with the new moon next after the spring equinox, and consisted normally of twelve months, or 354 days. The Jewish calendar, however, de- pended upon the course of the sun, since the festivaKs it appointed were in part agricultural celebrations. Some process of adjustment had then to be resorted to, and the obvious one was chosen of adding a thirteenth, or intercalary, month whenever the dis- crepancy between the ripening of the crops and the fixed dates of the commemorative feasts became glaringly apparent. Before the time of Solomon, the Jews appear to have begun their year in the autumn; and the custom, revived for civil purposes about the fifth century B. c, was adopted in the systematized religious calendar of the fourth century of our era.
Both the ritual and the civil day commenced in the evening, about half an hour after sunset. Its subdivisions were left indeterminate. The Old Testa- ment makes no mention of what we call hours; and it refers to the measurement of time, if at all, only in the narrative of the miracle wrought by Isaias in connection with the sundial of Achaz (IV Kings, xx, 9-11). In the New Testament, the Roman practice of counting four night-watches has superseded the antique triple division, and the day, as among the Greeks, consists of twelve equal parts. These are the "temporary hours" which still survive in the liturgy of the Church. Since they spanned the in- terval from sunrise to sunset, their length varied with the season of the year, from 49 to 71 minutes. Corresponding nocturnal hours, too, seem to have been partially used in the time of the Apostles (.A.cts, xxiii, 23).
As might have been expected, the Sacred Books convey no theory of celestial appearances. The descriptive phrases used in them are conformed to the elementary ideas naturally presenting themselves