Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/848

Rh 798 H Z O N that the spectrum differs from that of sunlight only in intensity. Some recent unpublished observations made in the tropics indicate that, while the spectrum is usually that described by Prof. Smyth, there are times when a bright line is seen. Too much stress, however, must not be laid on this, as the observations are by no means conclusive, and the apparent line may in reality be only a part of the continuous spectrum which is brighter than usual. The discussion of the real cause of the zodiacal light is rendered very difficult by the want of agreement in the observations that have been made upon it, and by the existence of a small number of apparently trustworthy observations of a very abnormal extension of the li^ht, as detailed above ; but certain conclusions may be safely arrived at. The theory that it is due to a ring of small bodies surrounding the earth seems to be entirely negatived, as pointed out by Proctor and others, by several of its features. The best observations leave no room for the parallactic displacement which would be observed if there were such a ring round the earth, and the absence of a large part of the luminous circle on ordinary occasions would be inexplicable on any such hypothesis. There can therefore be very little doubt that we must look for the cause of the light to the existence of a mass of small bodies moving in orbits round the sun, and that, as shown by the polarization and the spectrum, the light is chiefly, if not entirely, reflected sunlight. To account for all the observa tions, Proctor has shown that the bodies must travel in orbits of considerable eccentricity, carrying them far beyond the limits of what we may term the zodiacal disk. Under these conditions the constitution of the disk would become variable within exceedingly wide limits, and all the recorded variations might be fairly explained. The appearance of a complete arch, as seen by Liais and others, would indicate an extension far beyond the earth s orbit ; but, at present at least, this must be looked on as very exceptional. The spectroscopic evidence, so far as it goes, confirms this theory, since it indicates that the light is mainly reflected sunlight and, even if further observations should confirm the opinion that bright lines are sometimes present, this need in no way invalidate the conclusions that have been arrived at. Indeed, it seems highly probable especially if Mr Lockyer s views regarding meteorites are confirmed that bright lines should be seen in such a body of meteorites. Olbers long ago suggested that the corona was the brightest part of the zodi acal light, and there is nothing in recent observations to con tradict this view, for all observations go to show that the outer corona has no definite boundary, but shades off imper ceptibly and becomes invisible at a great distance from the sun. Dr Huggins, while holding that the corona is most probably due to the ceaseless outflow of extremely minute particles from the sun, thinks it not improbable that the zodi acal light may be in some way connected with this outflow. Dr Siemens, when discussing his theory of the conservation of solar energy, sought for an explanation of the zodiacal light in the dust which he supposed to be ejected from equa torial regions, rendered luminous partly by reflected sunlight, partly by phosphorescence, and partly by electrical action. With the increasing number of observatories at high altitudes it may fairly be hoped that before long astro nomers will be put in possession of such definite measure ments as will enable some at least of the points still under discussion to be finally settled, and that far more accurate observations will soon be available on which to construct a satisfactory theory. Bibliography. Childrey, Natural History of England (1659) and Britannia Baconica, p. 183 (1661); Cassini (D.), Nouv. Pheiiom. d une lumiere celeste [zodiacale] (1683) and Decouvcrte de la lumiere celeste qui paroist dans le zodiaque (1685) ; Hooke (R.), Explication of a Glade of Light, &c. (1685) ; Mairan, Observations de la lumiere zodiacale; Euler (L.), Sur la cause de la lumiere zodiacale (1746) ; Mairan, Sur la cause de la lumiere zodiacale (1747); Wolf (R. ), Beobachtungcn des Zodiacallichtes (1850-52); Brorsen, Ucbcr den Gegenschein des Zodiacallichts (1855) and in Schu macher, 998 ; Schmidt (J. F. J.), Das Zodiacallicht, Brunswick (1856), and in Astron. Nachr., Ixxiii. p. 199; Swob, Memoirs R. A. S., xxviii. p. 119; Jones (G.), in Gould, No. 84, in Monthly Notices R.A.S., xvi. p. 18, and in U. S. Exploring Expedition Narrative, vol. iii. (1856) ; Humboldt, in Monatsber. d. k. preuss Akad. d. Wiss., July 1855, also in M. Not. R.A.S., xvi. p. 16 ; Smyth (C. P.), in Trans. R.S.E.,x. p. 489 (1852), and in M. Not. R.A. S., xvii. p. 204 and xxxii. p. 277 ; Backhouse (T. W.), in M. Not. R.A.S., xxxvi. p. 1 and xli. p. 333 ; Tupman, in M. Not. R.A.S., xxxii. p. 74 ; Liais, in CompUs Rcndus, Ixiv. p. 262 (January 1872); Wright (A. W.), in Amer. Jour, of Science, cvii. p. 451 and cviii. p. 39 ; Angstrom, in Pogg. Annal., cxxxvii. p. 162 ; Searle (Arthur), in Proc. Amn: Acad., xix. p. 146 and vol. xi. p. 135 ; and Trouvelot, in Proc. Amcr. Acad., xiii. p. 183 (1877). (C. M. S.) ZOHAR. See KABBALAH. ZOLLNER, JOHANN CARL FRIEDRICH (1834-1882), astronomer and physicist, was born at Berlin on 8th November 1834. From 1872 he held the chair of astro physics at Leipsic university. He is the author of numer ous papers on photometry and spectrum analysis in Poggen- dorff s Annalen and Berichte der k. scichsischen Gesellschaft der Wissenschaften, of two works on astronomical photo metry (Grundziige einer allgemeinen Photometric des Him- mels, Berlin, 1861, 4 to, and Photometrische Untersuchungen, Leipsic, 1865, 8vo), and of a very strange book, Ueber die Natur der Cometen (Leipsic, 1872, 8vo). He died at Leipsic on 25th April 1882. Zollner s &quot;astrophotometer &quot; compares the light of a star as seen in a telescope with that of an artificial star produced by a paraffin lamp. From the latter the light passes through three Nicol s prisms, of which two can be turned so as to vary the intensity of the light, the latter being proportional to the square of the cosine of the angle through which the prisms are turned, which angle is read off on a small circle. In order to vary the coloxir of the arti ficial star, so as to make it resemble the natural star as much as possible, there is inserted between the first and second prisms a plate of left-handed quartz, cut perpendicular to the axis, the rota tion of which is read off on another circle. This instrument has been used by Zb llner, Lindemann, Engelmann, J. T. Wolff, and others for photometric observations of stars and planets ; but it labours under several defects, among which the difficulty of keeping the flame of the lamp at a constant height is the most serious. ZOMBOR, a royal free city of Hungary, the capital of the county of Bacs-Bodrog, lies about 1 20 miles south of Buda pest in a fertile plain, on the Francis Canal that connects the Danube and the Theiss. The town has some fine streets and squares, and several handsome buildings, among which may be mentioned the county and town halls, the theatre, and the Roman Catholic and Greek churches. Zombor is a station on the Alfold-Fiume Railway and the centre of the corn and cattle trade of an extensive area. The popu lation numbered 24,693 in 1880 and about 31,000 in 1886. ZONARAS, JOANNES, historian and theologian, flour ished at Constantinople in the 12th century. Under Alexius I. Comnenus he held the offices of commander of the bodyguard and private secretary to the emperor, but in the succeeding reign he retired to Mount Athos, where he spent the rest of his life in writing his books. He is said to have lived to the age of eighty-eight. His most important work, the Chronicon, is in eighteen books, and extends from the creation of the world to the death of Alexius (1118). The earlier part is largely drawn from Josephus ; for Roman history he chiefly followed Dion Cassius, whose first twenty books are only known to us through Zonaras. Of contemporary events which he himself witnessed he writes more briefly and meagrely than might have been expected. His history was con tinued by Nicetas Acominatus. Among the other works of Zonaras is an Exposition of the Apostolical Canons. The Chronicon or Annals was first printed at Basel in 1557 ; Du Gauge next edited it in 1686 ; and it also forms part of the Bonn collection of Byzantine writers, having been edited by Finder (2 vols.) in 1841-44. The latest edition is by Dindorf, with Du Cange s notes (6 vols. 8vo, Leipsic, 1868-75). The Opera Omnia Historica, Canonica, Dog- matica, were published by Migne in 2 vols. 4to, at Paris in 1865. A lexicon, also attributed to Zonaras, has been edited by Tittmann (Leipsic, 1808).