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 form and boundary of the region from which the light emanates, the next question is that of the matter sending it forth. The most plausible view is that we have to do with sunlight reflected from meteoric particles moving round the sun within the region of the lens. The polariscope and the spectroscope are the only instruments by the aid of which the nature of the matter can be inferred. The evidence afforded by these instruments is not, however, altogether accordant. In 1867, Ångström, observing at Upsala in March, obtained the bright auroral line (W.L. 5567), and concluded that in the zodiacal light there was the same material as is found in the aurora and in the solar corona, and probably through all space. Upsala, however, is a place where the auroral spectrum can often be observed in the sky, even when no aurora is visible, and it has generally been believed that what Ångström really saw was an auroral and not a zodiacal spectrum.

Professor A. W. Wright, of New Haven, also made careful observations leading to the conclusion that the spectrum differs from sunlight only in intensity. Some evidence has also been found by the same observer of polarization, showing that a considerable portion of the light must be reflected sunlight. The observations of Maxwell Hall also embraced some made with the spectroscope. He was unable to see any marked deviation of the spectrum from that of the sun; but it does not appear that either he or any other of the observers distinctly saw the dark lines of the solar spectrum. Direct proof that we have to do with reflected sunlight is therefore still incomplete.

The question whether the Gegenschein can be accounted for by the reflection of light from the same matter as the zodiacal band is still unsettled. Taking the general consensus of the observations it would seem that its light must be so much brighter than that of the band as to imply the action of some different cause. In this connexion may be mentioned the ingenious suggestion of S. Arrhehius, that the phenomenon is due to corpuscles sent off by the earth and repelled by the sun in the same way that they are sent off from a comet and form its tail. In other words, the light may be an exceedingly tenuous cometary tail to the earth, visible only because seen through its very great length. The view that no cause intervenes additional to that producing the zodiacal band is strengthened, though not proved, by a theorem due to F. R. Moulton of Chicago. He shows that, supposing the cloud of particles to move around the sun in nearly circular orbits immediately outside the earth, the perturbations by the earth in the motion of the particles will result in their retardation in that part of the orbit nearest the earth, and therefore in their always being more numerous in a given space in this part of the orbit than in any other. This view certainly accounts for some intensification of the light, to which may be added the intensification produced by the vertical reflection of the sunlight.

A new interest was given to the subject by the investigations of H. H. Seeliger, published in 1906, who showed that the observed excess of motion of the perihelion of Mercury may be accounted for by the action of that portion of the matter reflecting the zodiacal light which lies nearest to the sun. Plausible though his result is, the subject still requires investigation. It seems not unlikely that the final conclusion will be that instead of the reflecting matter being composed of solid particles it is an exceedingly tenuous gaseous envelope surrounding the sun and revolving on an axis the mean position of which is between that of the sun’s equator and that of the invariable plane of the solar system.

.—Childrey, Natural History of England (1659) and Britannia Baconica, p. 183 (1661); D. Cassini, ''Nouv. Phénom.'' d’une lumière céleste [zodiacale] (1683) and Découverte de la lumière céleste qui paroist dans le zodiaque (1685); R. Hooke, Explication of a Glade of Light, &c. (1685); Mairan, Observations de la lumière zodiacale; L. Euler, Sur la cause de la lumière zodiacale (1746); Mairan, Sur la cause de la lumière zodiacale (1747); R. Wolf, Beobachtungen des Zodiacallichtes (1850–52): Brorsen Ueber den Gegenschein des Zodiacallichts (1855) and in Schumacher, 998; J. F. J. Schmidt, Das Zodiacallicht (Brunswick, 1856) and in

''Astron. Nachr.'', lxxiii. p. 199; Jacob, Memoirs R.A.S., xxviii. p. 119; G. Jones, in Gould, No. 84, Monthly Notices R.A.S., xvi. p. 18, ''Amer. Journ. of Science, Series II., vol. 24, p. 274, and in U.S.'' Exploring Expedition Narrative, vol. iii. (1856); Humboldt, ''Monatsber,. d. k. preuss. Akad. d. Wiss.'' (July 1855). ''M. Not. R.A.S.'', xvi. p. 16; C. P. Smyth, ''Trans. R.S.E.'', xx. p. 489 (1852), M. Not. R.A.S., xvii. p. 204, xxxii. p. 277; T. W. Backhouse. M. Not. R.A.S., xxxvi. p. 1 and xli. p. 333; Tupman, ''M. Not. R.A.S.'', xxxi. p. 74; Liais, Comptes Rendus, lxlv. p. 262 (January 1872); A. W. Wright, ''Amer. Jour, of Science'', cvii. p. 451 and cviii. p. 39; Ångström, ''Pogg. Annal.'', cxxxvii. p. 162; Arthur Searle, Proc. ''Amer. Acad.'', xix. p. 146, vol. xi. p. 135, and Annals of the Harvard Observatory, vol. xix.; Trouvelot, ''Proc. Amer. Acad.'', xiii. p. 183 (1877); Barnard, Popular Astronomy, vii. (1899) p. 171; Bayldon, ''Pub. Ast. Soc. of the Pacific'', vol. xii. (1900); Maxwell Hall, U.S. Monthly Weather Review (March 1906); Newcomb, Astrophysical Journal (1905) ii.

 ZOFFANY, JOHANN (1733–1810), British painter, whose father was architect to the prince of Thurn and Taxis, was born in Frankfort-on-Main. He ran away from home at the age of thirteen and went to Rome, where he studied art for nearly twelve years. In 1758 he left for England, and after undergoing some hardships was brought into fashion by royal patronage, and in 1769 was included among the foundation members of the Royal Academy. He went to Florence in 1772 with an introduction from George III. to the grand duke of Tuscany, and did not return until 1779. During this second stay in Italy he met with much success, and was commanded by the empress Maria Theresa to paint a picture of the royal family of Tuscany; this work he executed so much to the satisfaction of the empress that in 1778 he was created a baron of the Austrian empire. He went next to India, where he lived from 1783 to 1790, to which period belong some of his best-known paintings; but the last twenty years of his life were spent in England. He died in 1810 and was buried in Kew churchyard. His portrait groups of dramatic celebrities are, perhaps, the most highly esteemed of his many productions; they have considerable technical merit and show much shrewd insight into character. Several of the best are in the Garrick Club, London.  ZOÏLUS (c. 400–320 ), Greek grammarian of Amphipolis in Macedonia. According to Vitruvius (vii., preface) he lived during the age of Ptolemy Philadelphus (285–247 ), by whom he was crucified as the punishment of his criticisms on the king. This account, however, should probably be rejected. Zoïlus appears to have been at one time a follower of Isocrates, but subsequently a pupil of Polycrates, whom he heard at Athens, where he was a teacher of rhetoric. Zoïlus was chiefly known for the acerbity of his attacks on Homer (which gained him the name of Homeromastix, “scourge of Homer”), chiefly directed against the fabulous element in the Homeric poems. Zoïlus also wrote against Isocrates and Plato, who had attacked the style of Lysias of which he approved. The name Zoïlus came to be generally used of a spiteful and malignant critic.

See U. Friedländer, De Zoilo aliisque Homeri Obtrectatoribus (Königsberg, 1895); J. E. Sandys, History of Classical Scholarship (2nd ed. 1906).  ZOISITE, a rock-forming mineral, consisting of basic calcium and aluminium silicate, Ca2(AlOH)Al2(SiO4)3, crystallizing in the orthorhombic system. It is closely related to (q.v.) both in the angles of the crystals and in chemical composition: a zoisite containing some iron replacing aluminium may be identical in composition with an epidote (“clinozoisite”) poor in iron. The crystals are prismatic in habit and are deeply furrowed parallel to their length; terminal planes are rare; there is a perfect cleavage parallel to the brachy-pinacoid. Columnar and compact masses are more common. The hardness is 6 and the specific gravity 3.25–3.37. The colour is often grey; a rose-red variety, known as “thulite,” occurs with sky-blue vesuvianite at Telemarken in Norway, and has been used to a limited extent as an ornamental stone. According to differences in the optical characters, two kinds of zoisite have been distinguished. Zoisite is a product of dynamo-metamorphism, and occurs as a constituent of some crystalline