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 VISION VITELLIUS 393 muscular apparatus of the eye. It is much more marked when the changes are made be- tween objects at short than at long distances, since the difference in the angular divergence of the rays is greater in the former case than in the latter. Accordingly, much greater ac- commodating power is required for shifting the sight from a distance of 6 in. to 12 in., than from 12 in. to 24 in. ; and for all dis- tances beyond 50 ft. the change required is comparatively trifling. A remarkable fact in regard to vision is that of the temporary per- sistence of visual impressions. If a bright . object be presented to the eye, and then sud- denly obscured, the impression of light re- mains upon the retina for a short interval of time after the luminosity has actually disap- peared ; and if the luminous body be again re- stored to its position before this interval has elapsed, we fail to see that it has disappeared at all. Thus the act of winking, by which the light is momentarily excluded from the eye, does not interfere with vision, because it is performed and terminated so rapidly that the images of external ejects appear to remain upon the retina until they are again actually perceptible. A lighted stick revolving with rapidity presents the appearance of a luminous circle; and the successive sparks thrown off from a knife grinder's wheel produce the im- pression of a continuous stream of fire. Close- ly connected with this part of the subject is the question of the amount of time required by the eye for the perception of light and the distinct vision of luminous objects. It is well known that if a darkened apartment, contain- ing objects in rapid movement, as for instance revolving wheels, be suddenly illuminated by an electric spark of sufficient intensity, the wheels will be perceived, but will be seen as if at rest. The duration of the spark is so short that the spokes of the wheel do not move far enough in the interval of illumination to con- fuse each other's outlines upon the retina ; and yet the illuminated bodies are perceived with perfect distinctness. Prof. 0. N. Kood ("Amer- ican Journal of Science and Arts," September, 1871) has shown that visual perception may take place within an interval of time exceed- ingly minute. In his experiments the illumi- nating agent was the electric spark from a Ley- den jar, passed between platinum points sepa- rated by a distance of -fa of an inch. The illuminated object was a glass plate ruled with parallel black and white lines, each -^^ of an inch in thickness. The image of this plate was thrown, through an achromatic lens, upon a mirror revolving upon its axis at the rate of 340 times a second, and thence reflected upon a surface of plain glass, where it was brought to a focus and viewed by a telescopic eye piece. The images of the black and white lines, mov- ing with the revolution of the mirror, would thus become mingled and confused if the illu- mination lasted long enough to allow them to pass over a space equal to their own diame- ters; and the duration of the electric spark was thus determined to be in some instances hardly more than ^,^.1$^ of a second. Yet with this illumination the letters on a printed page were plainly to be seen, and with a polari- scope the cross and rings around the axes of crystals could be observed with all their pe- culiarities. An illumination continuing forty billionths of a second is therefore sufficient for distinct vision. VISTULA (Pol. Wida; Ger. Weichsel), a river of central Europe, which has its sources in the Jablunka mountain, a branch of the Car- pathians, in the S. E. corner of Austrian Sile- sia, and traverses Galicia, Kussian Poland, and Prussia, passing Cracow, Sandomir, Pulawy, Warsaw, Modlin, Plock, Thorn, Culm, Grau- denz, and Marienburg. Near the village of its name in Silesia it has a fall of nearly 200 ft. It flows into the Baltic by three mouths, of which one is at Dantzic, and the other two open upon the sound called the Frisches Haff. Its length is about 650 m., and it is navigable to Cracow, about 550 m. Its principal afflu- ents are the Dunajec, San, Wieprz, Bug, and Drewenz from the right, and the Pilica, Bzura, and Brahe from the left. It is connected by canals with the Dnieper, the Oder, and the Niemen. VITEBSK, or Witebsk. I. A government of West Eussia, formerly belonging to the Lithu- anian provinces of Poland, bordering on Pskov, Smolensk, Mohilev, Minsk, Wilna, Courland, and Livonia ; area, 17,438 sq. m. ; pop. in 1870, 888,727. The principal river is the Duna. The soil is moderately fertile. Large numbers of domestic animals are reared, and there are ex- tensive forests of valuable timber. The Duna and its connecting canals afford channels for a flourishing export trade. The principal towns, besides the capital, are Polotzk, the oldest in White Russia, Dunaburg, Velish, and Nevel. II. A town, capital of the government, on both sides of the Duna at its confluence with the Viteba, 76 m. N. W. of Smolensk, with which, as well as with Riga, it is connected by rail ; pop. in 1867, 28,944, including many Jews. It is surrounded by ancient fortifications, and contains 15 Greek and Catholic churches, 10 convents, and a gymnasium. Mead and cloth are largely exported. VITELLICS, Anlns, a Roman emperor, born about A. D. 15, killed in Rome in 69. He became consul in 48, was subsequently pro- consul of Africa, and afterward legate of the sarde province under his brother. His vices made him a favorite of the emperors Tibe- rius, Caligula, Claudius, and Nero ; and when Galba ascended the throne he was placed in command of the legions of Lower Germany. In January, 69, he was saluted at Cologne with the title of imperator, and a civil war broke out between him and Otho, who had dethroned Galba. Vitellius was given chiefly to eating and drinking, and was totally unfit for the high position he had received; but his sol-