Page images
PDF
EPUB

de' Rossi (1821). His complete works on art appeared at Milan (12 vols., 1818-22), and his miscellaneous writings in Italian and French, edited by Giovanni Labus (3 vols., 1827-'32). II. Lonis Tullius Joachim, a French architect, son of the preceding, born in Rome, Feb. 11, 1791, died in Paris, Dec. 29, 1853. He completed his studies at the school of fine arts in Paris in 1817, and was connected with various works till 1825, when he became architect of the royal library, which he in vain strove to restore on a grand scale. His principal works were the fountain for the place Louvois, begun in 1835, and other fountains, monuments on the tombs of great warriors, and several palaces, especially the Collet palace on the quai d'Orsay. His masterpiece was the mausoleum of Napoleon at the Invalides.

VISCOUNT (Lat. vice-comes), a dignity in the British peerage, which ranks next below that of earl. The application of the title as a dignity dates from the time of Henry VI., though as a title of office it is much older. Anciently a vice-comes was the deputy of a count or earl, under whom he performed duties similar to that of a sheriff.

VISCUM. See MISTLETOE.

[ocr errors]

dles of rays very slightly divergent or sensibly parallel. Hence, the object brought much nearer the eye is still distinctly visible through a pin hole, the edges of which cut off the too divergent rays; or through a lens sufficiently convex to complement, in converging the pencils, the effect of the eye. Thus, for distinct vision, the image must be at the retina, and distinctly formed there. Again, since there are degrees of faint illumination not allowing of sight, it must be sufficiently bright or intense. Thus, in cases of cataract or opacities of the organ, distinct vision gradually becomes impossi ble, except by the stronger degree of brightness of the objects. Many stars are wholly unperceived until the light emitted by each of them has been collected and concentrated from over a larger space than that afforded by the pupil, as by use of a large convex lens. On the other hand, excessive brilliancy dazzles the eye, the impression not resulting in perception of the object, but often in pain, or in positive injury to the structure and the sensibility of the retina. Again, the image must have a sensible magnitude. When a single shoot of young corn at a distance is quite invisible, the several shoots of like size in a hill, impress

VISHNU. See INDIA, RELIGIONS AND RELI- ing a larger space on the retina, may come GIOUS LITERATURE of, vol. ix., p. 229. VISIGOTHS. See GоTHS.

clearly into view; but the rows are, on the same principle, more distinctly seen than the single VISION (Lat. visio, from videre, to see), a hill could be. Ehrenberg finds that the smallterm employed to denote, in different rela- est square magnitude visible near at hand to the tions, the power, the act or process, or the ob- naked eye, white on a black ground, or the reject of sight. The behavior of rays of light re- verse, is about of an inch. With effort, a flected or transmitted through various media, less spot may be seen; and if, like gold leaf, among which are the lens and humors of the the particle reflects light powerfully, it may be eye, and the manner in which images come to seen down to a fineness of of an inch. be formed at the retina, are considered under On the principle of extension already referred OPTICS. For the parts of the eye, and the ac- to, lines of much greater fineness are visible— tion of the humors and iris, see EYE.-The if opaque, and viewed toward the light, down mere action of rays of light upon the retina to the of an inch diameter, or about half is not sufficient to secure the actual percep- the thickness of the silkworm's fibre. The tion of the objects they proceed from. Of magnitude and extreme limit of distance and the other conditions, the most indispensable visibility of an object vary also with its brightfor distinct vision is that the divergent pen- ness, and hence with its color. A white object cil of rays emanating from any one point of in sunlight is seen at a distance of 17,250 times the object shall be brought to its proper focus its diameter; in the same light a red object of at the level of the retina. But if the object be so like size only about one half as far; a blue obnear the eye, say within 6 in., that the conver-ject at somewhat less distance than a red one. gent power of the lens fails to bring the pencils But all small or distant objects become more of light to a focus, or if the pencils from very readily visible when the background or surdistant objects in eyes too powerfully conver-rounding objects present to it the strongest congent are focalized and again dispersed before reaching the retina, no distinct perception takes place, but only a sense of light or brightness in greater or less degree. The least distance at which objects can be distinctly seen is termed the limit of distinct vision; it is for different eyes between 6 and 12 in. At remoter distances the several pencils admitted by the small opening of the pupil, about in. in diameter for an object at 6 in., may be regarded as consisting each of nearly parallel rays. For an object at 10 in., the angular divergence of each pencil is little over half a degree; and generally, it may be said that distinct vision is due to bun

trast, as in the case of a black object on a white ground, or a white object on a black ground. Though the human eye is capable of seeing objects both at great and small distances, yet most persons, wishing minutely to examine an object, place it at 8 to 10 in. before the eyes. But one whose eyes lack convergent power must remove the object further away to obtain distinct vision, and such a one is said to be longsighted; this condition is presbyopia, and is remedied by use of convex eye glasses. On the other hand, when from any cause too rapid convergence takes place, the object must be brought close to the eye, so as to carry its im

ner.

age back to the retina, and one thus affected is | tor placed in this position can see both the said to be near-sighted; this condition is myo- pupils, and is of course himself visible to the pia, and is remedied by concave eye glasses. animal. In some fishes, if not in most of Like any other lens, the crystalline has its optic the class, there appears to be really no limit centre; this coincides nearly with its centre of to the field of vision in any direction; so that form. The direction of every ray passing the animal can perceive rays of light coming through this is nearly the same after as before from every point of the sphere, excepting that transmission. Consequently, every such ray is actually occupied by his own body. But even an axis of a pencil of rays from some point of within the field of human vision there is only the object. The pencil of rays coming from the a single spot in its centre at which objects can point directly before the eye has its axis corre- be seen distinctly, that is, where their form sponding with the principal or "optic axis" of and outlines are accurately perceived. Thus, the eye, passing through very nearly the cen- when we stand in front of a row of upright tres of the pupil and lens, and meeting the stakes or poles, we can see those placed diretina in a direction perpendicular to its sur-rectly in front of the eye with perfect distinctface, at the middle point of the small circular ness, while those placed a little on either side, area which is the seat of most distinct vision. though perceptible, are indistinct. When we -In providing for accurate vision under differ- look steadily at the middle of a printed page ent circumstances, a very important element we see the distinct form of each separate letis the alternate contraction and dilatation of ter only immediately in front; above, below, the pupil. When exposed to a strong light so and on each side of this spot, at successive disintense that it would dazzle the eye and thus tances, the letters first become confused, then produce indistinctness of vision, the pupil con- run into each other, and finally the distinction tracts, shutting out a portion of the luminous of letters and words is lost, and we perceive beam, and thus reducing the amount of light only the lines and spaces. The straight line to that which is readily borne by the eye. On extending directly in front of each eye, upon the other hand, on first entering a darkened which alone objects are distinctly perceived, apartment with the pupils in a contracted con- is called the "line of distinct vision." Everydition, we may be incapable of perceiving sur- thing above or below this line, to the right or rounding objects; but within a few seconds the left, is perceived only in an imperfect manthe pupils enlarge, admitting a greater amount This is practically compensated for by of light to the eye, and everything in the apart- the mobility of the eyeballs, by which we are ment soon becomes distinctly perceptible. In enabled to direct the line of distinct vision to some of the lower animals, whose vision adapts all parts of the space in front, in rapid succesitself to extreme conditions of light and dark- sion. When reading, for instance, the eyes folness, as in the cat, the variations in size of the low the printed lines from left to right, seeing pupil are exceedingly marked; this opening each letter and word distinctly in succession. being reduced in broad sunlight to a nearly Consequently an object can be distinctly perimperceptible slit, while at night it is so ex- ceived by both eyes simultaneously only when panded that the borders of the iris almost dis- placed at a certain distance, namely, at that appear. Notwithstanding the perfection of point where the two visual axes, or the lines our visual apparatus, there is still only a small of distinct vision for the two eyes, cross each space directly in front of each eye within other. As the eyes are situated two or three which objects can be distinctly seen. Outside inches apart in their orbits, when they are of this space there is a field or circle of con- both directed toward the same object their siderable extent within which we can perceive, visual axes converge and meet at the situation though indistinctly, the presence of luminous of the object. Thus, although we look with objects; and this is called "the field of vis- two eyes, we see but a single object; because, ion." In man, when using both eyes immov- as the two lines of vision meet at a single ably fixed, the field of vision within its ex-point, the two distinct images exactly cover treme limits subtends an angle of about 180°; each other and so form but one. But either everything outside of this circle being invisi- within or beyond this point vision becomes ble to us, because the rays of light coming both double and indistinct. Thus, on holding from the sides and behind cannot enter the up a slender rod at a distance of one or two pupil. In many of the lower animals, partic-feet in front of the face, and in the same range ularly in birds and fishes, there is reason to believe that the field of vision is very much more extensive than this, owing to the lateral position of the two eyes, one being placed on each side of the head, and to the great prominence of the eyeball and its high refractive power, in consequence of which rays of light coming in an extremely oblique direction are bent inward and made to enter the pupil. It is certain, for example, that the ostrich can see objects directly behind him; for a specta

with any similar object, such as a door knob, on the opposite side of the room, if we direct both eyes to the rod we see it distinct and single, while the door knob appears as two indistinct images, one on each side. If we now direct the eyes to the door knob, it in turn becomes distinct and single, while the rod is doubled and confused in outline. This is because when we look at the nearer object so as to see it distinctly, the further one is still within the field of vision, and is therefore perceived; but

it is perceived with imperfect outlines, since | vex transparent body, of considerable density, it is outside the line of distinct vision. But for the right eye it is to the right of this line, and for the left eye to its left; and the two images therefore no longer occupy the same spot, and the object accordingly appears double. On this account we never see all parts of a landscape, even directly in front, with distinctness at the same time. The fore ground, the middle ground, and the distance are each examined separately; and when one is distinct, the others are always more or less confused.-One great advantage of the simultaneous action of the two eyes, as above described, is that we are thus enabled to judge in great measure of the distance of an object. As the angle of convergence of the two eyes varies according to the distance at which an object is placed, we instinctively appreciate the amount of this convergence and consequently the distance of the object. A still greater advantage is, that by using two eyes we appreciate the qualities of solidity and projection of foreign bodies. Vision with but one eye presents a simple expanse or variously shaded and colored picture in front of the visual organ. But since the two eyes are placed at a certain distance from each other, and their visual axes, as above mentioned, are convergent, it follows that for near objects it is not precisely the same figure which is perceived by each eye. If we look, for example, at a square box at the distance of a few feet, both eyes will see the front of the box equally well; but in addition the left eye will see a little of its left side in perspective, and the right eye will see a little of its right side. These two images, occupying the same spot at the convergence of the visual axes, are perceived as one; and it is by this combination of two different figures that we acquire the perception of the solidity of the object examined. We feel that it is not a flat picture, because in that case the same image would be presented to each eye; and however well it might be painted, such a picture could never deceive us, if looked at with both eyes simultaneously. It is on this principle that the stereoscope is constructed. (See STEREOSCOPE.)A change is also required in transferring the sight from near to remote objects, or vice versa, which is called the accommodation of the eye to distinct vision at different distances. The eye is a natural optical instrument, consisting essentially of a sensitive nervous expansion, the retina, intended to receive the impression produced by luminous rays, and of various refracting media, destined to converge these rays to a focus at the surface of the retina. The distinctness of vision depends upon the accuracy with which the rays of light, diverging from all parts of a luminous object, are brought to a focus exactly at the level of the sensitive membrane destined to receive them. The most important and efficient of the refracting parts of the eye is the crystalline lens, a doubly con

immediately behind the pupil and some distance in front of the retina.. As the original divergence of the luminous rays passing through the pupil varies with the distance of the object from which they emanate, the crystalline lens, if both its refracting power and its position remained the same, could not bring the rays to a focus at the same point behind it, for luminous objects at different distances. For example, suppose the lens to be so arranged that the rays from an object at the distance of 6 in. may be accurately focussed at the retina. If the object be now removed to a distance of 30 in., the divergence of its rays at their entrance through the pupil will be diminished; but, the refractive power remaining unaltered, they will now be converged more rapidly than before. They will accordingly meet and cross each other; and when they reach the lens they will have already become partially dispersed, the effect of this being to produce partial indistinctness of vision. This difficulty is met with in telescopes and spy glasses, and is obviated in them by shifting the relative distance of the object glass and the eye piece, when the instrument is turned from a near object to a remote one, or vice versa. In the eye the correction might be accomplished either by moving the lens backward and forward, so as to vary its distance in front of the retina, or by changing its refractive power, to correspond with the varying distances of visual objects. The experiments of Donders and others have shown conclusively that the latter method is that which is really employed by nature. If the eye be accommodated for vision of a distant object, and a lighted candle be then held in front of it and a little on one side, an observer will perceive three reflected images of its flame in the eyeball; namely, one from the anterior surface of the cornea, one from the anterior surface of the lens, and a third from the posterior surface of the lens. If the sight be now changed from the distant to a near object, the reflected image upon the cornea remains unaltered, but that from the anterior surface of the lens diminishes and moves slightly forward. The third reflection also diminishes a little, but is not altered in position. These changes show that in shifting the accommodation of the eye from distant to near objects, the lens increases the curvature of its surfaces and of course its refractive power, the bulging taking place principally in front. When the sight is shifted from near to remote objects, of course these changes are reversed. Thus, when we look at a near object, the refractive power of the lens is increased, and the diverging rays are more powerfully refracted; when we look at an object comparatively remote, the refractive action is diminished, and the slightly divergent rays are still brought to a focus exactly at the surface of the retina. This alteration in the form of the lens is accomplished by means of the internal

ters; and the duration of the electric spark was thus determined to be in some instances hardly more than 100.00.000 of a second. Yet with this illumination the letters on a printed page were plainly to be seen, and with a polariscope the cross and rings around the axes of crystals could be observed with all their peculiarities. An illumination continuing forty billionths of a second is therefore sufficient for distinct vision.

VISTULA (Pol. Wista; Ger. Weichsel), a river of central Europe, which has its sources in the Jablunka mountain, a branch of the Car

sia, and traverses Galicia, Russian Poland, and Prussia, passing Cracow, Sandomir, Pulawy, Warsaw, Modlin, Plock, Thorn, Culm, Graudenz, 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 affluents 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.

muscular apparatus of the eye. It is much | more marked when the changes are made between 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 accommodating 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 distances beyond 50 ft. the change required is comparatively trifling.-A remarkable fact in regard to vision is that of the temporary persistence of visual impressions. If a bright object be presented to the eye, and then sud-pathians, in the S. E. corner of Austrian Siledenly obscured, the impression of light remains upon the retina for a short interval of time after the luminosity has actually disappeared; and if the luminous body be again restored 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 ojects 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 impression of a continuous stream of fire.-Closely 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, containing 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 confuse each other's outlines upon the retina; and yet the illuminated bodies are perceived with perfect distinctness. Prof. O. N. Rood (“American Journal of Science and Arts," September, 1871) has shown that visual perception may take place within an interval of time exceedingly minute. In his experiments the illuminating agent was the electric spark from a Leyden jar, passed between platinum points separated by a distance of 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, moving with the revolution of the mirror, would thus become mingled and confused if the illumination lasted long enough to allow them to pass over a space equal to their own diame

VITEBSK, or Witebsk. I. A government of West Russia, formerly belonging to the Lithuanian 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 Düna. The soil is moderately fertile. Large numbers of domestic animals are reared, and there are extensive forests of valuable timber. The Düna and its connecting canals afford channels for a flourishing export trade. The principal towns, besides the capital, are Polotzk, the oldest in White Russia, Dünaburg, Velish, and Nevel. II. A town, capital of the government, on both sides of the Düna 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.

VITELLIUS, Aulus, a Roman emperor, born about A. D. 15, killed in Rome in 69. He became consul in 48, was subsequently proconsul of Africa, and afterward legate of the same province under his brother. His vices made him a favorite of the emperors Tiberius, 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

diers marched into Italy under Valens and Cacina, and near Bedriacum, in Cisalpine Gaul, completely defeated Otho, who killed himself. Most of the armies of the empire now acknowledged Vitellius, and he marched to Rome. But the eastern armies proclaimed Vespasian emperor; and Antonius Primus, acknowledging the latter, marched with the legions of Illyricum into Italy, and, after defeating the armies of Vitellius, reached Rome. The emperor was dragged through the streets and killed. VITELLUS. See EMBRYOLOGY, vol. vi., p. 561. | VITERBO, a town of central Italy, at the foot of Monte Cimino, an extinct volcano, in the province and 40 m. N. W. of the city of Rome; pop. in 1872, 20,637. It is surrounded by walls and towers, and contains a Gothic cathedral standing on the site of a temple of Hercules, and many other interesting churches and palaces. There are Etruscan antiquities, warm sulphur springs within 2 m. of the town, and sulphur refineries. Early Italian writers called it the city of beautiful fountains and women. Viterbo is supposed to occupy the site of the ancient Fanum Voltumn, where the Etruscan league held its assemblies. The present town dates from the 8th century. In the 11th century it was granted to the pope as part of the territory known as the patrimony of St. Peter. It was the capital of a papal delegation till 1870, when it was incorporated with the kingdom of Italy.

VITORIA, a town of Spain, capital of the Basque province of Alava, 29 m. S. S. E. of Bilbao; pop. about 19,000. The older parts are poorly built and dilapidated, but the new town has fine streets and squares. It has manufactures of stained paper, hats, brushes, combs, carriages, leather, mirrors, and articles in iron and copper. Its trade has declined since the removal of the custom house to the frontier. It probably existed under the Romans, and received its present name from Sancho the Wise of Navarre in commemoration of a victory over the Moors about 1180. On June 21, 1813, Wellington defeated here the French under Joseph Bonaparte and Jourdan, capturing 150 guns and $5,000,000 worth of plunder, the booty of the five years' occupation of the peninsula.

VITRIOL, Blac. See COPPER, vol. v., p. 319. VITRIOL, Oil of. See SULPHURIC ACID. VITRUVIUS POLLIO, Marens, a Roman architect, of whom it is only known that he probably served as a military engineer under Cæsar and Augustus. His treatise De Architectura is a compendium of the works of Greek writers, with much of his own knowledge and experience. The first edition was printed at Rome about 1486, and it has been many times edited and reprinted; the latest editions are those of J. G. Schneider (3 vols. 8vo, Leipsic, 1807-'8), Stratico (4 vols., Udine, 1825-'30), Marini (4 vols. fol., Rome, 1836), and Rose and MüllerStrübing (Leipsic, 1867). There are English translations by Robert Castell (2 vols. fol.,

|

London, 1730); by W. Newton, with notes and plates (2 vols. fol., 1771-'91); by W. Wilkins (4to, 1812); and by Joseph Gwilt (8vo, 1826). VIVES, Juan Luis, a Spanish scholar, born in Valencia in March, 1492, died in Bruges, Flanders, May 6, 1540. He studied in Paris and at the university of Louvain, where he early became professor of belles-lettres. In 1522 he published his commentaries on Augustine's De Civitate Dei, and dedicated it to Henry VIII., who thereupon invited him to England, and made him the tutor of his daughter Mary, and subsequently professor at Oxford. He was imprisoned for opposing the divorce of Catharine of Aragon, and on his release settled at Bruges. Budæus, Erasmus, and Vives were called the triumvirs of the republic of letters of the 16th century. His works were collected at Basel (2 vols. fol., 1555) and Valencia (8 vols. fol., 1782-'90). Among the most important are: De Corruptis Artibus; De Religione; commentaries on the "Dream of Scipio" and the "Bucolics" of Virgil; and several educational works. His commentary on the "City of God" was placed on the Index because he had given a place in heaven to Numa, Camillus, Cato, Seneca, and other heathens.

VIVIANI, Vincenzo, an Italian mathematician, born in Florence, April 5, 1622, died there, Sept. 22, 1703. He was a pupil of Galileo, then old, blind, and under the ban of the inquisition; and for three years he and Torricelli nursed him, until his death (Jan. 8, 1642). Subsequently he continued to study under the latter. He was admitted to the principal Florentine academies. From 1699 he was a member of the French academy and in the enjoyment of a considerable pension from Louis XIV., which he appropriated to a building in commemoration of Galileo; and in all his works he added to his name "pupil of Galileo." He published numerous mathematical works, but is best known by his restoration of the treatise of Aristæus, De Locis Solidis, and of the fifth book of Apollonius of Perga on the conic sections; the latter was entitled Divinatio in quintum Conicorum Apollonii Pergæi (1659), and the former Divinatio in Aristaum (1701).

VIVISECTION, (Lat. virus, alive, and sectio, a cutting), a term used to designate cutting operations performed on living animals for the purpose of acquiring physiological and surgical knowledge, and sometimes also applied to operations in which cutting is not employed. The practice of vivisection dates back to very early periods, and was known in the Alexandrian school. Among the earlier operations which led to positive physiological knowledge may be cited those of Galen, who demonstrated the existence of blood in the arteries by passing two ligatures around an artery in the living animal; but he formed no conception of the manner of communication between the arteries and the veins, and believed that the septum between the ventricles of the heart was perforated by small orifices. Harvey was

« PreviousContinue »