Page:The American Cyclopædia (1879) Volume XVI.djvu/410

 390 VISCOUNT VISION 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. Loots Tnllius Joarhim, 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 re- store on a grand scale. His principal works were the fountain for the place Louvois, be- gun in 1835, and other fountains, monuments on the tombs of great warriors, and several palaces, especially the Collet palace on the qnai d'Orsay. His masterpiece was the mau- soleum of Napoleon at the Invalides. VISCOUNT (Lat. vice-coma), a dignity in the British peerage, which ranks next below that of earl. The application of the title as a dig- nity dates from the time of Henry VI., though as a title of office it is much older. Anciently a vke-come* was the deputy of a count or earl, under whom he performed duties similar to that of a sheriff. VISITM. See MISTLETOE. VISHNU. See INDIA, RELIGIONS AND RELI- GIOUS LITERATURE OF, vol. ix., p. 229. VISIGOTHS. See GOTHS. VISION (Lat. //.//>, from tiderc, to see), a term employed to denote, in different rela- tions, the power, the act or process, or the ob- ject of sight. The behavior of rays of light re- flected or transmitted through various media, among which are the lens and humors of the eye, and the manner in which images come to be formed at the retina, are considered under OPTICS. For the parts of the eye, and the ac- tion of the humors and iris, see EYE. The mere action of rays of light upon the retina is not sufficient to secure the actual percep- tion of the objects they proceed from. Of the other conditions, the most indispensable for distinct vision is that the divergent pen- cil of rays emanating from any one point of the object shall be brought to its proper focus at the level of the retina. But if the object be so near the eye, say within 8 in., that the conver- gent power of the lens fails to bring the pencils of light to a focus, or if the pencils from very distant objects in eyes too powerfully conver- gent 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 dis- tances the several pencils admitted by the small opening of the pupil, about -fa 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- 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 pen- cils, 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 or- gan, distinct vision gradually becomes impossi- ble, except by the stronger degree of bright- ness of the objects. Many stars are wholly un- perceived 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- ing a larger space on the retina, may come clearly into view ; but the rows are, on the same principle, more distinctly seen than the single hill could be. Ehrenberg finds that the small- est square magnitude visible near at hand to the naked eye, white on a black ground, or the re- verse, is about T ^ y of an inch. With effort, a less spot may bo seen ; and if, like gold leaf, the particle reflects light powerfully, it may bo seen down to a fineness of T /^ of nn inch. On the principle of extension already referred to, lines of much greater fineness are visible if opaque, and viewed toward the light, down to the j-jVr f an inch diameter, or about half the thickness of the silkworm's fibre. The magnitude and extreme limit of distance and visibility of an object vary also with its bright- ness, and hence with its color. A white object in sunlight is seen at a distance of 17,250 times its diameter ; in the same light a red object of like size only about one half as far ; a blue ob- ject at somewhat less distance than a red one. But all small or distant objects become more readily visible when the background or sur- rounding objects present to it the strongest con- 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 ob- jects 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 long- sighted ; 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-