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to Distinct Vision. He shews that objects may be seen with sufficient distinctness, though the pencils of rays issuing from the points of them do not unite precisely in the same point on the retina; but that since, in this case, pencils from every point either meet before they reach the retina, or tend to meet beyond it, the light that comes from them must cover a circular spot upon it, and will therefore paint the image larger than perfect Vision would represent it. Whence it follows, that every object placed either too near or too remote for perfect Vision, will appear larger than it is by a penumbra of light, caused by the circular spaces, which are illuminated by pencils of rays proceeding from the extremities of the object.

The smallest distance of perfect Vision, or that in which the rays of a single pencil are collected into a physical point on the retina in the generality of eyes, Dr. Jurin, from a number of observations, states at 5, 6, or 7 inches. The greatest distance of distinct and perfect Vision he found was more difficult to determine; but by considering the proportion of all the parts of the eye, and the refractive power of each, with the interval that may be discerned between two stars, the distance of which is known, he fixes it, in some cases, at 14 feet 5 inches; though Dr. Porterfield had restricted it to 27 inches only, with respect to his own eye.

For other observations on this subject, see Jurin's Essay on Distinct and Indistinct Vision, at the end of Smith's Optics; and Robins's Remarks on the same, in his Math. Tracts, vol. 2, pa. 278 &c. See also an ingenious paper on Vision in the Philos. Trans. 1793, pa. 169, by Mr. Thomas Young.

Field of Vision. See Field.

VISUAL

, relating to sight, or seeing.

Visual Angle, is the angle under which an object is seen, or which it subtends. See Angle.

Visual Line. See Line.

Visual Point, in Perspective, is a point in the horizontal line, where all the ocular rays unite. Thus, a person standing in a straight long gallery, and looking forward; the sides, floor, and cieling seem to meet and touch one another in this point, or common centre.

Visual Rays, are lines of light, conceived to come from an object to the eye.

VITELLIO

, or Vitello, a Polish mathematician of the 13th century, as he flourished about 1254. We have of his a large Treatise on Optics, the best edition of which is that of 1572. Vitello was the first optical writer of any consequence among the modern Europeans. He collected all that was given by Euclid, Archimedes, Ptolomy, and Alhazen; though his work is of but little use now.

VITREOUS Humour, or Vitreus Humor, denotes the third or glassy humour of the eye; thus called from its resemblance to melted glass. It lies under the crystalline; by the impression of which, its forepart is rendered concave. It greatly exceeds in quantity both the aqueous and crystalline humours taken together, and consequently occupies much the greatest part of the cavity of the globe of the eye. Scheiner says, that the refractive power of this humour is a medium between those of the aqueous, which does not differ much from water, and of the crystalline, which is nearly the same with glass. Hawksbee makes its refractive power the same with that of water; and, according to Robertson, its specific gravity agrees nearly with that of water.

VITRUVIUS (Marcus Vitruvius Pollio)

, a celebrated Roman architect, of whom however nothing is known, but what is to be collected from his ten books De Architectura, still extant. In the preface to the sixth book he writes, that he was carefully educated by his parents, and instructed in the whole circle of arts and sciences; a circumstance which he speaks of with much gratitude, laying it down as certain, that no man can be a complete architect, without some knowledge and skill in every one of them. And in the preface to the first book he informs us, that he was known to Julius Cæsar; that he was afterwards recommended by Octavia to her brother Augustus Cæsar; and that he was so favoured and provided for by this emperor, as to be out of all fear of poverty as long as he might live.

It is supposed that Vitruvius was born either at Rome or Verona; but it is not known which. His books of architecture are addressed to Augustus Cæsar, and not only shew consummate skill in that particular science, but also very uncommon genius and natural abilities. Cardan, in his 16th book De Subtilitate, ranks Vitruvius as one of the 12 persons, whom he supposes to have excelled all men in the force of genius and invention; and would not have scrupled to have given him the first place, if it could be imagined that he had delivered nothing but his own discoveries. Those 12 persons were, Euclid, Archimedes, Apollonius Pergæus, Aristotle, Archytas of Tarentum, Vitruvius, Achindus, Mahomet Ibn Moses the inventor or improver of Algebra, Duns Scotus, John Suisset surnamed the Calculator, Galen, and Heber of Spain.

The architecture of Vitruvius has been often printed; but the best edition is that of Amsterdam in 1649. Perrault also, the noted French architect, gave an excellent French translation of the same, and added notes and figures: the first edition of which was published at Paris in 1673, and the second much improved, in 1684. —Mr. William Newton too, an ingenious architect, and late Surveyor to the works at Greenwich Hospital, published in 1780 &c, curious commentaries on Vitruvius, illustrated with figures; to which is added a description, with figures, of the Military Machines used by the Ancients.

VIVIANI (Vincentio)

, a celebrated Italian mathematician, was born at Florence in 1621, some say 1622. He was a disciple of the illustrious Galileo, and lived with him from the 17th to the 20th year of his age. After the death of his great master, he passed two or three years more in prosecuting geometrical studies without interruption, and in this time it was that he formed the design of his Restoration of Aristeus. This ancient geometrician, who was contemporary with Euclid, had composed five books of problems De Locis Solidis, the bare propositions of which were collected by Pappus, but the books are entirely lost; which Viviani undertook to restore by the force of his genius.

He broke this work off before it was finished, in order to apply himself to another of the same kind, and