endeavoured to exercise sole power in the state after the death | nevertheless, his anti-Austrian propaganda, he rendered good of his brother, but his young nephew Gian Galeazzo plotted against him and put him to death (1385). GIAN GALEAZZO, the most powerful of the Visconti, became joint ruler of the Milanese territories on the death of his father in 1378 and sole ruler on the death of his uncle seven years later. He founded the cathedral of Milan, built the Certosa and the bridge across the Ticino at Pavia, improved the university of Pavia and established the library there, and restored the university at Piacenza. His bureaucratic government was excellent; he was an able and economical administrator, and was reputed to be one of the wealthiest princes of his time. He was ambitious to reduce all Italy under the sway of the Visconti. He conquered Verona in 1387; and in the following year, with the aid of the Venetians, took Padua. He plotted successfully against the rulers of Mantua and Ferrara, and now that the whole of Lombardy lay prostrate before him he turned his attention to Tuscany. In 1399 he bought Pisa and seized Siena. The emperor Wenceslaus had already conferred on him the title of duke of Milan for 100,000 florins, reserving only Pisa, and refused to take arms against him. Gian Galeazzo took Perugia, Lucca and Bologna (1400-1), and was besieging Florence when he died of the plague (3rd of September 1402) at the age of fifty-five years. His sons, Giovanni Maria and Filippo Maria, were mere boys at the time of his death, and were taken under the protection of the celebrated condottiere Facino Cane de Cesale; but most of Gian Galeazzo's conquests were lost to his self-seeking generals. GIOVANNI MARIA was proclaimed duke of Milan in 1402, displayed an insane cruelty, and was killed in 1412 by Ghibelline partisans. FILIPPO MARIA, who became nominal ruler of Pavia in 1402, succeeded his brother as duke of Milan. Cruel and extremely sensitive about his personal ugliness, he nevertheless was a great politician, and by employing such powerful condottieri as Carmagnola, Piccinino and Francesco Sforza he managed to recover the Lombard portion of his father's duchy. From his marriage with the unhappy widow of the above-years' absence from active political life, he was chosen to be mentioned Facino Cane he received a dowry of nearly half a million florins. He died in 1447, the last of the Visconti in direct male line, and was succeeded in the duchy, after the shortlived Ambrosian republic, by Francesco Sforza, who had married his daughter Bianca in 1441 (see SFORZA). VALENTINA (13661408), a daughter of Gian Galeazzo and a sister of the preceding, married Louis of Orleans in 1387, and it was from her that Louis XII. of France derived his claims to the duchy of Milan. GABRIELE, an illegitimate brother, gained possession of Pisa and other towns, but was despoiled and beheaded (1407) by Charles VI.'s governor of Genoa, under whose protection he had placed himself. Among collateral branches of the Visconti family were the counts of Saliceto, counts of Zagnano, lords of Brignano, marquis of San Giorgio di Borgoratto, marquis of Invorio and Marquis Della Motta. Other branches attained to some prominence in the local history of Bari and of Tarento. Tebaldo Visconti of Piacenza became Pope Gregory X. in 1271. Among the Visconti lords of Fontaneto was Gasparo, who died in 1595 archbishop of Milan. An Ignatius Visconti was sixteenth general of the Jesuits (1751-55). There is a contemporary history of the principal members of the family by Paolo Giovio, bishop of Nocera, which may be had in several editions. See J. Burckhardt, The Civilization of the Renaissance in Italy, trans. by S. G. C. Middlemore (London, 1898); J. A. Symonds, Age of the Despots (New York, 1888); C. Magenta, 1 Visconti e gli Sforza nel Castello di Pavia (1883); A. Medin, I Visconti nella poesia contemporanea (Milan, 1891); F. Mugnier, "Lettres des Visconti de Milan in Mémoires et documents de la société savoisienne d'histoire et d'archéologie, vol. x. of the second series (1896). (C. H. HA.) service to the national cause, but being molested by the Austrian police, was obliged in 1859 to escape to Turin, and during the war with Austria of that year was appointed by Cavour royal commissioner with the Garibaldian forces. Elected deputy in 1860, he accompanied Farini on diplomatic missions to Modena and Naples, and was subsequently despatched to London and Paris to acquaint the British and French governments with the course of events in Italy. As a recompense for the tact displayed on this occasion, he was given by Cavour a permanent appointment in the Italian foreign office, and was subsequently appointed under-secretary of state by Count Pasolini. Upon the latter's death he became minister of foreign affairs (24th March 1863) in the Minghetti cabinet, in which capacity he negotiated the. September Convention for the evacuation of Rome by the French troops. Resigning office with Minghetti in the autumn of 1864, he was in March 1866 sent by La Marmora as minister to Constantinople, but was almost immediately recalled and reappointed foreign minister by Ricasoli. Assuming office on the morrow of the second battle of Custozza, he succeeded in preventing Austria from burdening Italy with a proportion of the Austrian imperial debt, in addition to the Venetian debt proper. The fall of Ricasoli in February 1867 deprived him for a time of his office, but in December 1869 he entered the Lanza-Sella cabinet as foreign minister, and retained his portfolio in the succeeding Minghetti cabinet until the fall of the Right in 1876. During this long period he was called upon to conduct the delicate negotiations connected with the Franco-German War, the occupation of Rome by the Italians, and the consequent destruction of the temporal power of the pope, the Law of Guarantees and the visits of Victor Emmanuel II. to Vienna and Berlin. Upon the occasion of his marriage with the daughter of the marquis Alfieri di Sostegno, grandniece of Cavour, he was created marquis by the king. For a time he remained a member of the parliamentary opposition, and in 1886 was nominated senator. In 1894, after eighteen Italian arbitrator in the Bering Sea question, and in 1896 once more accepted the portfolio of foreign affairs in the Di Rudini cabinet at a juncture when the disasters in Abyssinia and the indiscreet publication of an Abyssinian Green Book had rendered the international position of Italy exceedingly difficult. His first care was to improve Franco-Italian relations by negotiating with France a treaty with regard to Tunis. During the negotiations relating to the Cretan question and the Graeco-Turkish War, he secured for Italy a worthy part in the European Concert and joined Lord Salisbury in saving Greece from the loss of Thessaly. Resigning office in May 1898, on a question of internal policy, he once more retired to private life, but in May 1899 again assumed the management of foreign affairs in the second Pelloux cabinet, and continued to hold office in the succeeding Saracco cabinet until its fall in February 1901. During this period his attention was devoted chiefly to the Chinese problem and to the maintenance of the equilibrium in the Mediterranean and the Adriatic. In regard to the Mediterranean he established an Italo-French agreement by which France tacitly undertook to leave Italy a free hand in Tripoli, and Italy not to interfere with French policy in the interior of Morocco; and, in regard to the Adriatic, he came to an understanding with Austria guaranteeing the status quo in Albania. Prudence and sagacity, coupled with unequalled experience of foreign policy, enabled him to assure to Italy her full portion of influence in international affairs, and secured for himself the unanimous esteem of European cabinets. In recognition of his services he was created Knight of the Annunziata by Victor Emmanuel III. on the occasion of the birth of Princess Yolanda Margherita of Savoy (1st of June 1901). In February 1906 he was Italian delegate to the Morocco con VISCONTI-VENOSTA, EMILIO, MARQUIS (1829- ), Italian statesman, was born at Milan on the 22nd of January 1829. A disciple of Mazzini, he took part in all the anti-ference at Algeciras. Austrian conspiracies until the ineffectual rising at Milan on the 6th of February 1853, of which he had foretold the failure, induced him to renounce his Mazzinian allegiance. Continuing, An account of Visconti-Venosta's early life (down to 1859) is given in an interesting volume by his brother Giovanni ViscontiVenosta, Ricordi di Gioventd (Milan, 1904). VISCOUNT (through O. Fr. viscomte, mod. vicomte, from Low | ston is as old as 1478. The dignity was sparingly conferred in Lat. vice-comes, cf. Portug. visconde, Ital. visconte), the title of the fourth rank of the European nobility. In the British peerage it intervenes between the dignities of earl and baron. The title is now purely one of honour, having long been dissociated from any special office or functions. In the Carolingian epoch the vice-comites, or missi comitis, were the deputies or vicars of the counts, whose official powers they exercised by delegation, and from these the viscounts of the feudal period were undoubtedly derived. Soon after the counts became hereditary the same happened in the case of their lieutenants; e.g. in Narbonne, Nimes and Alby the viscounts had, according to A. Molinier, acquired hereditary rights as early as the beginning of the 10th century. Viscountcies thus developed into actual fiefs, with their own jurisdiction, domain and seigniorial rights, and could be divided or even transmitted to females. Viscounts, however, continued for some time to have no more than the status of lieutenants, calling themselves either simply vice-comites, or adding to this title the name of the countship from which they derived their powers. It was not till the 12th century that the universal tendency to territorialize the feudal dominions affected the viscountcies with the rest, and that the viscounts began to take the name of the most important of their patrimonial domains. Thus the viscounts of Poitiers called themselves viscounts of Thouars, and those of Toulouse viscounts of Bruniquel and Montelar. From this time the significance of the title was extremely various. Some viscounts, notably in the duchy of Aquitaine and the county of Toulouse, of which the size made an effective centralized government impossible, were great barons, whose authority extended over whole provinces, and who disputed for power on equal terms with counts and dukes. Elsewhere, on the other hand, e.g. in the Île de France, Champagne, and a great part of Burgundy, the vicomtes continued to be half feudatories, half officials of the counts, with the same functions and rank in the feudal hierarchy as the chatelains; their powers were jealously limited and, with the organization of the system of prévôts and baillis in the 12th century, practically disappeared. In the royal domains especially, these petty feudatories could not maintain them selves against the growing power of the crown, and they were early assimilated to the prévôls; thus there is no record of a vicomte at Paris after 1077. In Normandy, where from the first the central power had been strong, vicomtes appeared at a very early date as deputies of the counts (afterwards dukes) of the Normans: "They are both personal companions and hereditary nobles." When local Norman counts began in the 11th century, some of them had vicomtes under them, but the normal vicomte was still a deputy of the duke, and Henry I. largely replaced the hereditary holders of the vicomtés by officials. "By the time of the Conqueror the judicial functions of the viscount were fully recognized, and extended over the greater part of Normandy." Eventually almost the whole of Normandy was divided into administrative viscountcies or bailiwicks by the end of the 12th century. When the Normans conquered England, they applied the term viscounte or vicecomes to the sheriffs of the English system (see SHERIFF), whose office, however, was quite distinct and was hardly affected by the Conquest. Nearly four centuries later "viscount was introduced as a peerage style into England, when its king was once more lord of Normandy. John, Lord Beaumont, K.G., who had been created count of Boulogne in 1436, was made Viscount Beaumont, February 12, 1440, and granted precedence over all barons, which was doubtless the reason for his creation. Within a year the feudal vicomté of Beaumont in Normandy was granted to him and the heirs male of his body on the ground that he traced his descent from that district. In 1446 Lord Bourchier, who held the Norman countship of Eu, was similarly made a viscount. The oldest viscountcy now on the roll is that of Hereford, created in 1550; but the Irish viscountcy of Gorman the peerage of England till recent times, when the number of viscounts was increased by bestowing the dignity on retiring speakers (e.g. Viscounts Canterbury, Hampden, Peel, Selby) and ministers who accepted peerages (e.g. Viscounts Melville, Halifax, Knutsford, Llandaff, Cross, Ridley, Goschen, St Aldwyn, Morley of Blackburn, Wolverhampton). A viscount is " Right Honourable," and is styled "My Lord." His wife, also "Right Honourable," is a "viscountess," and is styled "My Lady." All their sons and daughters are "Honourable." The coronet first granted by James I. has on the golden circlet a row of fourteen small pearls set in contact, of which number in representations nine are shown. The scarlet parliamentary robe of a viscount has two and a half doublings of ermine. See A. Luchaire, Manuel des institutions françaises (Paris, 1892). bibliography on p. 282: Stapleton's Rotuli Scaccarit Normanniae; Powicke's The Angevin Administration of Normandy" (Eng. Hist. Rev. vols. xxi., xxii.); Lords' Reports on the Dignity of a Peer Courthope Nicolas's Historic Peerage. VISHNU (Sanskrit, " the worker," from root vish, "to work "), a solar deity, in later Hindu mythology a god of the first importance, one of the supreme trinity with Brahma and Siva, but in the Rig Veda only a minor deity. In the Vedic scriptures his only anthropomorphic characteristics are the frequently mentioned strides that he takes, and his being a youth vast in body. His essential feature is the three strides (vi-kram) with which he traverses the universe. Two of these steps are visible to men, but the third or highest is beyond mortal sight. These steps are symbolic of the rising, culminating and setting of the sun, or alternatively the course of the solar deity through the three divisions of the universe. To-day Vishnu is adored by the Vishnavite sects as the equal or even the superior of Brahma, and is styled the Preserver. He is represented with four arms, and black in colour; in one hand he holds a club and in the others a shell, a discus and a lotus respectively. He rides on the Garuda, half man and half bird, having the head, wings, beak and talons of an eagle, and human body and limbs, its face being white, its wings red and its body golden. In his character as preserver of men Vishnu has from time to time become incarnate to rid the world of some great evil (see also BRAHMANISM and HINDUISM). See A. A. Macdonell, Vedic Mythology (Strassburg, 1897): Sir W. Muir, Original Sanskrit Texts, iv. 63-298; Sir M. MonierWilliams, Brahmanism and Hinduism, iii. v. vi. VISION (from Lat. videre, to see), or SIGHT, the function, in physiology, of the organ known as the eye (q.v.). The sense of vision is excited by the influence of light on the retina, the special terminal organ connected with the optic nerve. By excitation of the retina, a change is induced in the optic nerve fibres, and is conveyed by these to the brain, the result being a luminous perception, or what we call a sensation of light or colour. If light were to act uniformly over the retina, there would be no image of the source of the light formed on that structure, and consequently there would be only a general consciousness of light, without reference to any particular object. One of the first conditions, therefore, of vision for useful purposes is the formation of an image on the retina. To effect this, just as in a photographic camera, refractive structures must be placed in front of the retina which will so bend luminous rays as to bring them to a focus on the retina, and thus produce an image. Throughout the animal kingdom various arrangements are found for this purpose; but they may be all referred to three types, namely―(1) eye-specks or eye-dots, met with-in Medusae, Annelidae, &c.; (2) the compound eye, as found in insects and crustaceans; and (3) the simple eye, common. to all vertebrates. The eye-specks may be regarded simply as expansions of optic nerve filaments, covered by a transparent membrane, but having no refractive media, so that the creature would have the consciousness of light only, or a simple luminous impression, by which it might distinguish light from darkness. The compound eye consists essentially of a series of transparent cone-like bodies, arranged in a radiate manner against the inner surface of the cornea, with which their bases are united, while their apices are connected with the ends of the optic filaments. As each cone is separated from its neighbours, it admits only a ray of light parallel with its axis, and its apex represents only a portion of the image, which must be made up, like a mosaic-work, of as many parts as there are cones in the eye. When the cones are of considerable length, it is evident, from their form and direction, their apices being directed inwards, that the oblique rays emanating from a luminous surface will be cut off, and that only those rays proceeding along the axis of the cone will produce an effect. Thus distinctness or sharpness of definition will be secured. The size of the visual field will depend on the form of the eye, the outermost cones marking its limits. Consequently the size of the visual field will depend on the size of the segment of the sphere forming its surface. The eyes of many insects have a field of about half a sphere, so that the creature will see objects before and behind it as well as those at the side. On the other hand, in many the eyes have scarcely any convexity, so that they must have a narrow field of vision. For anatomical details, and diseases of the eye, see EYE; the pathological aspects of vision itself are treated at the end of this article. 1. PHYSICAL CAUSES OF VISION A luminous sensation may be excited by various modes of irritation of the retina or of the optic nerve. Pressure, cutting or electrical shocks may act as stimuli, but the normal excitation is the influence of light on the retina. From a physical point of view, light is a mode of movement occurring in a medium, termed the aether, which pervades all space; but the physiologist studies the operation of these movements on the sentient organism as resulting in consciousness of the particular kind which we term a luminous impression. Outside of the body, such movements have been studied with great accuracy; but the physiological effects depend upon such complex conditions as to make it impossible to state them in the same precise way. Thus, when we look at the spectrum, we are conscious of the sensations of red and violet, referable to its two extremities: the physicist states that red is produced by 392 billions of impulses on the retina per second, and that violet corresponds to 757 billions per second; but he has arrived at this information by inductive reasoning from facts which have not at present any physiological explanation. We cannot at present trace any connexion, as cause and effect, between 392 billions of impulses on the retina per second and a sensation of red. Below the red and above the violet ends of the spectrum there are vibrations which do not excite luminous sensations. In the first case, below the red, the effect as a sensation is heat; and above the violet the result is that of chemical activity. Thus the method of dispersion of light, as is followed in passing a ray through a prism, enables us to recognize these general facts: (1) rays below the red excite thermal impressions; (2) from the lower red up to the middle of the violet, the thermal rays become gradually weaker until they have no effect; (3) from the lower red to the extreme violet, they cause luminous impressions, which reach their greatest intensity in the yellow; and (4) from about the end of the yellow to far beyond the extreme violet, the rays have gradually a less and less luminous effect, but they have the power of exciting such chemical changes as are produced in photography. In general terms, therefore, the lower end of the spectrum may be called thermal, the middle luminous, and the upper actinic or chemical; but the three merge into and overlap one another. It may be observed that the number of vibrations in the extreme violet is not double that of the low red, so that the sensibility of the eye to vibrations of light does not range through an octave. The ultra-violet rays may act on the retina in certain conditions, as when they are reflected by a solution of sulphate of quinine, constituting the phenomenon of fluorescence. Far above the violet are the Röntgen radiations and probably others. 2. OPTICAL ARRANGEMENTS, OF THE EYE 1. General. When light traverses any homogeneous transparent medium, such as the air, it passes on in a straight course with a certain velocity; but if it meet with any other transparent body of a different density, part of it is reflected or returned to the first medium, whilst the remainder is propagated through the second medium in a different direction and with a different velocity. Thus we may account for the phenomena of reflection of light (q.v.) and of refraction (q.v.). Let ab, in fig. 1, be a plane surface of some transparent substance, say a sheet of glass; a ray, cd, perpendicular to the surface, will pass through without refraction; but an oblique ray, ef, will be sent in the direction ch. If the ray eh had passed from a dense into a rarer medium, then the direction would have been eg. It might also be shown that the sine of the angle of incidence always bears a certain ratio, to the sine of the angle of refraction; this ratio is termed the index of refraction. Thus, if a ray pass from air into water, the sine of the angle of incidence will have to the sine of the angle of the refraction the ratio of 4:3, or 1. FIG. 1.-Refraction of Light. Before a ray of light can reach the retina, it must pass through a number of transparent and refractive surfaces. The eye is a nearly spherical organ, formed of transparent parts situated behind each other, and surrounded by various membranous structures, the anterior part of which is also transparent. The transparent parts are (1) the cornea; (2) the aqueous humour, found in the anterior chamber of the eye; (3) the crystalline lens, formed by a transparent convex body, the anterior surface of which is less convex than the posterior; and (4) the vitreous humour, filling the posterior chamber of the eye. The ray must therefore traverse the cornea, aqueous humour, lens and vitreous humour. As the two surfaces of the cornea are parallel, the rays practically suffer no deviation in passing through that structure, but they are bent or refracted during their transmission through the other media. From the optical point of view, the eye may be regarded as a dioptric system consisting of various refractive media. In such a system, as shown by K. F. Gauss, there are six cardinal points, which have a certain relation to each other. These are→ becomes, after its refraction, parallel to the axis, and every ray (1) Two focal points: every ray passing through the first focal point which before refraction is parallel to the axis passes after its refraction to the second focal point; (2) two principal points: every ray which passes through the first point before refraction passes after refrac tion through the second, and every ray which passes through any point of a plane elevated on a perpendicular axis from the first principal point (the first principal plane) passes through the corresponding point of an analogous plane raised upon the axis at the second principal point (the second principal plane); and (3) two principal planes just alluded to. The distance of the first principal nodal points, which correspond to the optical centres of the two point from the first focal point is called the anterior focal length, and the term posterior focal length is applied to the distance of the posterior focal point from the second principal point. Listing has of the cornea for the cardinal points in an ideal eye:given the following measurements in millimetres from the centre Anterior focal point 12-8326 First nodal point Posterior focal point 22.6470 Second nodal point. First principal point 2.1746 Anterior focal length Second principal point. 2.5724 Posterior focal length. A view of such an ideal eye is shown in fig. 2. The remaining measurements of such an eye are as follows: 19-1-3379 If=1.4545 =1·3379 7.2420 7.6398 15.0072 20-0746 The optical constants of the human eye may be still further on the pupil. Thus it may be circular, or oval, or even trisimplified by assuming that the two principal points and the two angular. If the pencil is focused in front of the retina, as at DEF HV FIG. 2.-Transverse Section of an Ideal or Schematique Eye. A, summit of cornea; SC, sclerotic; S. Schlemm's canal; CH. choroid; I, iris; M, ciliary muscle; R, retina: N, optic nerve: HA, aqueous humour; L, crystalline lens, the anterior of the double lines on its face showing its form during accommodation; HV, vitreous humour; DN, internal rectus muscle; DE, external rectus; YY', principal optical axis; 4, visual axis, making an angle of 5° with the optical axis; C, centre of the ocular globe. The cardinal points of Listing: HH, principal points; KiK, nodal points; FiF, principal focal points. The dioptric constants according to Giraud-Teulon: H, principal points united; 2, principal foci during the repose of accommodation; 1. principal foci during the maximum of accommodation; O, fused nodal points. nodal points respectively are identical. Thus we may construct a reduced eye, in which the principal point is 2.3448 mm. behind the cornea and the single nodal point is 1-4764 mm. in front of the posterior surface of the lens. The refracting surface, or lens, has a radius of 5 mm and is 3 mm. behind the cornea; and the index of refraction is that of the aqueous humour, or 1, or 1·3379. 2. The Formation of an Image on the Retina. This may be well illustrated with the aid of a photographic camera. If properly focused, an inverted image will be seen on the glass plate at the back of the camera. It may also be observed by bringing the eyeball of a rabbit near a candle flame. The action of a lens in forming an inverted image is illustrated by fig. 3, where the pencil of rays proceeding from a is brought to a focus at a', and those from b at b'; consequently the image of ab is inverted as at b'a'. The FIG. 3.-Inversion by Action of a Lens. three characteristic features of the retinal image are: (1) it is reversed; (2) it is sharp and well defined if it be accurately focused on the retina; and (3) its size depends on the visual angle. If we look at a distant object, say a star, the rays reaching the eye are parallel, and in passing through the refractive media they are focused at the posterior focal point-that is, on the retina. A line from the luminous point on the retina passing through the nodal point is called the line of direction. If the luminous object be not nearer than, say, 60 yds. the image is still brought to a focus on the retina without any effort on the part of the eye. Within this distance, supposing the condition of the eye to be the same as in looking at a star, the image would be formed somewhat behind the posterior focal point, and the effect would be an indistinct impression on the retina. To obviate this, for near distances, accommodation, so as to adapt the eye, is effected by a mechanism to be afterwards described. FIG. 4.-Formation of Circles of Diffusion. d, or behind it as at f, or, in other words, if the retina, in place FIG. 5.-Diagram illustrating the Experiment of Scheiner. Let C be a lens, and DEF be screens placed behind it. Hold in front of the lens a card perforated by two holes A and B, and allow rays from a luminous point a to pass through these holes. The point o on the screen E will be the focus of the rays emanating from a; if a were removed farther from the lens, the focus would be on F, and if it were brought near to C, the focus would then be on D. The screens F and D show two images on the point a. If, then, we close the upper opening in AB, the upper image m the retina be substituted for the screens D and F. the contrary on F and the lower image n on D disappear. Suppose now that will take place, in consequence of the reversal of the retinal image. If the eye be placed at o, only one image will be seen; but if it be placed either in the plane of F or D, then two images will be seen, be circles of diffusion and indistinctness, and only in the plane E as at mm, or nn; consequently, in either of these planes there will will there be sharp definition of the image. To understand the formation of an image on the retina, suppose a line drawn from each of its two extremities to the nodal point and continued onwards to the retina, as in fig. 6, where the visual angle is x. It is evident that its size will depend on the size of the fo object and the distance of the object from the eye. Thus, also, objects of different sizes, c, d, e in fig. 6, may be in- B cluded in the same visual angle, as they are at different distances from the eye. The size of the retinal image may d be calculated if we know the size of the object, its distance from the nodal point o, and the distance of the nodal point from the posterior focus. When rays, reflected from an object or coming from a lumin- Let A be the size of the object, B its distance from the ous point, are not brought to an accurate focus on the retina, nodal point, and C the distance of o from the retina, the image is not distinct in consequence of the formation of or 15 mm.; then the size of the retinal image x=(A+15)/B. circles of diffusion, the production of which will be rendered The smallest visual angle in which two distinct points evident by fig. 4. From the point A luminous rays enter may be observed is 60 seconds; below this, the two senthe eye in the form of a cone, the kind of which will dependsations fuse into one; and the size of the retinal image FIG. 6.-The Visual Ang OPTICAL ARRANGEMENTS) VISION corresponding to this angle is 004 mm., nearly the diameter 3. The Optical Defects of the Eye.-As an optical instrument, the eye is defective; but from habit, and want of attention, its defects are not appreciated, and consequently they have little or no influence on our sensations. These defects are chiefly of two kinds-(1) those due to the curvature of the refractive surfaces, and (2) those due to the dispersion of light by the refractive media. M K FIG. 7-Spherical Aberration. F (a) Aberration of Sphericity.-Suppose, as in fig. 7, MA K to be a refractive surface on which parallel rays from L to S impinge, it will be seen that those rays passing near the circumference are brought to a focus at F, and those passing near the centre at Fintermediate rays Thus on the portion of the axis being focused at N. between F and F there will be a series of focal points, and the effect will be a blurred and bent image. In the eye this defect is to a large extent corrected by the following arrangements: (1) the iris cuts off the outer and more strongly refracted rays; (2) the curvature of the cornea is more ellipsoidal than spherical, and consequently those farthest from the axis are least deviated; (3) the anterior and posterior curvatures of the lens are such that the one corrects, to a certain extent, the action of the other; and (4) the structure of the lens is such that its power of refraction diminishes from the centre to the circumference, and consequently the rays farthest from the axis are less refracted. (b) Astigmatism.-Another defect of the eye is due to different This defect meridians having different degrees of curvature. is known as astigmatism. It may be thus detected. Draw on a sheet of white paper a vertical and a horizontal line with ink, crossing at a right angle; at the point of distinct vision, it will be found impossible to see the lines with equal distinctness at the same time; to see the horizontal line distinctly the paper must be brought near the eye, and removed from it to see the vertical. In the cornea the vertical meridian has generally a shorter radius of curvature, and is consequently more refractive than the horizontal. The meridians of the lens may also vary; but, as a rule, the asymmetry of the cornea is greater than that of the lens. The optical explanation of the defect will be understood with the aid of fig. 8. Thus, suppose the vertical meridian C A D to be more strongly curved than the horizontal F A E, the rays which fall on CAD will be brought to a focus G, and those falling on F AE at B. If we divide the pencil of rays at successive points, G, H, I, K, B, by a section perpendicular to A B, the various forms it would present at these points are seen in the figures underneath, so that if the eye were placed at G, it would see a horizontal line a a'; if at H, an ellipse with the long axis a a' parallel to A B; if at I, a circle; if at K, an ellipse, with the long axis, b c, at right angles The degree of to A B; and if at B, a vertical line b c. astigmatism is ascertained by measuring the difference of refraction in the two chief meridians; and the defect is corrected by the use of cylindrical glasses, the curvature of which, added to that of the minimum meridian, makes its focal length equal FIG. 8.-Diagram illustrating Astigmatism. α B (c) Aberration of Refrangibility.-When a ray of white light FIG. 9.-Diagram illustrating the Dispersion of |