organs in higher animals, but the compound eyes are extremely elaborate and complex in their structure. They are two in number, appearing as hemispherical masses on the sides of the head. When examined with the microscope, their surface is seen to be divided into an enormous number of hexagonal facets, which are in fact corneæ. In the ant, there are only 50 of these facets in each eye; in the common house-fly, 4000; in butterflies, upwards of 17,000; and in some of the beetles more than 25,000. Each cornea is found to belong to a distinct eye, provided with a nervous apparatus, and exhibiting a lens, iris, and pupil. Strauss Durckheim, who has carefully studied these structures in the cockchafer, suggests that, the eyes of insects being fixed, nature has made up for their want of mobility by their number, and by turning them in all directions; so that it might be said that these little animals have a distinct eye for every object.

Compound eyes of similar structure occur in many of the crustaceans.

3. Having now described the anatomical structure of the eye in man and certain of the lower animals, we are able to proceed to the consideration of the uses of the various parts of this organ. Assuming a general knowledge of the ordinary laws of geometrical optics (see DIOPTRICS, LENS, &c.), we will trace the course of the rays of light proceeding from any luminous body through the different media on which they impinge. If a luminous object, as, for example, a lighted candle, be placed at about the ordinary distance of distinct vision (about ten inches) from the front of the eye, some rays fall on the sclerotic, and being reflected, take no part in vision; the more central ones fall upon the cornea, and of these some also are reflected, giving to the surface of the eye its beautiful glistening appearance; while others pass through it, are converged by it, and enter the aqueous humour, which probably exerts no perceptible effect on their direction. Those which fall on and pass through the outer or circumferential part of the cornea are stopped by the iris, and are either reflected or absorbed by it; while those which fall upon its more central part pass through the pupil, and are concerned in vision. In consequence of its refractive power, the rays passing through a comparatively large surface of the cornea are converged so as to pass through the relatively small pupil and impinge upon the lens, which, by the convexity of its surface, and by its greater density towards the centre, very much increases the convergence of the rays passing through it. They then traverse the vitreous humour, whose principal use appears to be to afford support to the expanded retina, and are brought to a focus upon that tunic, forming there an exact but inverted image of the object.

This inversion of the image may be easily exhibited in the eye of a white rabbit or other albino animal, after removing the muscles, &c., from the back part of the globe. The flame of a candle held before the cornea may be seen inverted at the back of the eye, increasing in size as the candle is brought near, diminishing as it retires, and always moving in a direction opposite to that of the flame.

The adaptation of the eye to distinct vision at every distance beyond that of a few inches, is extremely remarkable, and numerous attempts have been made to explain the mechanism by which its focal length admits of alteration under the influence of the will. One view that has met with much support is, that the focal length is modified by a slight movement of the lens. In the eye of the bird there is a structure termed the ciliary muscle, which obviously approximates the lens to the cornea when a short field of view is required, and although the corresponding structure is only slightly

developed in man and mammals, it is probably sufficiently strong to produce the slight action required; while for the vision of distant objects the lens is carried back towards the retina by the elasticity of the connecting tissues. It would appear, however, from the recent researches of Cramer, Helmholtz, Allen Thomson, and others, that the accommodation is effected rather by a change in the form than in the position of the lens. It has been experimentally proved, that when the eye is turned from a distant to a near object, the antero-posterior diameter of the lens becomes elongated, and the anterior surface becomes more convex, while the opposite changes take place in turning the eye from a near to a distant object. According to Helmholtz, the radius of curvature of the anterior surface of the lens diminishes on turning the eye to a near object from ten to six millimetres (from about 0'4 to 0.24 of an inch), while the most projecting point of the same surface is brought forward about 0·2 of an inch.

Whichever view be adopted, the ciliary muscle takes an active part in the process. According to the observations of Hueck, the focal distance may be changed about three times in a second. The accommodation from a near to a distant object is effected much more rapidly than the converse process.

There are two well-known forms of defective vision in which this power of adaptation is very much limited-viz., short-sightedness or myopia, and long-sightedness or presbyopia. The limitation, however, is not due to a defect in the muscular apparatus to which we have referred, but to an abnormality either in the curves or in the density of the refracting media. In short-sightedness from too great a refractive power from either cause, the rays from objects at the ordinary range of distinct vision are brought too soon to a focus, so as to cross one another, and begin to diverge before they fall on the retina; the eye in this case being able to bring to the proper focus on the retina only those rays which were previously diverging at a large angle from a very near object. The correction for this deficiency is accomplished by interposing between the eye and indistinctly-seen objects a concave lens, with a curvature just sufficient to throw the images of external objects at the ordinary distance of distinct vision backwards upon the retina. In long-sightedness, on the other hand, there is an abnormal diminution of the refractive power from too flat a cornea, a deficient aqueous humour, or a flattening of the lens, so that the focus is behind the retina. This defect is corrected by convex lenses, which increase the convergence of the rays of light. Long-sightedness, as its name presbyopia indicates, usually comes on at a comparatively advanced period of life, while short-sightedness is most commonly met with in young persons; but both these rules present occasional exceptions; and the common belief that the latter affection naturally disappears after the middle period of life, is altogether erroneous.

We have already noticed the most essential use of the iris-viz., its power, under the influence of light upon the retina, of modifying the size of the pupil, so as to regulate the amount of light entering the eye. But this is not its only use; one of its offices being to prevent the passage of rays through the circumferential part of the lens, and thus to obviate the indistinctness of vision which would arise from spherical aberration (the unequal refrac tion of the rays passing through the centre and near the margin of the lens), in the same manner as the diaphragms employed by the optician. But there are additionally two other means by which this spherical aberration is prevented, which so well

(1.) The surfaces of the dioptric parts of the eye are not spherical, but those of the cornea and posterior surface of the lens are hyperbolical, and that of the anterior surface of the lens elliptical-configurations found by theory fitted to prevent spherical aberration. This discovery was made at a time when it was not known but that the dioptric parts of the eye had spherical surfaces.

(2.) The density of the lens diminishing [as we have already shewn] from the centre to its periphery, the circumferential rays are less refracted than they would have been by a homogeneous lens with similar surfaces. This elegantly simple contrivance has been hitherto inimitable by human art.'-The Actonian Prize Treatise, 1851, p. 50.

Chromatic aberration, which is caused by the unequal refrangibility of the primitive rays of which white light is composed, when transmitted through an ordinary lens, whereby coloured fringes are produced, is practically corrected in the eye, although it is doubtful whether it is entirely absent. The provision, however, on which the achromatism depends has not been determined with certainty, probably because we do not yet know the relative refractive and dispersive powers of the cornea and humours of the eye. Sir David Brewster denies that the chromatic aberration receives any correction in the eye, and maintains that it is imperceptible only in consequence of its being extremely slight.

4. We have hitherto been considering the eye as an optical instrument which projects pictures of external objects on the retina; we now come to the action of the nervous tunic, the retina, and its adaptation to the physical construction of the eye. When the retina or the optic nerve is stimulated, we have the sensation of light, whatever may be the nature of the stimulus employed--as, for example, if it be a blow on the eye in the dark, or irritation of the optic nerve from some morbid condition. The sensation of light, then, consists in a recognition by the mind of a certain condition of these nervous structures, and this condition may be induced by the application of any stimulus; the ordinary stimulus obviously being the rays of light which fall upon the retina. There must, however, be a certain amount of light for the purpose of vision. Every one knows that it is difficult and painful to discern objects in a very faint light; and, on the other hand, that on suddenly entering a brilliantly lighted room from the dark, everything appears confused for one or two seconds. There is, however, a gradual adaptation of the retina to different amounts of light. Persons long immured in dark dungeons acquire the power of distinctly seeing surrounding objects; while those who suddenly encounter a strong light, are unable to see distinctly until the shock which the retina has experienced has subsided, and the iris has duly contracted. In protecting the retina from the sudden effects of too strong a light, the iris is assisted by the eyelids, the orbicular muscle, and, to a certain extent, by the eyebrows. Moreover, the dark pigment of the choroid coat acts as a permanent guard to the retina, and where it is deficient, as in albinos, an ordinary light becomes painful, and the protective appendages, especially the eyelids, are in constant use.

The persistence, during a certain time, of impressions made on the retina, facilitates the exercise of sight. A momentary impression of moderate intensity continues for a fraction of a second; but if the impression be made for a considerable time, it endures for a longer period after the removal of the object. Thus, a burning stick, moved rapidly in a circle

before the eyes, gives the appearance of a continuous ribbon of light, because the impression made by it at any one point of its course remains on the retina until it again reaches that point. It is owing to this property that the rapid and involuntary act of winking does not interfere with the continuous vision of surrounding objects; and, to give another illustration of its use, if we did not possess it, the act of reading would be a far more difficult performance than it now is, for we should require to keep the eye fixed on each word for a longer period, otherwise the mind would fail fully to perceive it. Again, in consequence of the retention of sensations by the retina, the image of an object may continue to be seen, especially in certain morbid states of the system, and in twilight, for some seconds after the eyes have been turned away from it, and this physi ological phenomenon has probably given origin to many stories of ghosts and visions. Thus, if a person has unconsciously fixed his eyes, especially in the dusk, on a dark post or stump of a tree, he may, on looking towards the gray sky, see projected there a gigantic white image of the object, which may readily be mistaken for a supernatural appearance. These ocular spectra are always of the complementary colour to that of the object. Thus, the spectrum left by a red spot is green; by a violet spot, yellow; and by a blue spot, orange. However great may be the velocity of a luminous body, it can always be seen; but if an opaque body move with such rapidity as to pass through a space equal to its own diameter in a less time than that of the duration of the retinal impression, it is altogether invisible; and hence it is, for example, that we cannot see bullets, &c., in the rapid part of their flight.

A small portion of the retina, corresponding to the entrance of the optic nerve, is incapable of exciting the sensation of vision when it receives! the image of an object. According to Volkmann, this small invisible spot exactly corresponds in size with the artery lying in the centre of the optic nerve. If the blind spot' had been situated in the axis of the eye, a blank space would always have existed in the centre of the field of vision, since the axes of the eyes in vision correspond. But as it is, the blind spots do not correspond when the eyes are directed to the same object; and hence the blank which one eye would present is filled up by the other eye. Mariotte, early in the last century, first described the existence of these blind spots. Any one may satisfy himself of their existence by the following simple experiment. Let two small black circles be made upon a piece of paper, about four or five inches apart, then let the left eye be closed, and the right eye be strongly fixed upon the left-hand circle. If the paper be then moved backwards and forwards, a point will be found at which the righthand circle is no longer visible, although it reappears when the paper is either brought nearer or removed further. Although no other part of the retina possesses the complete insensibility presented by the blind spot, it is probable that its anterior portions have very little to do with vision. When using only one eye, we direct it towards the object we wish to inspect, in such a way as to throw the image to the back of the globe; and when the eye is thus fixed, objects near the boundary of the field of vision are less distinctly seen than those at its centre.

The extent of the field of vision for a single eye, the head being fixed, has been calculated by Dr Young. He found that the eyeball was capable of a movement of 55 degrees in every direction, so that a single eye may have perfect vision of any point within a range of 110 degrees.

We have not yet referred to the longitudinal range, or greatest distance of human vision; indeed,

this range varies so extremely that it is difficult to assign an arbitrary limit to it. Many uncivilised races, as the North American Indians, and the inhabitants of the vast Asiatic steppes, possess powers of sight which would appear almost incredible if they had not been thoroughly and frequently corroborated. Our information is more definite regarding the limits of human vision in regard to the minuteness of the objects of which it can take cognizance. Ehrenberg has carefully studied this subject, and has arrived at the following results. The side of the smallest square magnitude usually visible to the naked eye-either of white particles on a black ground or conversely-is about th of an inch; and with the greatest condensation of light and effort on the part of the observer, squares with a side as small as th of an inch may be recognised, but without sharpness or certainty. Bodies smaller than these, when observed singly, cannot be discerned by the naked eye, but may be seen when placed in a row. Much smaller particles may, however, be distinctly seen, if they powerfully reflect light; thus, gold-dust, which in none of its diameters exceeded th of an inch, is easily discernible in common daylight. The delicacy of vision is far greater for lines than for minute areas, since opaque threads of th of an inch may be discerned when held towards the light.

Various topics which the reader might perhaps have expected to find noticed, such, for instance, as 'single vision with two eyes,' 'the appreciation of solid forms by the sense of vision,' correct vision with an inverted image on the retina,' &c., which belong fully as much to metaphysics as to physiology, will be discussed in a future article on VISION. In the meantime, we may refer those who desire information on these points to Professor Bain's treatise on The Senses and the Intellect.

eye

EYE, DISEASES OF THE. The diseases of the enumerated by the surgeon are very numerous, partly from the variety of the tissues and parts of which it is formed, partly because the exposed situation and transparency of the eye enable the diseases to be seen. Nearly all its parts are liable to inflammation and its consequences. See OPHTHALMIA. The eyelids are liable to various diseases, as growths of several kinds, most of which the surgeon may remove; inflammation, as bleareye (ophthalmia tarsi); to be misdirected inwards or outwards, Entropium and Ectropion (q. v.); and the upper eyelid may fall down (ptosis) from palsy of the common motor oculi nerve. The eyelashes may grow in upon the eye (trichiasis), and produce serious results. When plucked out, they grow again; and if they still grow in upon the eye after this palliative treatment has been tried several times, the surgeon has to cut down on their roots, and destroy them. The duct which conveys away the tears to the nose is liable to inflammation and obstruction, causing watery eye. See LACHRYMAL ORGANS, DISEASES OF. The cornea is liable to opacity in various degrees. The mere nebula or cloudy condition, either limited or general, may pass off, and leave the cornea again clear; but the white mark, which is the cicatrix or scar of an ulcer, is permanent, although it may become smaller by the disappearance of the surrounding haze. The pupil may be closed as the result of iritis, or of operations for cataract, and an artificial pupil may be made by either of the three methods-incision, excision, or separation-but the operation is seldom attended with success. For opacities of the crystalline lens, see CATARACT. For an account of diseases of the nervous parts of the eye, see AMAUROSIS. Various affections of vision may arise from peculiar or altered conditions of the refracting humours of the eye-as

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near-sightedness (myopia), far-sightedness (presbyopia), the appearance of bodies (musca) floating in or before the eye; and there may be double vision (diplopia), with two eyes or with one. See VISION, ABNORMAL CONDITIONS OF. The parts between the eye and its bony orbit may be the seat of inflammation, abscess, or tumour, making the eye protrude. The movements of the eyeballs may be affected from palsy of the motor nerves, or from contraction of the lateral recti muscles, causing inward or outward squinting. See STRABISMUS The eye may lose all feeling, from palsy of the fifth pair of nerves. The whole of the same side of the face, nostril, and mouth, will be in the same condition, and the eye becomes inflamed and disorganised. Substances thrown against the eye may injure it. Quicklime is rapidly destructive to the eye, slaked lime and mortar less so. When one of these, or any other caustic, has got into the eye, sweet oil is the best thing to introduce, until the surgeon arrives to remove them. If it is oil of vitriol (sulphuric acid) that has been the cause of the injury, a weak solution of soda may be used in the first place to neutralise the acid. In gunpowder explosions near the eye, besides the burn, the particles are driven into the surface of it, and will cause permanent black stains over the white of the eye, unless they are carefully removed at the time. When chips of glass, stone, &c., are driven into the interior of the eye, there is little hope of it being saved from destructive inflammation. When only partially sunk into the cornea, as is often the case with sparks of hot iron, or 'fires,' as they are called, the rubbing of the projecting part on the eyelid causes great pain, and the surgeon has not much difficulty in removing them. Most commonly these, or other foreign bodies,' as particles of dust, sand, seeds, flies, &c., merely get into the space between the eyeball and the lids, almost always concealed under the upper, as it is the larger, and sweeps the eye. They cause great pain, from the firmness and sensitiveness of the papillary surface of the lid, soon excite inflammation, and their presence, as the cause, is apt to be overlooked. The lid must be turned round to find them. To do this, pull the front or edge of the lid forwards by the eyelashes, held with the finger and thumb, and at the same time press down the back part of the lid with a small pencil or key. The lid will readily turn round, when the body may be seen about its middle, and may be removed with the corner of a handkerchief. Another plan, which the person himself may try, is to pull forward the upper lid by the eyelashes, and push the lashes of the lower lid up behind it, when the foreign body may be brushed out. After the bodies are removed, a feeling as if they were still there may remain for some time.

EYE, a parliamentary and municipal borough in the north of Suffolk, near the source of the Waveney, 20 miles north of Ipswich. Its streets are rather narrow and irregular. Pop. (1861) 2430. It sends one member to parliament, the parliamentary borough including eleven parishes. Eye, in Anglo-Saxon, means island; the river surrounding the town. There was formerly a castle and priory here. (1871-pop. of m. b., 2396; of p. b., 6721.)

EYEBRIGHT (Euphrasia), a genus of plants of the natural order Scrophulariacea, having a tubular calyx, the upper lip of the corolla divided, the lower of three nearly equal lobes, the cells of the anthers spurred at the base, a two-celled capsule and striated seeds. Some of the species are rootparasites. The only British species is the COMMON E. (E. officinalis), a little plant of at most six or eight inches in height, with ovate serrated leaves,

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THE sixth letter in the Latin and English alphabets, corresponding to the Vau of the Hebrew, and the Digamma (q. v.) of the old Greek alphabet. See ALPHABET. F and v are called labio-dentals, from the organs employed in producing them; they belong to the class of consonants called Aspirates (q. v.), and bear the same relation to each other that exists between the unaspirated labials p and b. In Latin, had a peculiar sound, different from that of Greek 4, as we learn from Cicero and other Latin writers. What the sound was, we do not exactly know, but it approached to the nature of a strongly breathed h, as is indicated by the fact, that in the Sabine dialect it sometimes takes the place of h, as Sab. fircus Lat. hircus (a he-goat); and the Latins made use both of faba and haba for a bean.' This affinity is also shewn in modern Spanish, where h takes the place of the Latin f; as Lat. femina, Sp. hembra; fl becomes, in Spanish, ll, as Lat. flamma Sp. llama. F, in English and other Teutonic tongues, corresponds to p in Greek and Latin; as Lat. and Gr. pater Eng, father; Gr. pod-, Lat. ped- Eng. foot; Lat. piscEng. fish; Gr. pur Eng. fire; Lat. vulp-Eng. wolf. In some words, v takes the place in German of ƒ in English; as Ger. vater = Eng. father; Ger. vier Eng. four. In the Aberdeenshire dialect, f takes the place of wh, as fat for what; fup for whip. This seems to be a relic of the Teutonic pronunciation of w (v), still to be observed in the Cockney pronunciation of vill for will, ven for when; but why the sharpening of the labial into ƒ should be confined to one circumscribed district of Scotland, and to the case of w followed by h, it is hard to say. Fin Lat. and Greek becomes b in Eng.; as Gr. and Lat. ferEng. bear; Lat. frater Eng. brother. See Letter B. More remarkable are the interchanges between ƒ and the series d, th, t. Lat. foris Gr. thura, Eng. door; Lat. fera = Gr. ther, Eng. deer; Eng. red, Sans. ruthira, Gr. eruthros, Lat. rutilus, rufus, ruber. In Russian, Feodor, Afanasja Theodor, Athanasia. In words originally common to both Greek and Latin, the Greek is represented in Lat. by f; as Gr. Onun = Lat. fama. But in spelling Greek words with Latin letters, the Romans, after the time of Cicero, were careful to represent e, not by f, which had a somewhat different power, but by ph. This mode of spelling words derived from Greek is still adhered to in English, German, and French, although the distinction in sound has long been lost sight of. The distinction began to disappear in the Latin itself in the time of the later Roman emperors, when inscriptions shew such spelling as Afrodite for Aphrodite; and this simplification is followed in modern Italian, Spanish, and Portuguese. Ph is sometimes erroneously used in words having no connection with Greek; as Adolphus, for the Teutonic Adolf or Adalolf-i. e., 'noble wolf.'

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F, in Music, is the fourth note of the natural diatonic scale of C, and stands in proportion to C as 4 to 3, and is a perfect fourth above C as fundamental note. F major, as a key, has one flat at its signature-viz., B flat. F minor has four flats the same as A flat major, of which it is the relative minor.

FAAM, or FAHAM (Angræcum fragrans), an orchid, native of India and the Mascarene Isles, of its leaves, which is owing to the presence of much prized in the East for the delightful fragrance Coumarin (q. v.), and resembles that of the Tonka Bean and of Vernal Grass. In the Isle of Bourbon, an infusion of F. leaves is in great repute as a cure In France, it has been successfully employed, under for pulmonary consumption and as a stomachic. the name of Isle of Bourbon Tea, as an expectorant, anti-spasmodic and stomachic.

FABACEÆ. See LEGUMINOSA.

son.

FA'BER is the name of two artists, father and John F., the elder, was born in Holland, where he acquired a knowledge of the art of mezzotinto-engraving. Subsequently, he came to England, and died at Bristol, May 1721. His works do not exhibit much talent.-The younger F., also called John, obtained, however, a high reputation as an engraver in mezzotinto. His principal works are the portraits of the Kit-Cat Club, and the Beauties of Hampton Court, several of which are executed with great freedom, vigour, and beauty. F. lived in London, where he is believed to have died in 1756.

FABER, REV. GEORGE STANLEY, a learned and voluminous divine of the Anglican Church, was the eldest son of the Rev. Thomas Faber, and was born 25th October 1773. He entered University College, Oxford, in 1789, where he achieved a brilliant academical reputation. Before his 21st year, he was elected Fellow and Tutor of Lincoln College. In 1796, he took his degree of M.A.; was Bampton Lecturer for 1801, in which capacity he delivered the lectures subsequently published under the title of Hora Mosaica; and in 1805 became vicar of Stockton-on-Tees, in the county of Durham. After several changes, he received from Bishop Van Mildert, in 1832, the mastership of Sherburn Hospital, near the city of Durham, where he died 27th January 1854. F. wrote upwards of forty works, several of which, especially those upon prophecy, have enjoyed a very extensive popularity. All his writings are marked by 'strong masculine sense, extensive classical erudition, and a hearty love of hypothesis.' The principal are-The Genius and Object of the Patriarchal, the Levitical, and the Christian Dispensations (1823, 2 vols.); The Diffi culties of Infidelity (1824); The Sacred Calendar of Prophecy (1828, 3 vols.); The Primitive Doctrine of Election (1836), reckoned by some critics the most valuable of all F.'s writings; The Primitive Doctrine of Justification (1837); and Eight Dissertations