Calico-printing is the art of applying colours to cloth after it has come from the hand of the weaver in such a manner as to form patterns or figures. The art was originally brought to Britain from India, and was sometimes practised on linen, woollen, and silk, but most frequently upon that species of cotton cloth called calico. The process was originally accomplished by means of hand-blocks made of wood on which patterns or parts of patterns for each different colour were cut. These blocks were of various dimensions, according to the nature of the work, and where several colours were employed in one pattern, a block for each colour was necessary.
As an improvement in the method of printing from wooden blocks, especially where delicacy of outline was required, engraved copperplates were introduced about 1760; but the greatest improvement was effected by the introduction of cylinder printing about 1785, which had almost superseded the other methods, except for particular styles by 1900. The machinery then generally used consisted of various modifications of the cylinder printing-machine, in which a number of separate engraved cylinders were mounted, corresponding to the number of colours to be printed. Formerly the cloth had to pass once through the machine for every colour; but later, by an arrangement of machinery equally ingenious and effective, any number of cylinders were fitted on one machine, which acted on the cloth one after the other, and by this means the pattern was finished with a corresponding number of colours in the same time that was formerly employed to give one.
A great variety of methods have been employed in calico-printing, but they all fall under the general heads of dye-colours and steam-colours. Under the first head are included all the styles in which the pattern is printed on the cloth by a mordant - a substance which may have little or no colour itself, but has an affinity for the fibre on the one hand, and for the colouring matter on the other - the dye or colouring matter being subsequently fixed by dyeing on such parts of the cloth as have been impregnated with the mordant, and thus bringing out the pattern.
In steam-colour printing the colouring material is applied to the cloth direct from the printing-cylinder, and subsequently fixed by steaming. In steam-colours there is no limit to the number and variety of shades which may be produced, each colour-box on the cylinder printing-machine containing the whole ingredients essential to the production and fixation of a separate and distinct shade of colour. This process was superseding most of the other styles by the end of the Victorian era, the brilliant coal-tar colours so extensively used being almost entirely fixed by steaming.
The bodies used for fixing were tin mordants, tannic acid, etc, which were mixed with the dye-colours and printed together. The effects of calico-printing are varied by numerous other operations, such as the discharge-style, in which the cloth is first dyed all over, then printed in a certain pattern with discharge-chemicals, which either produce a pattern of some other colour, or one purely white, as in the Turkey-red bandanna handkerchiefs. The resist-style, in some respects, is the reverse of the discharge-style; the process being to print a pattern in certain chemicals, which will enable those parts to resist the action of the dye subsequently applied to all other parts of the cloth. After the prints have undergone the printing process they are submitted to a series of finishing operations, the object of which is to give to the fabrics a pleasing appearance to the eye. Research Calico-Printing
Estates of Scotland was the name given to a body of similar constitution to the English Parliament, but with important differences, the king himself, as well as his officers, being responsible to the estates for wrongs done. They held the power of declaring war, or entering on a peace or treaty, and with them rested the right of declaring, with or without the consent of the king, resolutions of the assembly to be law. To prevent a bill being hurried through parliament it was submitted to and discussed by a committee called the Lords of the Articles. If sanctioned by this committee the bill was passed on to the whole house for approval. Another committee appointed by the estates was called the Auditors of Complaints, whose duty was to hear appeals against the decisions of the king's judges, and, if necessary, to reverse their sentences. Research Estates of Scotland
The committee reported various devices during several years. William Barton, of Philadelphia, was appointed to submit designs. Sir John Prestwich, an English antiquarian, suggested a design to John Adams in 1779.
Combining the various designs of William Barton and John Prestwich, a seal was adopted on June the 20th, 1782. Arms: Paleways of thirteen pieces argent and gules; a chief azure; the escutcheon on the breast of the American eagle displayed proper, holding in his dextertalon an olive branch and in his sinister a bundle of thirteen arrows; and in his beak a scroll with the motto: E Pluribus Unum. Crest: a glory breaking through a cloud proper and surrounding thirteen stars. Reverse: A pyramid unfinished. In the zenith an eye in a triangle, surrounded with a glory proper, over the eye the words, Annuit Coeptis. Beneath the pyramid, MDCCLXXVI, and the words, Novus Ordo Seculorum. Research Seal of the United States
Tic-tac is a method of signalling in use on racecourses. Its chief object is to communicate the betting movements in Tattersall's to book-makers in the smaller rings and on the course itself when outside betting is permitted.
The tic-tac is operated in a triangular manner, the betting movements in Tattersall's being communicated by one of the operators to a second stationed on the grandstand, who passes on the message to a third person, who in turn, communicates the information to the bookmaker for whom he works.
The signals are made by touching various parts of the head, arms and body, and although more or less universally used, are varied at times to deceive outsiders by introducing what is colloquially called the 'twist', i.e. the signs are made the reverse way and the numbers assigned to horses on the race cards are reversed or otherwise altered.
Customarily the signs used run from the right shoulder, over to the left shoulder, and up either arm, a few of them being as follows: One, right shoulder touched with right hand; two, right ear with right hand; three, right side top of head with right hand; six, left shoulder with left hand; ten, both hands placed together; 7-to-4 against, tips of left hand fingers, with right hand; 5-to-2 against, breast pocket with right hand. Research Tic-Tac
A tournament was originally an exhibition of combat between mounted men. Named from the tournament, or quick turning of the horses, it appeared in Europe in the 11th century, and quickly grew in popularity, both in France and England, where, in the reign of Henry II, it became necessary to prohibit tournaments owing to extravagant indulgence in them by the younger nobility. Later, tournaments were allowed to be held under royal licence, und a classic account of one in the time of Richard I is given in the book Ivanhoe, in which the customs and rules of such assemblies are graphically indicated. Froissart is also a mine of information regarding tournaments.
The essential feature of a tournament was the single combat of knight with knight, each striving to unhorse or incapacitate his opponent, the usual weapon being the lance. Sometimes, however, one body of knights fought against another body. They took place on most occasions of rejoicing.
Participation in tournaments was usually reserved for those of noble birth, and about the end of the 13th century a new set of rules was embodied in a statute of arms for tournaments, which laid down, among other provisions, that disputes arising out of a tournament were to be settled by a court of honour composed of princes and earls. By the 16th century the tournament had degenerated until it was merely a form of pageant. The term is now applied to many kinds of athletic contests.
In the 18th century novel Ivanhoe, the author, Walter Scott describes the arena and audience of a fictional 12th century tournament as follows:
On the verge of a wood, which approached to within a mile of the town of Ashby, was an extensive meadow, of the finest and most beautiful green turf, surrounded on one side by the forest, and fringed on the other by straggling oak-trees, some of which had grown to an immense size. The ground, as if fashioned on purpose for the martial display which was intended, sloped gradually down on all sides to a level bottom, which was enclosed for the lists with strong palisades, forming a space of a quarter of a mile in length, and about half as broad. The form of the enclosure was an oblong square, save that the corners were considerably rounded off, in order to afford more convenience for the spectators. The openings for the entry of the combatants were at the northern and southern extremities of the lists, accessible by strong wooden gates, each wide enough to admit two horsemen riding abreast. At each of these portals were stationed two heralds, attended by six trumpets, as many pursuivants, and a strong body of men-at-arms for maintaining order, and ascertaining the quality of the knights who proposed to engage in this martial game.
On a platform beyond the southern entrance, formed by a natural elevation of the ground, were pitched five magnificent pavilions, adorned with pennons of russet and black, the chosen colours of the five knights challengers. The cords of the tents were of the same colour. Before each pavilion was suspended the shield of the knight by whom it was occupied, and beside it stood his squire, quaintly disguised as a salvage or silvan man, or in some other fantastic dress, according to the taste of his master, and the character he was pleased to assume during the game. The central pavilion, as the place of honour, had been assigned to Brian be Bois-Guilbert, whose renown in all games of chivalry, no less than his connexions with the knights who had undertaken this Passage of Arms, had occasioned him to be eagerly received into the company of the challengers, and even adopted as their chief and leader, though he had so recently joined them. On one side of his tent were pitched those of Reginald Front-de-Boeuf and Richard de Malvoisin, and on the other was the pavilion of Hugh de Grantmesnil, a noble baron in the vicinity, whose ancestor had been Lord High Steward of England in the time of the Conqueror, and his son WilliamRufus. Ralph de Vipont, a knight of St John of Jerusalem, who had some ancient possessions at a place called Heather, near Ashby-de-la-Zouche, occupied the fifth pavilion. From the entrance into the lists, a gently sloping passage, ten yards in breadth, led up to the platform on which the tents were pitched. It was strongly secured by a palisade on each side, as was the esplanade in front of the pavilions, and the whole was guarded by men-at-arms. The northern access to the lists terminated in a similar entrance of thirty feet in breadth, at the extremity of which was a large enclosed space for such knights as might be disposed to enter the lists with the challengers, behind which were placed tents containing refreshments of every kind for their
tion, with armourers, tarriers, and other attendants, in readiness to give their services wherever they might be necessary.
The exterior of the lists was in part occupied by temporary galleries, spread with tapestry and carpets, and accommodated with cushions for the convenience of those ladies and nobles who were expected to attend the tournament. A narrow space, betwixt these galleries and the lists, gave accommodation for yeomanry and spectators of a better degree than the mere vulgar, and might be compared to the pit of a theatre. The promiscuous multitude arranged themselves upon large banks of turf prepared for the purpose, which, aided by the natural elevation of the ground, enabled them to overlook the galleries, and obtain a fair view into the lists. Besides the accommodation which these stations afforded, many hundreds had perched themselves on the branches of the trees which surrounded the meadow; and even the steeple of a country church, at some distance, was crowded with spectators.
It only remains to notice respecting the general arrangement, that one gallery in the very centre of the eastern side of the lists, and consequently exactly opposite to the spot where the shock of the combat was to take place, was raised higher than the others, more richly decorated, and graced by a sort of throne and canopy, on which the royal arms were emblazoned. Squires, pages, and yeomen in rich liveries, waited around this place of honour, which was designed for Prince John and his attendants. Opposite to this royal gallery was another, elevated to the same height, on the western side of the lists; and more gaily, if less sumptuously decorated, than that destined for the Prince himself. A train of pages and of young maidens, the most beautiful who could be selected, gaily dressed in fancy habits of green and pink, surrounded a throne decorated in the same colours. Among pennons and flags bearing wounded hearts, burning hearts, bleeding hearts, bows and quivers, and all the commonplace emblems of the triumphs of Cupid, a blazoned inscription informed the spectators, that this seat of honour was designed for 'La Royne de las Beaulte et des Amours'. But who was to represent the Queen of Beauty and of Love on the present occasion no one was prepared to guess.
Meanwhile, spectators of every description thronged forward to occupy their respective stations, and not without many quarrels concerning those which they were entitled to hold. Some of these were settled by the men-at-arms with brief ceremony; the shafts of their battle-axes, and pummels of their swords, being readily employed as arguments to convince the more refractory. Others, which involved the rival claims of more elevated persons, were determined by the heralds, or by the two marshals of the field, William de Wyvil, and Stephen de Martival, who, armed at all points, rode up and down the lists to enforce and preserve good order among the spectators.
Gradually the galleries became filled with knights and nobles, in their robes of peace, whose long and rich-tinted mantles were contrasted with the gayer and more splendid habits of the ladies, who, in a greater proportion than even the men themselves, thronged to witness a sport, which one would have thought too bloody and dangerous to afford their sex much pleasure. The lower and interior space was soon filled by substantial yeomen and burghers, and such of the lesser gentry, as, from modesty, poverty, or dubious title, durst not assume any higher place. It was of course amongst these that the most frequent disputes for precedence occurred.
Walter Scott goes on to describe the rules of the tournament, which he says were imparted to the audience by the monarch:
First, the five challengers were to undertake all comers.
Secondly, any knight proposing to combat, might, if he pleased, select a special antagonist from among the challengers, by touching his shield. If he did so with the reverse of his lance, the trial of skill was made with what were called the arms of courtesy, that is, with lances at whose extremity a piece of round flat board was fixed, so that no danger was encountered, save from the shock of the horses and riders. But if the shield was touched with the sharp end of the lance, the combat was understood to be at 'outrance', that is, the knights were to fight with sharp weapons, as in actual battle.
Thirdly, when the knights present had accomplished their vow, by each of them breaking five lances, the Prince was to declare the victor in the first day's tourney, who should receive as prize a warhorse of exquisite beauty and matchless strength; and in addition to this reward of valour, it was now declared, he should have the peculiar honour of naming the Queen of Love and Beauty, by whom the prize should be given on the ensuing day.
Fourthly, it was announced, that, on the second day, there should be a general tournament, in which all the knights present, who were desirous to win praise, might take part; and being divided into two bands of equal numbers, might fight it out manfully, until the signal was given by Prince John to cease the combat. The elected Queen of Love and Beauty was then to crown the knight whom the Prince should adjudge to have borne himself best in this second day, with a coronet composed of thin gold plate, cut into the shape of a laurel crown. On this second day the knightly games ceased. But on that which was to follow, feats of archery, of bull-baiting, and other popular amusements, were to be practised, for the more immediate amusement of the populace.
At the flourish of clarions and trumpets, they started out against each other at full gallop; and such was the superior dexterity or good fortune of the challengers, that those opposed to Bois-Guilbert, Malvoisin, and Front-de-Boeuf, rolled on the ground. The antagonist of Grantmesnil, instead of bearing his lance-point fair against the crest or the shield of his enemy, swerved so much from the direct line as to break the weapon athwart the person of his opponent - a circumstance which was accounted more disgraceful than that of being actually unhorsed; because the latter might happen from accident, whereas the former evinced awkwardness and want of management of the weapon and of the horse. The fifth knight alone maintained the honour of his party, and parted fairly with the Knight of St John, both splintering their lances without advantage on either side.
The shouts of the multitude, together with the acclamations of the heralds, and the clangour of the trumpets, announced the triumph of the victors and the defeat of the vanquished. The former retreated to their pavilions, and the latter, gathering themselves up as they could, withdrew from the lists in disgrace and dejection, to agree with their victors concerning the redemption of their arms and their horses, which, according to the laws of the tournament, they had forfeited. The fifth of their number alone tarried in the lists long enough to be greeted by the applauses of the spectators, amongst whom he retreated, to the aggravation, doubtless, of his companions' mortification.
During intervals of the tournament it appears that heralds inspired the contestants and audience by announcements and chants inspiring chivalry and bravery such as:
'Love of ladies, splintering of lances! stand forth gallant knights, fair eyes look upon your deeds!' Research Tournament
In an organism, circulation is the flowing of sap or blood through the veins or channels, by means of which the perpetual and simultaneous movements of composition and decomposition manifested in organic life are carried on. Although Galen, who had observed the opposite directions of the blood in the arteries and veins, may be said to have been upon the very point of discovering the circulation, the discovery was reserved for William Harvey, who in 1628 pointed out the continuity of the connections between the heart, arteries, and veins, the reverse directions taken by the blood in the different vessels, the arrangements of valves in the heart and veins so that the blood could flow only in one direction, and the necessity of the return of a large proportion of blood to the heart to maintain the supply.
In 1661 Malpighi exhibited microscopically the circulation in the web of a frog's foot, and showed that the blood passed from arteries to veins by capillaries or intermediate vessels. This finally established the theory with regard to animals, but the movements of sap in vegetables were only traced with difficulty and after numerous experiments.
Many physiologists were reluctant to ascribe the term 'circulation' to this portion of the economy of plants; but though sap, unlike the blood, does not exhibit movements in determinate vessels to and from a common centre, a definite course is observable. In the stem of a dicotyledonous tree, for example, the sap describes a sort of circle, passing upwards from the roots through the newer woody tissue to the leaves, where it is elaborated under the action of air and light; and thence descending through the bark towards the root, where what remains of it is either excreted or mixed with the new fluid, entering from the soil for a new period of circulation.
In infusorial animalcules the movement of the fluids of the body is maintained by that of the animal itself and by the disturbing influence of nutritive absorption. In the Coelentera (zoophytes, etc) the movement receives aid besides from the action of cilia on the inner walls of the body. The Annelids, as the earth-worm, possess contractile vessels traversing the length of the body. The Insects, Crustaceans, Myriapods, and Spiders have a dorsal tube, a portion of which may be specially developed as a heart. The blood is driven to the tissues, in some cases along arterial trunks, being distributed not in special vessels, but simply through the interstices of the tissues. From the tissues it is conveyed, it may be by special venous trunks to a venous sinus which surrounds the heart and opens into it by valvular apertures. The Mollusca have the heart provided with an auricle and a ventricle, as in the snail and whelk; two auricles, one on either side of the ventricle, as in the fresh-water mussel; or two auricles and two ventricles, as in the ark-shells. Among the ascidians, which stand low in that division of animals to which the molluscs belong, the remarkable phenomenon is encountered of an alternating current, which is rhythmically propelled for equal periods in opposite directions.
All vertebrated animals (except Amphioxus) have a heart, which in most fishes consists of an auricle and ventricle, but in the mud-fishes (Lepidosiren) there are two auricles and one ventricle; and this trilocular heart is found in the amphibians, and in most reptiles except the crocodiles, which, like birds and mammals, have a four-chambered organ consisting of two auricles and two ventricles. In these two last-named classes the venous and arterial blood are kept apart; in the trilocular hearts the two currents are mixed in the ventricle. Research Circulation
Robert Chambers was an English historical and miscellaneous writer. He was born in 1802 at Peebles and died in 1871. The younger of two brothers originally composing the publishing firm of W & R Chambers his father was a muslin weaver. Along with his brother William who was his senior by two years, he received his education at the Peeblesparish school and in the High School of Edinburgh. His family experiencing a reverse of fortune, he got together all the books belonging to his mother and himself, their value being about 2 pounds, and at the age of sixteen commenced business as a bookseller in Edinburgh.
His elder brother William established himself in the neighbourhood as a printer, and they united in projecting and issuing a short-lived periodical called the Kaleidoscope, Robert Chambers being editor and chief contributor, and William Chambers printer. Robert's illustrations of the Author of Waverley and his Traditions of Edinburgh (1823) won a ready popularity, and various other works followed in quick succession from this period until 1832: Popular Rhymes of Scotland (1826); Picture of Scotland (1827); Histories of the Scottish Rebellions; and a Life of James I.
He next edited Scottish Ballads and Songs; a Biographical Dictionary of Eminent Scotsmen; and on the 4th of February, 1832, the brothers commenced Chambers's Edinburgh Journal, which achieved an immense success. Erom this time W & R Chambers united in the publishing business, and issued a series of works for the entertainment and instruction of the people. Robert Chambers contributed numerous essays to the Journal, besides editing or compiling many instructive works of a high class, including the Cyclopaedia of English Literature; the Domestic Annals of Scotland; Ancient Sea-Margins; and the Book of Days. He also edited a valuable edition of Burns. Research Robert Chambers
Addiction is a state of dependence caused by the habitual use of drugs, alcohol, or other substances. It is characterised by uncontrolled craving, tolerance, and symptoms of withdrawal when access is denied. Habitual use produces changes in body chemistry and treatment must be geared to a gradual reduction in dosage. Initially, only opium and its derivatives (morphine, heroin, codeine) were recognised as addictive, but many other drugs, whether therapeutic (such as tranquillisers) or recreational (such as cocaine and alcohol), are now known to be addictive. Research points to a genetic predisposition to addiction; environment and psychological make-up are other factors. Although physical addiction always has a psychological element, not all psychological dependence is accompanied by physical dependence. A carefully controlled withdrawal programme can reverse the chemical changes of habituation. A cure is difficult because of the many other factors contributing to addiction. Research Addiction
The bronchial arterioles and venules supply blood to the alveolar sacs for regeneration and carry the regenerated blood back to the heart, respectively. The arterioles branch off of the pulmonary artery, which originates at the heart. These arterioles lead to smaller vessels called metarterioles which, in turn, lead to tiny capillaries in the alveolar tissue. The semipermeable membrane of the capillary wall allows oxygen to transport from the air to the blood cells (binding to the hemoglobin in blood), while allowing excess carbon dioxide and other waste gases to transport from the blood to the air to be exhaled. The capillaries then carry the blood cells to larger vessels, called metavenules, which lead to venules and then to the pulmonary vein. The pulmonary vein returns this regenerated blood to the heart to be pumped throughout the body. It is worthwhile to note that, in most graphic representations, as in the body itself, oxygen-poor blood is blue or dark purple, while oxygen-rich blood is bright red. In the lungs, however, the reverse is true. Blood passing through the pulmonary artery and arterioles is oxygen-poor, while the blood passing back to the heart through the pulmonary vein and venules is oxygen-rich. Research Bronchial Arterioles
The eye is the organ of vision of animals, consisting in man of the globe of the eye, the muscles which move it, and of its appendages, which are the eyelids and eyebrows, and the lachrymal apparatus. The walls of the globe of the eye are formed principally of two fibrous membranes; one white and opaque - the sclerotic (from the Greek skleros meaning hard) - which envelops two-thirds of the globe posteriorly; and the other transparent, and resembling a horny plate, whence its name, cornea (from the Latin. corneus, meaning horny). The sclerotic is a tough fibrous coat, and is the part to which the phrase 'white of the eye' is applied. In the front of the globe the sclerotic is abruptly transformed into the transparent portion (the cornea), which is circular, and which forms a window through which one can see into the interior.
A mucousmembrane, the conjunctiva, so named because it unites the eye to the lid, spreads over the anterior portion of the globe, and then folds back on itself and lines the internal surface of the, eyelids. On the internal surface of the sclerotic is a vascular membrane called the choroid. This is essentially the blood-vessel coat of the eyeball. The front part of the choroid terminates about the place where the sclerotic passes into the cornea in a series of ridges, the ciliary processes. The circular space thus left in front by the termination of the choroid is occupied by the iris, a round curtain, the structure seen through the cornea, differently coloured in different individuals. In its centre is a round hole, the pupil, which appears as if it were a black spot. The iris forms a sort of transverse partition dividing the cavity of the eyeball into two chambers, a small anterior chamber filled with the aqueous humour, and a large posterior chamber filled with vitreous humour. The iris consists of a framework of connective tissue, and its posterior surface is lined by cells containing pigment which gives the colour to the eye. In its substance are bundles of involuntary muscular fibres, one set being arranged in a ring round the margin of the pupil, the other set radiating from the pupil like the spokes of a wheel. In a bright light the circular fibres contract and the pupil is made smaller; but in the dark these fibres relax and cause the pupil to dilate more or less widely, thus allowing only that quantity of luminous rays to enter the eye which is necessary to vision.
Just behind the pupil is the crystalline lens, resembling a small, very strongly magnifying glass, convex on each side, though more so behind. The greater or less convexity of the surfaces of the lens determines whether the vision is long or short. The internal surface of the choroid, or rather the pigmentary layer which covers it, is lined by the retina or nervous tunic upon which the objects are depicted that we see.
The ocular globe is put in motion in the orbit by six muscles, grouped two by two, which raise or lower the eye, turn it inward or outward, or on its antero-posterior axis. In these movements the centre of the globe is immovable, and the eye moves round its transverse and vertical diameters. These three orders of movements are independent of each other, and may be made singly or in combination, in such a manner as to direct the pupil towards all points of the circumference of the orbit.
Each eye is furnished with two eyelids, moved by muscles, which shield it from too much light and keep it from being injured. They are fringed with short fine hairs called eyelashes; and along the edge of the lids is a row of glands similar to the sebaceous glands of the skin. The eyebrows, ridges of thickened integument and muscle, situated on the upper circumference of the orbit and covered with short hairs, also regulate to some extent the admission of light by muscular contraction. In reptiles, some fishes (sharks, etc), in birds, and in some mammals a third eyelid or nictitating membrane is present, and can be drawn over the surface of the eye so as to clear it of foreign matters, and also to modify the light.
The lachrymal apparatus is composed of, firstly, the lachrymal gland, which lies in a depression of the orbitalarch; secondly, of the lachrymal canals, by which the tears are poured out upon the conjunctiva a little above the border of the upper lid; thirdly, the lachrymal ducts, which are destined to receive the tears after they have bathed the eye, and of which the orifices or lachrymal points are seen near the internal commissure of the lids; fourthly, the lachrymal sac, in which the lachrymal ducts terminate, and which empties the tears into the nasalcanal.
The tears, by running over the surface of the conjunctiva, render it supple and facilitate the movements of the globe and eyelids by lessening the friction. The influence of moral or physical causes increases their secretion, and when the lachrymal ducts do not suffice to carry them off they run over the lids.
The retina renders the eye sensible of light, and we may therefore consider it as the essential organ of vision. The function of the other portions is to converge the luminous rays to a focus on the surface of the retina, a condition necessary for distinct vision and the clear perception of objects. The visual impressions are transmitted from the retina to the brain by means of the optic nerve. The two optic nerves converge from the base of the orbit toward the centre of the base of the skull, where there is an interlacement of their fibres in such a manner that a portion of the right nerve goes to the left side of the brain, and a part of the left nerve to the right side; this is called the chiasma or commissure of the optic nerves. The principal advantage of having two eyes is in the estimation of distance and the perception of relief. In order to see a point as single by two eyes we must make its two images fall on corresponding points of the retinas; and this implies a greater or less convergence of the optic axes according as the object is nearer or more remote.
According to one estimate, four-fifths of everything we know reaches the brain through our eyes. The eyes transmit constant streams of images to the brain by electrical signals. The eyes receive information from light rays. The light rays are either absorbed or reflected. Objects that absorb all of the light rays appear black, whereas those that reflect all the light rays appear white. coloured objects absorb certain parts of the light spectrum and reflect others. When you look at something, the light rays reflected from the object enter the eye. The light is refracted by the cornea and passes through the watery aqueous humor and pupil to the lens. The iris controls the amount of light entering the eye. Then the lens focuses the light through the vitreous humor onto the retina, forming an image in reverse and upsidedown. Light- sensitive cells in the retina transmit the image to the brain by electrical signals. The brain perceives the image the right side up.
To accommodate the eye to different distances the lens is capable of altering itself with great precision and rapidity. When we look at a near object the anterior surface of the lens bulges forward, becoming more convex the nearer the object; the more distant the object the more the lens is flattened. When the transparency of the cornea, the crystalline lens, or any of the humours, is destroyed, either partially or entirely, then will partial or total blindness follow, since no image can be formed, upon, the retina; but although all the humours and the cornea be perfectly transparent, and retain their proper forms, which is likewise necessary to distinct vision, yet, from weakness or inactivity of the optic nerve, or injury of the central ganglia with which it is connected, weakness of sight or total blindness may ensue. Defective vision may also arise from the crystalline lens being so convex as to form an image before the rays reach the retina (a defect known as short sight or myopia), in which case distinct vision will be procured by interposing a concave lens between the eye and the object of such a curvature as shall cause the rays that pass through the crystalline lens to meet on the retina; or the lens may be too flat, as is the case in old age, a defect which is corrected by convex lenses.
In the lower forms of life the organs of sight appear as mere pigment spots. Ascending higher, simple lenses or refracting bodies occur. Insects, crustaceans, etc, have large masses of simple eyes or ocelli aggregated together to form compound eyes - the separate facets or lenses being optically distinct, and sometimes numbering many thousands. In the molluscs well-developed eyes approaching in structure those of the highest animals are found; and in all vertebrate animals the organ of vision corresponds generally to what has been described, though they vary much in structure and adaptation to the surroundings of the animal. Research Eye
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Calico-Printing
