Page:The New International Encyclopædia 1st ed. v. 12.djvu/264

* LIGATUBE. 238 LIGHT. their circulation, niid for bringing about a cessa- tion of l)loiHl-llnv tlirongli an aneurism. The usefulness of the ligature deiJends upon the fact that when the internal coat of an artery is torn oil' ur injured, the hlood clots, the lluidity of the bh)oil (K']iending upon the integrity of this coat. Ligatures are llierefore ajiplied wilh suHicient force to rupture the internal and niid- <lle coats and constrict lirnily the outer eiiat of the artery. Clotting takes place as far as the nearest branch, and this portion of the vessel is ultimately converted into a fibrous cord. In- jured arteries of any size are tied on both sides of the woiuul to prevent possible recurrent hem- orrhage from coiuiecting branches. When the injured point is not accessible the ligature is sometimes ])Iaced at a distance from the wound. In the treatment of aneurisms, the ligature is placed upon the artery, when jjossible, between the heart and the sac, and at some distance from the aneurism, for the reason that the artery in the vicinity of the aneurism is too diseased to resist being cut through by a ligature. This operation was devised by Hunter. Brasdor aft- erwards conceived the idea of tying on the distal .side of the aneurism; a barrier to the blood- current is just as cllectively formed, collateral branches above the aneurism enlarge, the blood is diverted and a clot forms in the aneurismal sac. Another device is that of Wardrop, which consists in tying one of the branches of the arlcry below the aneurism. The ini|)ortant operations in arterial ligature are those involving the sul)clavian. innominate, carotid, and iliac arteries, and the abdominal aorta. The latter vessel has been tied on nine or ten occasions and the operation has always re- sulted" fatally. It was first performed by .Sir Astley Cooper in 1817, the patient surviving forty-eight hours; the latest case, reported (1002) by Keen, an American surgeon, survived forty-eight days. These operations are done only in eases of ilesperate need, cither in the event of an aneurism about to rn]iture or alarming hemorrhage from penci rating wounds. See Amurism ; CinciTLATXOx; Brasdou's Operation. LIGATURE. In music, a slur or tie. hence a term which indicates that a group or series of notes is to be executed coherently; if in vocal music, that they are to be sung with one breath. The term is used in instrumental nuisic to mark the phrasing. In mensurable music ligature denotes a group of notes that are to be executed on one syllable. See Mensurable ^Ir.sir. LIGHT (AS. Iroht. Goth, luihnp, OHG. lioht, Oct. Lirht : connected with Lat. lucere, to be bright. Gk. Xei/itAj, Iriilxnn, bright, Ir. Inrjic, lightning, OChurch Slav, lurhn. beam of light, Skt. rue, to shine). The sensation of which one becomes conscious through the optic nerves. In pencral the cause of this sensation is the en- trance into the eye of ether-waves whose wave- Tuunbcrs lie between certain limits; and fhere- fiue such unscientific expressions as 'light enters the eye.' 'light-waves,' etc.. are often used. From the standpoint of physics one is concerned with the problem of the production of these ether- waves, their propagation and their entrance into the eye; after reaching the retina their further action becomes the study of physiology. .^ny proper physical explanation of light must show that all the phenomena of ether-waves both in the pure ether and inside material bodies may be deduced from certain dynamical equa- tions; and with our present knowledge of matter and the ether this can be done with a fair degree of satisfaction. There are, however, many phenomena which do not require f<u' the ex- planation of their main features any exact knowledge of the properties of ether and matter, but which may be referred to certain general mechanical principles. Thus, the most important J phenomena of interference and dill'raetion may ' be explained by the general ])rinciplcs of wave- motion; and the most .striking features of lenses, mirrors, and juisms may be shown to follow from the treatment of 'rays of light,' assuming the laws for rellection and refraction of rays. That portion of the subject of liglit which may be explained by the use of the idea of rays is called 'Gebmetrieal Optics;' that portion which uses the idea of wave-motion is called 'Physical 0])tics;' and the complete treatment which explains the laws of rays and of waves and connects all the phenomena of dispersion, rellection. magnetic rotation, etc., by om- set of dynamical equations is called the 'Dynamical Theory of Light.' HISTOHICAL SKETCH. Geometrical optics is not entirely a modern science, for the Greeks and their disciples the i Arabs were acquainted with the law of rellection j and its more innnediati^ consequences. Aristotle. Euclid, Archimedes, Hero, and Ptolemy knew that light was transmitted in straight lines; but with the important exception of Aristotle, and some of his followers, the ancient philosophers believed that rays proceeded from the eye to the object, instead of in the contrary direction. Ptolcmj' was well acqiiainted with atmospheric refraction. Alhazi'n (early part of eleventli century) and Vitellio the Pole (1200) were almost the only students of this science duriiiL' the Middle Ages, and their additions to it were unimportant. The lens, though known from early antiquity, was not applied as an aid to defective eyesight till after the time of Roger Bacon. Jansen, Jletius, and Galileo separately invented the telescope (q.v.) about the beginning of the seventeenth century: and with it the last-mentioned philosopher (see Galileo) made various important astronomical discoveries. Kepler, shortly after, gave the true theory of the telescope, explained the method of finding the focal length of lenses, and applied it to iind the magnifying power of the telescope, besides pointing out the mode of constructing an instrument better adapted for astronomical purposes than that of (Jalileo; he also made some useful experiments on the nature of colors, and showed that images formed on (lie retina of the eye are inverted, a fact previously discov- ered by Maurolycus of Messina. From this period the science of optics steadily advanced through the labors of De Dominis, Snell (the dis- coverer of the law of refraction in 1021). Des- cartes. Fermat, Barrow. Mariotte, and Boyle. Up to the time of Xewton it was generally be- lieved that color was produced by refraction, but he showed by a beautiful series of experiments that refraction only separates the colors already existing in white light. Tn his hands the theory and construction of the telescope underwent many valuable improvements, and in 1072 the description of his rclleeting telescope was sub-