Page:Cyclopaedia, Chambers - Volume 2.djvu/626

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and to do the fame Office to the Retina which the Quickfilver docs to a Looking-Glafs ; efpecially in Animals wherein it is black. See Choroides.

Buc from an Experiment of a Cat plunged in Water, M. Me- ry conceiv'd a different Opinion.— He obfervM the Retina to dif- abfolutely appear on that Occafion, as well as all the other Hu- mours of the Eye ; but the Choroides itill to appear diftinfily, and even with all the lively Colours it has in that Animal.— Hence he concluded, that the Retina was as tranfparent as the Humors, but the Choroides, opake : Confequenrly the Retina was not a proper Inftrument to terminate and (top the Cones of Rays, or to receive the Images of Objefls; but that the Light mult pafs through it, and could only be ftop'd on the Choroides; which therefore would become the principal Organ of Vifion.

The black Colour of the Choroides in Man is extremely fa- vourable to this Sentiment: The principal Organ mould feem to require that the Action of the Light Ihould terminate on it, as it arrives ; which 'tis certain it here does in the black that abforbs all the Rays, and reflects none ; and it mould alfo feem neceffa- ry that the Action of the Light Ihould be ftronger on the Or- gan of Sight than any where elfc : Now, 'tis certain that the Light being received and ablbrbed in a black Body, muft excite a grea- ter Vibration there than any where elfe; and hence it is that black Bodies are inflamed by a burning Glafs much fooner than white ones. See Blackness, &c.

The Situation of the Choroides behind the Retina is another Circumftance on its Side; M. Mery having obferved the fame Pofition of the principal Organ behind a mediate Organ in the o- ther Senfes ; which makes a happy Analogy.— Thus the Cuticle extended over the Skin, is the mean Organ of feeling ; but the Cutis underneath is the principal Organ. The like is obferved in the Ear, Nofc, drc.

The Retina, therefore, (liould feem, a kind of mediate or fe- condary Organ, ferving to break the too ftrong Impreffion of the Light on the Choroides, or to preferve it ; which is the ufe afcribed to the Cuticle.— Add to all this, that the Retina is infen- fible, as having its Origin from the medullary Subftance of the Brain, which is fo too; and the Choroides, on the contrary, ve- ry feniible, as ariling from the Via Mater, which is certainly fo in a great Degree. See Nerve, Medulla, Meninx.

This iaft Argument being doubted of, M. Mery was engaged to prove it ; which he did before the Royal Academy, where he fhew'd that the Optic Nerve is not compoled like the other Nerves, of Fibres; that 'tis only a Train of the Medulla inclofed in a Canal, out of which it is eafily fc parable. See Optic Nerve.

This Structure of the Optic Nerve, hitherto unknown, fhews that the Retina can be no Membrane ; 'tis only a Dilatation of the Medulla, inclofed under two Membranes, and a Medulla is no proper Subftance to be the Seat of a Senfation.— It can fcarce ferve for any thing but to filtrate the Spirits neceffary for the Adion of Vifion. — The Vibration whereby the Senfation it felf is ef- fected, muft be made on a Part more folid, more firm, and more fufceptiblc of a brisk Impreffion.

RETINUE, the Attendants or Followers of a Prince, or Per- fon of Quality ; chiefly in a Journey.

In Law, thofe Perfons are faid to be of a Nobleman's Retinue who belong to him in Quality either of Servants or Retainers. See Retainer.

RETIRADE, in Fortification, a kind of Retrenchment made in the Body of a Baftion, or other Work, which is to be difpu- ted Inch by Inch, after the firft Defences are difmantled. See Retrenchment, &c.

It ufually confifts of two Faces, which make a re-entering An- gle. See Re-entering.

When a Breach is made in a Baftion, the Enemy may alfo make a Retirade, or a new Fortification behind it. See Bas-

Ition. RETIRED Flank, in Fortification. See Flank.

RETORT, in Chymillry, a kind of crooked Matrafs, or a round, bellied Veflel, either of Earth or Glafs, with a llender crooked Beak, or Nofe, to which the Recipient is to be fatten- ed. See Matrass.

When the Retort is of Glafs, 'tis ufually lin'd with a Lute or Paft an Inch thick; to enable it to bear the Fire the better. See Fire and Heat.

The Retort ferves to draw Spirits and Oils from Woods, Gums, Minerals, Earths, and other Matters which require a ftrong Fite. See Spirit, <&c.

The Retort is a kind of Compendium or Improvement of the Cucurbit and Bok-head; anfwering all the Purpofes of both, with- out the Affiftance of a Capital or Head, which che other frequent- ly require. See Distillation, &c.

RETRACTATION, the Act of unfaying whataPerfon had faid, or wrote. See Recantation.

Gal/ileo made a public Retaliation of hisDoBrine of the World, tie Mmido, after its being cenfured and condemned by the Popes.

Among St. Augufliu's Works is a Book of Retractations ; where, however, the Word is to be underftood in a new Senfe; not as if he recanted or unfaid any thing he had taught, but only treated of the fame Matter, or handled the fame Subject: a fe- cond Time. This Senfe the Word will very well bear; being a compound of re and trails, I handle, treat of.

RETRACTION, in Anatomy, the Contraftion or morteir ing of a Part. See Contraction.

The RetraOion of the Nerves takes away the Ufe of the Limbs. See Nerve.

The Word is form'd from the Latin, retrahere, to draw back. >

RETRACTS, among Horfemen, pricks in the Horfe's Feet, ariling from the Fault of the Farrier in driving Nails that are weak, ill-pointed, or are driven amifs. See Shoe.

Thefe, unlefs timely prevented, fetter, and prove very dan- gerous.— When the Farrier, in mooing, perceives the Horfe to fhrink at every blow on the Nail ; 'tis a fign of a Retratl, and the Nail is to be pull'd out again, which is done without any harm.

When the Horfe halts immediately after he is mod, 'tis con- cluded fome of the Nails prefs the Veins, or touch him in the Quick.

To find where the Grievance lies, they knock the Nails round with 4 Hammer, till the Horfe's Ihrinkine upon hitting a particu- lar Nail, difcovers the Place.

Some Farriers give this as a Rule, that throwing Water on the Hoot, the Place where he is hurt will be dry lboner tlren any of the reft. The Places where the Horfes are moll ufually pri.k'd, are the Heel in the Fore-foot, and the Toe in the Hind-foot. See Hoof.

RETRACTOR, Ala Nafi, in Anatomy, a pair of Mufcles, call'd alfo Elevator Labit fuferioris. Sej Elevator.

RETRAHENS Auricula™, in Anatomy, a pair of Mufcles of the external Ear; confiding of a Parcel of fielhy Fibres, which in fome Bodies are divided into three diftinft Mufcles, a- rifing from the Or Temfor.de, and fix'd to the hind Part of the Concha. See Auricle.

But theie Mufcles are fo finall in Men, that the Auricle is fel- dom moveable at all. Sec Ear.

RETRAXIT, in Law, is where the Plaintiff comes into Court in Peribn, alone, or with the Defendant; and declares he will proceed no further.

A Retraxit is peremptory, and a perpetual Bar; and may be pleaded as fuch to the Plaintiff in the fame Action for ever.

RETREAT, in War, a retiring or moving back again of an Army, or Part thereof.

What they call a Retreat in the Armies is really a Flight ; on- ly a Flight made by Defign, and with Conduft.

The Skill and Ability of the General, is known by his Retreats, more than his Engagements. — The Retreat of ten '.houfand Greeks under the Command of Xenophon, has been admired in ail An- tiquity. To found a Retreat, fecure a Retreat, &c. Retreat, or Relay, in Mafonry, a little Rc-cefs or Dimi- nution of the thicknefs of a Wall, Rampart, &c. in Proportion as 'tis rais'd. See Wall, ef c

The Retreat is properly the Diminution of a Wall, without Side ; or the Contraction of its upper Courfes more than the Foundation.— Where the Foundation is very large, they ufually make two or three Retreats.— Parapets are always built with Retreats.

Retrenchment, litrerally fignifies fomething cut off a thing, and taken from it ; in which Senfe it coincides with Subltradti- on. Diminution, e^r.— The Word is French, Relranchement* form'd of re and trancher, to cut.

RETRENCHMENT, in Architeflurc, Carpentry, &c. is ufed not only for what is cut off from a Piece when too large, in order to a better proportioning it, or fome oiher Convenience ; but alfo for the Prefectures taken out of Streets, publick Ways, &c. to render them more even, and in a Line.

By a gradual Retrenchment of the ordinary Quantity of Food, a Man may bring himfelf to a great Degree of Abitinence. See Abstinence, Fasting, Food, &c.

The Reformation of the Calendar in 1582, occafioned a Re- trenchment of ten Days which had crept into the Account more than there ihould have been. See Calendar.

The Frugality fo much boafted of among the antient Romans, St. Evemnond obferves, did not fo much confift in a voluntary Abitinence or Retrenchment of things fuperfluous, as a coarfe and fordid Way of employing or ufing them.

Retrenchment, in War, the Fortification of a Camp ; or any Kind of Work cait up to ftrengthen or defend a Poft againft the Enemy. See Fortification, Defence, Work.

Such are Ditches, with Parapets, Gabions, Fafcines, &c. for a covering, ebc. See Ditch, grc.

The Enemy came with Defign to oblige them to raife the Siege, but could not force the retrenchments.

Retrenchment is particularly ufed for a fimple Retirade made on a Horn-work or Baftion; when 'tis intended to Difpute the Ground Inch by Inch.

'Tis ufually a re-entering Angle, whole Faces Flank each other; and fortified with Ditches, Parapets, Gabions, ire. See Re- tirade.

RETRIBUTION, a handfome Prcfent, Gratuity, or Ac- knowledgment, given in lieu of a format Saiaiy or Hire, to Per- fons imploy'd in Affairs that do not fo immediately fall under Eftimation, nor wichin the common Commerce/of Money.

Thofe