prevent minifters from encroaching on the liberties of the people, will be readily granted; but when the fame individuals violently oppofe meafures when out of place, which they themselves zealously fupported when in power, they degrade the character of oppofition, and render themselves and their caufe contemptible.

* To OPPRESS. v. a. [oppreffus, Lat.] 1. To crush by hardship or unreasonable severity.Ifrael and Judah were oppreffed together. Jer.

Alas! a morta: moft oppreft of those Whom fate has loaded with a weight of woes.

Pope.

2. To overpower; to subdue.— Nature, being oppreft, commands the mind To fuffer with the body, Shak. The fun, opprefs'd, is plung'd in thickeft gloom. Thomson. * OPPRESSION. n. f. [oppression, Fr. from opprefs. 1. The act of oppreffing; cruelty; feverity.If thou feeft the oppreffions of the poor, marvel not at the matter, for he that is higher than the highest regardeth. Ecclef. 2. The ftate of being oppreffed; mifery.

Need and oppreffion flare within thine eyes.

Shak. Cæfar himself has work, and our oppreffion Exceeds what we expected. Shak. 3. Hardship; calamity.-When we fee any under particular oppreffion, we fhould look upon it as the common lot of human nature. Addison. 4. Dullness of spirits; laffitude of body.-Drowfinefs, oppreffion, heavinefs, and laffitude, are figns of a too plentiful meal. Arbuthnot.

* OPPRESSIVE. adj. [from oppress.] 1. Cruel; inhuman; unjustly exactious or severe. 2. Heavy; overwhelming.

Help me to fupport that feeble frame, That nodding totters with oppressive woe, And finks beneath, its load.

Rowe.

To eafe the foul of one oppreffive weight, This quits an empire that embroils a state. Pope. * OPPRESSOR. n. S. [oppresseur, Fr. from opprefs.] One who haraffes others with unreasonable or unjuft feverity.

I from oppressors did the poor defend. Sandys. The cries of orphans, and th' oppreffor's rage, Had reach'd the stars.. Dryden. -Power when employed to relieve the oppreffed, and to punish the oppreffor, becomes a great bleffing. Swift.

* OPPROBRIOUS. adj. [from opprobrium, Lat.] 1. Reproachful; difgraceful; caufing infamy; fcurrilous.-Himfelf pronounceth them bleffed, that should for his name's fake be subject to all kinds of ignominy and opprobrious malediction. Hooker. They fee themfelves unjustly afperfed, and vindicate themselves in terms no lefs opprobrious than those by which they are attacked. Addifon. 2. Blafted with infamy.

I will not here defile

Think you, this little pratting York
Was not incenfed by his fubtle mother,
To taunt and scorn you thus opprobriously?

Sbak. * OPPROBRIOUSNESS. n. f. [from opprobrious] Reproachfulness; fcurrility. OPPS, a town of Pennsylvania, in Northampton county: 6 miles SE. of Bethlehem.

* To OPPUGN. v. a. [oppugno, Lat.] To oppofe; to attack; to refift.-Ye be by no neceffity bound to oppugn them. Hooker. They said the manner of their impeachment, they could not but conceive, did oppugn the rights of parliament. Clarendon.

Nothing can oppugn his love. Hudibras. -The ingredients reclude oppilations, mundify the blood, and oppugn putrefaction. Harvey. OPPUGNANCY. n. S. [from oppugn.] Op

*

pofition.

Hark what difcord follows, each thing meets In mere oppugnancy.

* OPPUGNER. n. [from oppugn. One who opposes or attacks.-The modern and degenerate Jews be, upon the score of being the great patrons of men's free will, not causelessly esteemed the great oppugners of God's free grace. Boyle.

OPPURG, a town of Upper Saxony, in Neuftadt; miles WSW. of Neuftadt, and 9 W. of Auma.

OPS, in mythology, the daughter of Cœlus and Terra, the fifter and wife of Saturn, and mother of Jupiter, Neptune, and Piuto, &c. She is alfo called CYBELE, RHEA, BONA DFA, MAGNA MATER, TELLUS, THYA, &c. (See thefe articles.) Lempriere confounds her with June, Proferpina, and even Minerva; but the mythology of the ancient Greeks and Romans is a fufficient mafs of confufion and inconfiftency, without confounding the mother with her daughters and grand-daughters. Her festivals were the OPALIA.

OPSA, a town of Lithuania, in Wilna. OPSAL, a town of Norway, in Aggerhuys. * OPSIMATHY. n. S. [ofiμadia.] Late educacation; late erudition.

OPSIOKOVA, a town of Ruffia, in Novogorod, on the Tchagoda; 80 miles NNE of Novogorod.

OPSO, a town of Maritime Austria, in Istria; 7 miles ENE, of Capo d'Iftria.

* OPSONATION. n. f. [opfonatio, Lat.] Catering; a buying of provifions. Di&.

OPSOPUS. See OBSOPÆUS, N° 1 and 2. * OPTABLE. adj. [optabilis, Lat.] Defirable; to be wished.

(1.) OPTATIVE. adj. [optativus, Lat.] Expreffive of defire. [In grammar.] The verb undergoes in Greek a different formation to fignify wishing, which is called the optative mood. Clarke.

(2.) The OPTATIVE MOOD, in the Greek grammar, is that which ferves to exprefs an ardent defire or wish for fomething. In most languages, except the Greek, the optative is only expreffed by prefixing to the fubjunctive an adverb of withing, as utinam, in Latin; plut à Dieu, in French; and would to God, in English..

OPTATUS, Bp. of Melevia, a town of Numidia, in Africa, flourished in the 4th century, under

der Valentinian and Valens. He wrote a book on the Schifm of the Donatifts, about A. D. 370, againft Parmenian, Bp. of that fect, which was published by Du Pin at Paris, in fol. 1700. He also wrote The Sacred Geography of Africa. He died A. D. 384. OPTICAL. adj. [xx] Relating to the fcience of opticks.-It feems not agreeable to what anatomifts and optical writers deliver, touching the relation of the two eyes to each other. Boyle. (1.) OPTIC ANGLE, the angle which the optic axes of both eyes make with one another, as they tend to meet at fome distance before the eyes.

(2.) OPTIC AXIS, the axis of the eye, or a line going through the middle of the pupil and the centre of the eye..

* OPTICIAN. 8. f. [from optick.] One skilled in opticks.

(1.) OPTICK. adj. [oxlix; optique, Fr.] 1. Vifual; producing vifion; fubfervient to vifion.May not the harmony and difcord of colours arise from the proportions of the vibrations, propa

DEFINITIONS.

gated through the fibres of the optick nerves into the brain, as the harmony and difcord of founds arife from the proportions of the vibrations of the air? Neauton's Opticks. 2 Relating to the fcience of vifion. Where our master handleth the contractions of pillars, we have an optick rule, that the higher they are, the lefs fhould be always their diminution aloft, because the eye itself doth contract all objects, according to the distance. Wotton.

(2.) * OPTICK. n. f. An inftrument of fight; an organ of fight.-Can any thing escape the perfpicacity of eyes which were before light, and in whofe opticks there is no opacity? Brown.Our corporeal eyes we find Dazzle the opticks of our mind.

Denham.

Quickly cold indiff'rence will enfue, When you love's joys through honour's optick view.

Prior.

Say what the use, were finer opticks given T' inspect a mite, not comprehend the heaven? Pope.

to be a vibration of that fluid, or any thing else. We shall therefore proceed to

lows:

OPTICKS. n. J. [oxlxn.] The fcience of the nature and laws of vifion.-No spherical body, of what bignefs foever, illuminates the whole fphere of another, although it illuminate fomething more than half of a leffer, according unto the doctrine of opticks. Brown's Vulgar Errours.-Thofe who defire fatisfaction muft go to the admirable treatise of opticks by Sir Ifaac Newton. Cheyne.

3. OPTICS is by others more accurately defined, That science which treats of the element of light, and the various phenomena of vision.

HISTORY OF OPTICS.

SECT. I. DISCOVERIES concerning LIGHT. 4. The element of light has occupied much of the attention of thinking men ever fince the phenomena of nature have been objects of rational inveftigation. The discoveries that have from time to time been made concerning it, are fo fully inferted under the article LIGHT, that there is little occafion for any addition here. The nature of that fubtile element is indeed very little known as yet, notwithstanding all the endeavours of philofophers; and whatever fide is taken with regard to it, whether we suppose it to confift of an infinity of small particles propagated by a repulfive. power from the luminous body, or whether we fuppose it to confift in the vibrations of a fubtile fluid, there are prodigious difficulties, almoft, if not totally infuperable, which will attend the explanation of its phenomena. See CHEMISTRY, Index. In many parts of this work the identity of light and of the electric fluid is afferted: this, however, doth not in the leaft interfere with the phenomena of optics; all of which are guided by the fame invariable laws, whether we fuppofe light

TION of LIGHT.

5. The ancients, though they made very few optical experiments, nevertheless knew, that when light paffed through mediums of different densities, it did not move forward in a ftraight line, but was bent, or refracted, out of its courfe. This was probably fuggefted to them by the appearance of a ftraight stick partly immersed in water; and we find many questions concerning this and other optical appearances in Ariftotle; to which, however, his anfwers are infignificant. Archimedes is faid to have written a treatise concerning the appearance of a ring or circle under water, and therefore could not have been ignorant of the common phenomena of refraction. But the ancients were not only acquainted with these more ordinary appear ances of refraction, but knew alfo the production of colours by refracted light. SENECA fays, that if the light of the fun fhines through an angular piece of glass, it will show all the colours of the rainbow. These colours, however, he fays, are falfe, fuch as are seen in a pigeon's neck when it changes its pofition; and of the fame nature, he fays, is a fpeculum, which, without having any colour of its own, affumes that of any other body.

6. It appears alfo, that the ancients were not unacquainted with the magnifying power of glass globes filled with water, though they do not feem to have known any thing of the reafon of this power; and the ancient engravers are fuppofed to have made ufe of a glafs globe filled with water to magnify their figures, and thereby to work to more advantage. That the power of transparent bodies of a spherical form in magnifying or burning was not wholly unknown to the ancients, is further probable from certain gems preserved in the cabi

nets

nets of the curious, which are fuppofed to have belonged to the Druids. They are made of rock cryftal, of various forms, amongst which are found fome that are lenticular and others that are fpheri. cal: and though they are not fufficiently wrought, to perform their office as well as they might have done, if they had been more judiciously executed, yet it is hardly poffible that their effect, in magnifying at leaft, could have escaped the-notice of those who had often occafion to handle them; if, indeed, in the spherical or lenticular form, they were not folely intended for the purposes of burning. One of thefe, of the spherical kind, of about an inch and an half diameter, is preserved among the foffils given to the university of Cambridge by Dr Woodward.

7. The first treatife of any note, written on op. tics, was by the celebrated aftronomer CLAUDIUS PTOLOMEUS, who lived about the middle of the 2d century. The treatise is loft; but from the accounts of others we find that he treated of aftronomical refractions. Though refraction in general had been observed very early, it is poffible that it might not have occurred to any philofopher much before his time, that the light of the fun, moon, and ftars, mult undergo a fimilar refraction, in confequence of falling obliquely upon the grofs atmosphere that furrounds the earth; and that they muft thus be turned out of their rectilinear course, so as to cause those luminaries to appear higher in the heavens than they would otherwife do. The firft aftronomers were not aware that the intervals between ftars appear lefs near the horizon than near the meridian; and, on this account, they must have been much embarraffed in their obfervations. But it is evident that Ptolemy was aware of this circumstance, by the caution that he gives to allow fomething for it, upon every recourfe to ancient obfervations.

8. This philofopher alfo advances a very fenfible hypothefis to account for the remarkably greater apparent fize of the fun and moon when feen near the horizon. The mind, he fays, judges of the fize of objects by means of a preconceived idea of their distance from us; and this diftance is fancied to be greater when a number of objects are interpofed between the eye and the body we are view. ing; which is the cafe when we fee the heavenly bodies near the horizon. In his Almageft, how ever, he ascribes this appearance to a refraction of the rays by vapours, which actually enlarge the angle under which the luminaries appear; juft as the angle is enlarged by which an object is feen

from under water.

9. In the 12th century, the nature of refraction was largely confidered by ALHAZEN, an Arabian writer; infomuch that, having made experiments upon it, at the common furface between air and water, air and glafs, water and glass or cryftal; and being prepoffeffed with the ancient opinion of cryftalline orbs in the regions above the atmofphere, he even fufpected a refraction there alfo, and fancied he could prove it by aftronomical obfervations. This author deduces from hence several properties of atmospherical refraction; as that it increases the altitudes of all objects in the heavens; and he first advanced, that the ftars are fometimes feen above the horizon, by means of

refraction, when they are really below it. This obfervation was confirmed by Vitellio, B. Waltherus, and especially by the excellent obfervations of Tycho Brahe. Alhazen obferved, that refraction contracts the vertical diameters and distances of the heavenly bodies, and that it is the cause of the twinkling of the ftars. But we do not find that either he, or his follower Vitellio, knew any of its juft quantity. Indeed it is too fmall to be determined, except by very accurate inftruments; and therefore we hear little more of it till about 1500, when great attention was paid to it by Bernard Walther, Mæftlin, and others, but chiefly by Tycho Brahe.

10. ALHAZEN fupposed that the refraction of the atmosphere did not depend upon the vapours in it, as was probably the opinion of philosophers before his time, but on the different transparency; by which, as Montucla conjectures, he meant the denfity of the grofs air contiguous to the earth, and the æther or fubtile air that lies beyond it. In examining the effects of refraction, he endeavours to prove, that it is fo far from being the cause of the heavenly bodies appearing larger near the horizon, that it would make them appear lefs; two ftars, he fays, appearing nearer together in the horizon than near the meridian. This phenomenon he ranks among optical deceptions. We judge of diftance, he fays, by comparing the angle under which objects appear, with their fuppofed diftance; fo that if these angles be nearly equal, and the diftance of one object be conceived greater than that of the other, it will be imagined to be larger. And the sky near the horizon, he says, is always imagined to be further from us than any other part of the concave furface. Roger Bacon afcribes this account of the horizontal moon to Ptolemy; and as fuch it is examined and objected to by B. Porta.

11. In the writings of this BACON, whose genius perhaps equalled that of his great namefake, Lord Verulam, we find the first distinct account of the magnifying power of glaffes; and it is not improbable, that what he wrote upon this fubject gave rife to that most useful invention of spectacles. For he fays, that if an object be applied clofe to the bafe of the larger fegment of a sphere of glafs, it will appear magnified. He also treats of the appearance of an object through a globe, and fays that he was the first who obferved the refraction of rays into it.

12. In 1270, VITELLIO, a native of Poland, published a treatife of optics, containing all that was valuable in Alhazen, and digefted in a much more intelligible and methodical manner. He obferves, that light is always loft by refraction, in confequence of which the objects feen by refracted light always appear lefs luminous; but he does not attempt to eftimate the quantity of this lofs. He reduced into a table the refult of his experiments on the refractive powers of air, water, and glass, correfponding to different angles of incidence. In his account of the horizontal moon he agrees exactly with Alhazen; obferving, that in the horizon the feems to touch the earth, and appears much more diftant from us than in the zenith, on account of the intermediate space containing a greater variety of objects upon the vifible

furface

farface of the earth. He afcribes the twinkling of the ftars to the motion of the air in which the light is refracted; and to illuftrate this hypothefis, he obferves, that they twinkle ftill more when viewed in water put in motion. He alfo fhows, that refraction is neceffary, as well as reflection, to form the RAINBOW; because the body which the rays fall upon is a tranfparent fubftance, at the furface of which one part of the light is always reflected and another refracted. But he feems to confider refraction as ferving only to condenfe the light, and thereby enabling it to make a ftronger impreffion upon the eye. He alfo makes fome ingenious attempts to explain refraction, or to afcertain the law of it. He alfo confiders the foci of glass spheres, and the apparent fize of objects, feen through them; though upon these subjects he is not at all exact. It is fufficient indeed to fhow the ftate of knowledge, or rather of ignorance, at that time, to obferve, that both Vitellio and his mafter Alhazen endea. vour to account for objects appearing larger when they are feen under water, by the circular figure of its furface; fince, being fluid, it conforms to the figure of the earth.

13. Contemporary with Vitellio was ROGER BACON, a man of very extenfive genius, and who wrote upon almost every branch of science; yet in this branch he does not feem to have made any confiderable advances beyond Alhazen. Even fome of the wildeft and moft abfurd of the opinions of the ancients have had the sanction of his authority. He affents to an opinion adopted by many of the ancients, and most philofophers till his time, that vifual rays proceed from the eye. In his Specula Mathematica, he added fome obfervations on the refraction of the light of the ftars; the apparent fize of objects; the extraordinary fize of the fun and moon in the horizon; but in all this he is not very exact.

14. From this time to that of the revival of learning in Europe, we have no farther treatise on refraction, or on any other part of optics. One of the firft who diftinguished himself in this way was MAUROLYCUS, teacher of mathematics at Meffina. In a treatise, De Lumine et Umbra, published in 1575, he demonftrates that the cryftalline humour of the eye is a lens that collects the rays of light iffuing from the objects, and throws them upon the retina, where is the focus of each pencil. From this principle he discovered the reafon why fome people were short-fighted and others long-fighted; and why the former are relieved by concave, and the others by convex glaffes.

15. About the fame time that Maurolycus made fuch advances towards the discovery of the nature of vifion, JOHN BAPTISTA PORTA of Naples dif covered the camera obfcura, which throws ftill more light on the fame fubject. His house was conftantly reforted to by all the ingenious perfons at Naples, whom he formed into what he called an academy of fecrets; each member being obliged to contribute fomething that was not generally known, and might be useful. Thus he was furnished with materials for his Magia Naturalis, which contains his account of the camera obfcura, and the first edition of which was published when he was not quite 15 years old. He alfo gave the VOL. XVI. PART Í.

firft hint of the magic lantern, which Kircher afterwards followed, and improved. His experiments with the camera obfcura convinced him that vifion is performed by the intromiffion of fomething into the eye, and not by vifual rays proceeding from the eye, as had been formerly imagined; and he was the firft who fully fatisfied himself and others upon this subject. Indeed the refemblance between experiments with the camera obfcura and the manner in which vifion is performed in the eye, was too ftriking to escape the obfervation of a lefs ingenious perfon. But when he fays that the eye is a camera obfcura, and the pupil the hole in the window-fhutter, he was fo far mistaken as to fuppofe that it was the cryftalline humour that corresponds to the wall which receives the images; nor was it discovered till 1604, that this office is performed by the retina. He makes a variety of juft obfervations concerning vifion; and particularly explains feveral cafes in which we imagine things to be without the eye, when the appearances are occafioned by fome affection of the eye itself, or fome motion within the eye. He obferves alfo, that, in certain circumftances, vifion will be aflifted by convex or concave glaffes; and he seems alfo to have made fome fmall advances towards the difcovery of telescopes. He takes notice, that a round and flat surface plunged into water will appear hollow, as well as magnified, to an eye perpendicularly over it; and he very well explains, by a figure, the manner in which it is done.

16. All this time, however, the great problem concerning the measuring of refractions had remained unfolved. Alhazen and Vitellio, indeed, had attempted it; but failed, by attempting to measure the angle itself inftead of its fine. At laft it was discovered by SNELLIUS, profeffor of mathematics at Leyden. This philofopher, however, did not perfectly understand his own discovery, nor did he live to publish any account of it himself. It was afterwards explained by profeffor HORTENSIUS, both publicly and privately, before it appeared in the writings of DESCARTES, who published it under a different form, without making any acknowledgment of his obligations to Snellius, whofe papers, Huygens affures us, from his own knowledge, Defcartes had feen. Before this time Kepler had published a New Table of refracted Angles, determined by his own experiments, for every degree of incidence. Kircher had done the fame, and attempted a rational or phyfical theory of refraction, on principle.

17. DESCARTES undertook to explain the cause of refraction by the refolution of forces, on the principles of mechanics. In confequence of this, he was obliged to suppose that light paffes with more eafe through a dense medium than through a rare one. The truth of this explanation was first queftioned by M. Fermat, counsellor to the parliament of Thouloufe, and an able mathematician. He afferted, contrary to the opinion of Descartes, that light fuffers more refiftance in water than in air, and more in glass than in water; and he maintained, that the refiftance of different mediums with refpect to light is in proportion to their denfities, M. LEIBNITZ adopted re fame general idea; and thefe gentlemen argued upon the Sa

fubject

fubject in a manner which did not afford fatisfaction; and a little time fhowed the fallacy of their different hypothefes.

18. At a meeting of the ROYAL SOCIETY, Aug. 31. 1664, an experiment for measuring the refraction of common water was made with a new inftrument which they had prepared for that purpofe; and, the angle of incidence being 40°, that of refraction was found to be 30. About this time alfo we find the firft mention of mediums not refracting the light in an exact proportion to their denfities. For Mr BOYLE, in a letter to Mr OLDENBURGH, dated Nov. 3. 1664, cbferves, that in fpirit of wine, the proportion of the fines of the angles of incidence to the fines of the angles of refraction was nearly the fame as 4 to 3; and that, as fpirit of wine occafions a greater refraction than common water, fo oil of turpentine, which is lighter than spirit of wine, produces not only a greater refraction than common water, but a much great ́er than falt water. And at a meeting held Nov, 9. the fame year, Dr Hooke brought in an, account of an experiment that he made with pure and clear falad oil, which was found to have pro duced a much greater refraction than any liquor which he had then tried; the angle of refraction that anfwered to an angle of incidence of 30° being found no less than 40° 30', and the angle of refraction that answered to an angle of incidence of 20 being 29° 47′.

19. M. DE LA HIRE alfo made feveral experiments to ascertain the refractive power of oil with respect to that of water and air, and found the fine of the angle of incidence to that of refraction to be as 60 to 42; which, he obferves, is a little nearer to that of glafs than to that of water, though oil is much lighter than water, and glafs much heavier. The members of the Royal Society finding that the refraction of falt water exceeded that of fresh, purfued the experiment farther with folutions of vitriol, faltpetre, and alum, in water; when they found the refraction of the folution of vitriol and faltpetre a little more, but that of alum a little lefs, than common water.

20. Dr HOOKE made an experiment before the Royal Society, Feb. 11. 1663, which clearly proves that ice refracts the light lefs than water; which he took to be a good argument that the lightnefs of ice, which caufes it to fwim in water, is not caufed only by the finall bubbles which are vifible in it, but that it arifes from the uniform conftitution or general texture of the whole mafs. M. de la Hire alfo took a good deal of pains to determine whether, as was then the common opinion, the refractive power of ice and water were the fame; and he found, as Dr Hooke had done be fore, that ice réfracts lefs than water.

21. By a moft accurate and elaborate experiment made in 1698, in which a ray of light was tranfmitted through a Torricellian vacuum, Mr LOWTHORP found, that the refractive power of air is to that of water as 36 to 34,400. He concludes his account of the experiment with obferving, that the refractive power of bodies is not proportioned to the denfity, at least not to the gravity, of the refracting medium. For the refractive power of glafs, to that of water, is as 55 to 34, whereas its gravity is ae 87 to 34; that is, the squares of their refrac

tive powers are very nearly as their respective gravities. And there are fome fluids, which, though they are lighter than water, yet have a greater power of refraction. Thus the refractive power of fpirit of wine, according to Dr Hooke's experiment, is to that of water as 36 to 33, and its gravity reciprocally as 33 to 36, or 364. But the refractive powers of air and water seem to observe the fimple proportion of their gravities directly.

22. CASSINI the younger was prefent when Mr Lowthorp made the above-mentioned experiment before the Royal Society; and upon his return home, having made a report of it to the members of the Royal Academy of Sciences, thofe gentlemen endeavoured to repeat the experiment in 1700; but they did not fucceed.-For, as they faid, beams of light paffed through the vacuum without fuffering any refraction. The Royal So ciety being informed of this, were defirous that it might be put paft difpute, by repeated and wellattefted trials; and ordered Mr HAUKSBEE to make an inftrument for the purpofe, by the direc tion of Dr HALLEY. It confifted of a strong brafs prifm, two fides of which had fockets to receive two plane glaffes, whereby the air in the prifm might either be exhaufted or condensed, The prifm had alfo a mercurial gage fixed to it, to difcover the density of the contained air; and was contrived to turn upon its axis, in order to make the refractions equal on each fide when it was fixed to the end of a telefcope. The refracting angle was near 64°; and the length of the telescope was about 10 feet, having a fine hair in its focus. The event was as follows:

23. Having chosen a proper and very distinct erect object, whofe diftance was 2588 feet, June 15. O. S. 1708, in the morning, the barometer being then at 29.74, and the thermometer at 60, they firft exhaufted the prifm, and then applying it to the telescope, the horizontal hair in the focus covered a mark on the object diftin&ly seen through the vacuum, the two glaffes being equally inclined to the vifual ray. Then admitting the air into the prifm, the object was feen to rife above the hair gradually as the air entered, and in the end the hair was obferved to hide a mark 101 inches below the former mark. This they often repeated, and with the fame fuccefs. After this, they applied the condenfing engine to the prifm; and having forced in another atmosphere, fo that the denfity of the included air was double to that of the outward, they again placed it before the telefcope, and, letting out the air, the object, which before feemed to rife, appeared gradually to defcend, and the hair at length refted on an object higher than before by the fame interval of 10 inches. This experiment they likewife frequently repeated without any variation in the event. They then forced in another atmosphere; and upon difcharging the condensed air, the ob ject was feen near 21 inches lower than before.

24. Now the radius in this cafe being 2588 feet, 104 inches will fubtend an angle of one minute and 8 feconds, and the angle of incidence of the vifual ray being 32° (becaufe the angle of the glafs planes was 64), it follows from the known laws of refraction, that as the fine of 39° is to that of 31°, 59′, 26" differing from 32° by 34" the half