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to fact in the moft common and fimple cases. In rays ODM, Odm, which are one higher than thie microscopes, he says, it is impossible that the eye other, in their progress to the eye;

whereas the fhould judge the object to be nearer than the dif- image at E is made by the rays ODM, Oef, tance at which it has viewed the object itfelf, in which enter the eye laterally. This, says he, proportion to the degree of magnifying. For may serve to explain the difficulty of F. Tacquet, when the microscope magnifies much, this rule Barrow, Smith, and many other authors, and would place the image at a distance, of which which Newton himself confidered as a very diffithe fight cannot possibly form any opinion, as be. cult problem, though it might not be abfolutely ing an interval from the eye at which no object insoluble. can be seen. In general, he believes, that who. 484. G. W. Kraft has ably supported the ever looks at an object through a convex glass, opinion of Dr Barrow, that the place of any point, and then at the obješt itself without the glass, will seen by reflection from the surface of any medium, find it to appear nearer in the latter cafe, though is that in which rays issuing from it, infinitely near it be magnified in the glass; and in the fame trial to one another, would meet; and confidering the with the concave glass, though by the glass the case of a distant object viewed in a concave mir. object be diminished, it will appear nearer through ror, by an eye very near to it, when the image, the glass than without it. But the moft convin- according to Euclid and other writers, would be cing proof, that the apparent distance of the image between the eye and the object, and the rule of is not determined by its appareat magnitude, is Dr Barrow cannot be applied; he says that in this the following experimente If a double convex case the speculum may be considered as a plane, glass be held upright before fome luminous object, the effe&t being the same, only the image is more as a candle, there will be seen two images, one obscure. erect, and the other inverted. The firft is made 485. Dr PORTERFIELD gives a distinct and comfimply by refledion from the neareft surface, the prehenfive view of the natural methods of judging fecond by reflection from the farther furface, the concerning the distance of objects. The conforrays undergoing a refraction from the first surface mation of the eye, he says, can be of no use to both before and after the reflection. If this glass us with respect to objects that are placed without has not too short a focal distance when it is held the limits of diftinct vision. As the object, how. near the object, the inverted image will appear ever, does then appear more or less confused, aclarger than the other, and also nearer ; but if the cording as it is more or less removed from those glafs be carried off from the object, though the limits, this confufion aflists the mind in judging eye remain as near to it as before, the inverted of the distance of the object; it being always el. image will diminih so much faster than the other, teemed so much the nearer, or the farther off, by that, at length, it will appear very much less than how much the confusion is less or greater. But it, but ftill nearer. Here, says Mr Robins, two this confufion bath its limits also, beyond which images of the fame object are seen under one view, it can never extend; for when an obje&t is placed and their apparent ditances immediately compa. at a certain distance from the eye, to which the red; and here, it is evident, that those distances breadth of the pupil bears no fenfible proportion, have no necessary connection with the apparent the rays of light that come from a point in the obmagnitude. He also shows how this experiment ject, and pass the pupil, are so little diverging, that may be made ftill more convincing, by fticking a they may be considered as parallel. For a picture piece of paper on the

middle of the lens and view- on the retina will not be fenfibly more confused, ing it through a short tube.

though the object be removed to a much greater 483. M. BOUGUER adopts the general maxim of distance. The most universal, and the most fure Dr BARROW, in suppofing that we refer objects means of judging of the distance of objects is, be to the place from which the pencils of rays seem- says, the angle made by the optic axis. For our ingly converge at their entrance into the pupil. two eyes are like two different stations, by the But when rays ifsue from below the surface of a affiftance of which distances are taken: and this is veffel of water or any other refracting medium, the reason why those persons who are blind of one he finds that there are always two different plaeye fo frequently miss their mark in pouring ces of this seeming convergence; one of them of liquor into a glass, snuffing a candle, and such the rays that issue from it in the same vertical cir. other actions, as require that the distance be excle, and therefore fall with different degrees of actly diftinguished. To prove the usefulness of obliquity upon the surface of the refracting me. this method of judging of the distance of objects, dium; and another, of those that fall upon the fur- he directs to fuspend a ring in a thread, so that its face with the fame degree of obliquity, entering the fide may be towards us, and the hole in it to the eye laterally with respect to one another. Some. right and left hand; and taking a small rod, crooktimes, he says, one of these images is attended to by ed at the end, retire from the ring 2 or 3 paces, the mind, and sometimes the other, and different and having with one hand covered one of our eyes, images may be observed by different persons. An to endeavour with the other to pass the crooked object plunged in water, affords an example, he end of the rod through the ring. This, says he, says, of this duplicity of images. If BAb (Plate appears very easy; and yet, upon trial, perhaps CCLVII, fig. 5.). be part of the surface of water, we shall not fucceed once in 100 times, especially and the object be at 0, there will be two images if we move the rod a little quickly. The use of of it in two different places ; one at G, on the this second method of judging of diftances De caustiç by refraction, and the other at E, in the Chales limited to 120 feet; beyond which, he says, perpendicular AO, which is as much a caustic as we are not sensible of any differenee in the angle ibe other line. The former image is visible by the of the optic axis.

486. A 3d method of judging of the diftance of apparent distance is diminished by the parts thas objects, confifts in their apparent magnitudes, on do not appear in it. This is the reason that the which so much dress was laid by Dr Smith. banks of a river appear contiguous to a diftans From this change in the magnitude of the image eye, when the river is low and not seen. For the upon the retina, we easily judge of the distance fame reason a large lake, or an arm of the fear of objects, as often as we are otherwise acquaint- such as the Frith of Forth, appears to be much ed with the magnitude of the objects themselves ; narrower, than an equal extent of level ground, but as often as we are ignorant of the real magni- diverfified with houses, fields, plantations, &c. tude of bodies, we can never, fron: their appa 491. Dr PORTERFIELD wery well explains fever rent magnitude, form any judgment of their dif- ral fallacies in vision depending upon our mista. tance. From this we may see why we are so fre. king the distances of objects. Of this kind, he quently deceived in our estimates of distance, by says, is the appearance of parallel lines, and long any extraordinary magnitudes of obje&s seen at vistas consisting of parallel rows of trees; for they the end of it; as, in travelling towards a large ci- seem to converge more and more as they are farty, or a castle, or a cathedral church, or a moun- ther extended from the eye. The reason of this, tain larger than ordinary, we fancy them to be he says, is because the apparent magnitudes of nearer than we find them to be. This also is the their perpendicular intervals are perpetually dimi. reason why animals, and all small obje&ts, seen in nithing, while, at the same time, we iniftake their valleys, contiguous to large mountains, appear distance. Hence we may fee why, when two paexceedingly small, because both appear nearer to rallel rows of trees stand upon an ascent, whereby us than they really are.

the more remote parts appear farther off than they 487. Dr JURIN clearly accounts for our ima. really are ; Lecause the line that measures the gining objects, when seen from a high building, to length of the viftas now appears under a greater be smaller than they are, and smaller than we fan- angle than when it was horizontal; the trees, in cy them to be when we view them at the same such a cafe, will seem to converge less, and somedistance on level ground. It is, says he, because times, inftead of corverging, they will be thought we have no distinct idea of dirtance in that direc. to diverge. For the same reason that a long vifta tion, and therefore judge of things by their pictures appears to converge more and more the farther it upon the eye only: but custom will enable us to is extended from the eye, the remoter parts of a judge rightly even in this case. Let a boy, says horizontal walk or a long floor will appear to afne, who has never been upon any high building, cend gradually; and objects placed upon it, the go to the top of the monument, and look down more remote they are the higher they will appear, into the street ; the objects seen there, as men and till the last be seen on a level with the eye; wherehorses, will appear fo Imall as greatly to surprise as the ceiling of a long gallery appears to descend him. For this reason, ftatues placed upon very towards a horizontal line, drawn from the eye of high buildings ought to be made of a larger size the spectator. For this reason, also, the surface than those which are seen at a nearer distance; of the sea, seen from an eminence, seems to rife because all persons, except architects, are apt to higher and higher the farther we look; and the imagine the height of such buildings to be much upper parts of bigh buildings seem to stoop, or less than it really is.

incline forwards over the eye below, because they 488. The 4th method by which Dr Porter- seem to approach towards a vertical line proceed. FIELD says that we judge of the distance of objects, ing from the spectator's eye. is the force with which their colour strikes our 492. Our author also hows the reason why a eyes. For if we be assured that two objects are windmill, seen from a great distance, is fontetimes of a similar and like colour, and that one appears imagined to move the contrary way from what it more bright and lively than the other, we judge , really does, by our taking the near end of the that the brighter objed is the nearer of the two. fail for the more remote. The uncertainty we

489.. The sth method consists in the different sometimes find in the courfe of the motion of a appearance of the small parts of objects. When branch of lighted candles, turned round at a disthese parts appear distind, we judge that the ob- tance, is owing, he says, to the same cause; ag ject is near; but when they appear confused, or also our sometimes mistaking a convex for a con, when they do not appear at all, we judge that it is care surface, more especially in viewing seals and at a greater distance. For the image of any object, impressions with a convex glass or a double microor part of an object, diminishes as the distance of scope ; and lastly, that, upon coming in a dark it increases.

night into a street, in which there is but one row 490. The 6th and last method by which we of lamps, we often mistake the fide of the street judge of the distance of objects is, that the eye they are on. does not represent to our mind one object alone, 493. Much more light was thrown upon this but at the same time all those that are placed be curious fubject by. M. BOUGUER. The proper twixt us and the principal object, whole distance method of drawing the appearance of two rows we are considering; and the more this distance is of trees, that shall appear parallel to the eye, is a divided into separate and distinct parts, the great problem which has exercited the ingenuity of feer it appears to be. For this reason, distances upon veral philofophers and mathematicians. That the uneven surfaces appear less than upon a plane: apparent magnitudes of objects decreases with the for the inequalities of the surfaces, such as hills, angle under which they are feen, has been acknowand holes, and rivers, that lie low and out of sight, ledged; as well as, that it is only by custom and either do not appear, or hinder the parts that lie experience, that we learn t, form a judgment bebind them from appearing; and to the whole both of magnitudes and distances. But in the apVOL. XVI, PART II.



the eye.

plication of these maxims to the above-mentioned more inclined to the horizon than AR. These re. problem, all persons, before M. Houguer, made marks are applicable to different plapes exposed to use of the real distance iniiead of the apparent the eye at the same time. For if BH,(PL. CCLVII, one: by which only the mind can form its judy- fig. 7.) be the front of a building, at the distance ment. It is manifest, that, if any circunsi ances of AB from the eye, it will be reduced in appearcontribute to make the distance appear otherwise ance to the distance A b; and the front of the than it is in reality, the apparent magnitude of building will be bh, rather inclined towards the the onject will be attested by it ; for the same rea fpectator, unless the distance be inconsiderable. fon, ihat, if the magnitudde te mir:pprehended, 497. After many more observations upon this the idea of the distance will vary.

fubject, M. Bouguer adds, that when a man 494. M. BOUGver obferves, that very great stands upen a level plane, it does not seem to rise diitances, and those that are considerably less fenfibly but at fome distance from him. The apthan they, make nearly the fame impresion upon parent plane, therefore, has a curvature in it, at

We, therefore, always imagine great ihat distance, the form of which is not very easy distances to be less than they are; and for this to determine; so that a man standing upon a level reason the ground plan of a long vista always ap- plane, of vali extent, will imagine that he stands pears to rise. The visual rays come in a determin in the centre of a bason. This is also, in fome nate direction; but as we imagine them to terini. measure, the case with a person standing upon the nate sooner than they do, we neceffarily conceive level of the sea. that the place frog which they ittue is elevated. 498. He observes, that there is no difficulty ia Every large plane, therefore, as AB,A! 297,79.6.) drawing lines according to these rules, so as to have viewed by an eye at (), will seem : fic in luchado any given effect upon the eye, except when some rection as Ab; and contiquently lines, in order to parts of the prospect are very near the spectator, appear truly parallel on the plane Ab,must be drawn and others very diftant from him; because, in this so as that they would appear parallel on the plane cafe, regard must he had to the conical or conoidal Ab, and be from thence projected to the plane AB. figure of a surface. A right line paffing at a small

495. M. BOUGUER thows several geometrical dittance from the observer, and below the level of methods of determining this inclination; and fays, his eye, in that case almost always appears fenfibly that by 'these means he has often found it to be curved at a certain distance from the eye ; and al4° or so, though fometimes only 20 or 2. The most all figures in this case are subject to some determination of this angle, he observes, is varia. complicated optical alteration to which the rules ble; depending upon the manner in which the of perspective bave not as yet been extended. If ground is illuminated and the intensity of the light. a circle be drawn near our feet, and within that The colour of the foil is also not without its influ- part of the ground which appears level to us, it

But what is very remarkable, yet may be will always appear to be a circle, and at a very depended upon, is, that if we look towards a ri- considerable diftance it will appear an ellipse; but fing ground, the difference between the apparent between these two situations, it will not appear ground plan and the true one will be much more to be either the one or the other, but will be like considerable, so that they will sometimes make one of thote ovals of Descartes which is more an angle of 25° or 30°. Of this he had made fre- curved on one of its fides than the other. On quent observations. Mountains, be says, begin these principles a parterre, which appears diftort. to be inacceslible when their fides make an angle ed when it is seen in a low situation, appears perfrom 35° to 37° with the horizon, as then it is fectly regular when it is viewed from a balcony or not possible to climb them but by means of stones any other eminence. Still, however, the apparent or shrubs, to serve as steps to fix the feet on. In irregularity takes place at a greater distance, while these cases, both he and his companions always the part that is near the fpečtator is exempt from agreed, that the apparent inclination of the side of it. If AB, (PL. CCLIV. fig. 12.) be the ground the mountain was 60° or 70°.

plane, and Aa be a perpendicular, under the eye, 496. These deceptions are represented in Plate the liigirer it is fituated, at 0, to the greater dif. CCLVI, fig. 18, in which, when the ground plan tance will T, tbe place at which the plane begins AM, or AN, is much inclined, the apparent to have an apparent ascent along Tb, be removed. ground-plan Am, or An, makes a very large angle 499. Ali the varieties that can occur with respect with it. On the contrary, if the ground dips be- to the visible MOTION of ovjects, are thus fuclow the level, the inclination of the apparent to cinctly summed up by Dr PORTERFIELD, under at the true ground-plan diminishes, till, at a certain heads :--1. An object moving very fwiftly is not degree of the slope, it becomes nothing at all; the feen, unless it be very luminous. Thus a cannontwo plans AP and Ap being the same, so that pa- ball is not seen if it is viewed transversely : but if rallel lines drawn upon them would always appear it be viewed according to the line it describes, it so. If the inclination below the horizon is carried may be feen, because its picture continues long on beyond the situation AP, the error will increase; the same place of the retina; which, therefore, and what is very remarkable, it will be on the con- receives a more sensible impresiion from the obje&. trary fide; the apparent plan Ar being always be- 2. A live coal swung briskly round in a circle aplow the true plan AR, so that if a person would pears a continued circle of fire, because the ima' draw upon the plan AR lines that shall appear pa. pressions made on the retina by light, being of a rallel to the eye, they must be drawn converging, vibrating, and consequently of a lasting nature, do and not diverging, as is usual on the level ground; not prefently perish, but continue till the coal per. because they must be the projections of two lines forms its whole circuit, and returns again to its imagined to be parallel, on the plap Ar, which is former place. 3. If two objects, unequally diftant



from the eye, move with equal velocity, the more afcribe the whole motion to the eye, though it be. remote one will appear the flower; or, if their ce- Tongs entirely to the object; and when the eye is lerities be proportional to their distances, they will in motion, though we are ferfible of its motion, appear equally swift. 4. If two objects, unequally yet, if we do not imagine that it moves fo fwiftly distant from the eye, move with unequal veloci. as it really does, we ascribe only a part'rf the tics in the same direction, their apparent velocities motion to the eye, and the rest of it we areribe are in a ratio compounded of the direct ratio of to the object, though it be truly at rest. This last, their true velocities, and the reciprocal one of he says, happens in the present case, when the eye their distances from the eye. 5. Å visible object turns round; for though we are tensible of the moving with any velocity appears to be at rett, if motion of the eye, yet we do not apprehend that the space described in the interval of one second it moves so fast as it really docs; and therefore be imperceptible at the distance of the eye. Hence the bodies about appear to move the contrary a near object moving very lowly, as the index of way, as is agreeable to experience. But the great a clock, or a remote one very swiftly, as a planet, difficulty till remains, viz. Why, after the eye seems to be at reft. 6. An'object moving with ceases to move, objects should, for some time, fill any degree of velocity will appear at reft, if the appear to continue in motion, though their picspace it runs over in á fecond of time be to its dif- turcs on the retina be truly at rest, and do not at tance from the eye as I to 1400. 9. The eye pro- all change their place. This, he imagined, proceeding straight from one place to another, a late- ceeds from a millake we are in with respect to the ral object, not too far off, whether on the right or eye, wlie, though it he absolutely at rest, we left, will seem to move the contrary way. 8. The never beleis conceive as moving the contrary way eye proceeding straight from one place to another, to that in which it moved before; from whicii and being ferifible of its motion, distant objects mistake, with respect to the motion of the eye, the will seem to move the same way, and with the ojects at rest will appear to move the same way same velocity. Thus, to a periön rumming east- which the eye is imagined to move; and, conwards, the moon on his right hand appears to fequently, will seen to continue their motion for move the same way, and with eçusł swiftness; fome rime after the eye is at rest. for, by reason of its distance, its image continues 501. This is ingenious, but perhaps not just. fixed upon the same place of the retina ; from an account of this matter, which seems more fawhence we imagine that the obje& moves along tisfactory, has been given to the public by Dr with the eye. 9. If the cye and the object move WELLS. “ Sorre of the older writers upon optics both the same way, only the eye much swifter (says he)imagined the prisive spirits to be contained than the object, the lat will appear to go back in the head, as water is in a veffel; which, therewards. 10. If two or more objects move with the fore, when once put in motion by the rotation of same velocity, and a third remain ai reit, the move our bodies, must continue in it for some time af. able ones will appear fixed, and the guie!cert in ter this has ceased; and to this real circular movemotion the contrary way. Thus clouds moving ment of the visive spirits, while the body is at rest, very (wiftly, their parts seem to prelurve their tie they at:ributed the apparent motions of objects in tuation, and the moon to more the contrary way. gidúinefs. Decuales saw the weakness of this 11. If the eye be moved with great velocity, lateral hypothefis; and conjectured, that the phenomeobjects at reft appear to move the contrary way. non might be owing to a real movement of the Thus to a person sitting in a coachi, and 'riding eyes; but produced no fact ir proof of his opinion. briskly through a wood, the trees seem to retire Dr PORTERFIELD, on the contrary, suppoled the the contrary way; and to people in a thip, &c. difficulty of explaining it to consist in showing, the shores fee.n to recede,

wly objects at rest appear in motion to an eye 500. At the conclufion of these observations, which is also at rest. The solution he offered of the Doctor endeavours to explain another, pheno: this representation of the phenomenon, is not only menon of motion, which, though very coinmon extremely ingenious, but is, I believe, the only and well known, had not been explained in a fatis probable one which can be given. It does not apfactory manner. If a perfon turris fuiitly round, ply, bowever, to the fact irtich truly exitts; for all objects abcut will seem to move round in a cira ihe eye is not at res, as he imagined. The last cle the contrary vay; and this deception conti- gunci I know of who has touched upon this subnues, not only while the perfop himot movis jest is Dr CBRWIN. His words are, “ When any round, but for some time after he ceiles to move, one iurns round rapidly on one foot till he bewhen the eye, as well as the objects, is at a folnté comes dizzy, and falls upon the ground, the specrest. The reaton why ohjeet's appear to move tra of the ambient objects continue to present round the cotrary way, when the eye turns round, themselves in rotation, or appear to librate, and is not to difficult to explair ; for though, proper he seems to behold them for some time inmotion.” ly speaking, motici is not feon, as not being in I do not pretend to understand his opinion fully; itself the immediate object of light; yet by the light but if such an apparent motion of the surrounding we easily know when the inage changes its place objects depends in any way upon their spectra, or on the retira, and thence conclude that either the illusive representations, occasioned by their former object, the eye, or both, are moved. But by the impressions upon the retina, no fimilar motion fight alone we can never determine how far this would be observed, were we to turn ourselves motion belongs to the object, to the cyc, or to round with our eyes shut, and not to open them both. If we imagine the eye at rest, we afcribe till we became giddy. But whoever will make the whole motion to the objeét, though it be truly the experiment, will find, that objects about him It reft. If we imagine the object at relt, we appear to be equally in 'ınótion, when he has be

Fff 2


come giddy by turning himself round, whether Let AB( represent the luminous this has been done with his eyes open or (but. I object to which the fight is directed, CD tbe more Ihall now propose my own opinion upon this sub. diltant opaque body, GH the nearer, and Ef the ject:

diameter of the pupil. Join ED, FD, EG, FG, 307. “ If the eye be at reit, we judge an object and produce them till they meet AB in K, N, M, to be in' motion, when its picture fails in succeeding and L. It is plain that the parts AN, MB, of the times upon different parts of the retina; and if the luminous object cannot be seen. But taking any eye be in motion, we judge an object to be at reft, point a between N and K, and drawing a D , 18 long as the change in the place of its picture lince the portion d F of the pupil is filled with upon the retina, holds a certain correspondence light flowing from that poiiit, it must be visible. with the change of the eye's position. Let us Any point b, between a and K, muft fill FF, a now suppose the eye to be in motion, while, from greater portion of the pupil, and therefore muft fome disorder in the system of sensation, we are appear brighter. Again, any points, between b either without those feelings which indicate the and K, must appear brighter than h, because it fills various positions of the eye, or are not able to at.

a greater portion g F with light. The point Kit. tend to them. In such a state of things an object felf, and every other point in the 1pace KL, must at rest mult appear to be in motion, since it sends appear very luminous, since they lend entire penin succeeding times its picture to different parts of cils of rays EKF, ELF, to the eye ; and the visithe retina. And this seems to be what happens in ble brighiness of every point from L towards M, giddiness.” Dr Wells then relates the particu- mult decrease gradually, as from Kto N; that is, Lars of a case of giddiness, which happened to the spaces KN, LM, will appear as dim shadowy himself, and produced several optical deceptions; borders, or fringes, adjacent to the edges of the from which he infers, that “all these phenomena opaque bodies. Whey the exige G is brought to demonftrate, that there was a real motion in my touch the right line KF, the penumbras unite; eyes at the «ime I imagined them to be at reft; and as soon as it reaches NDF, the above pbellofor the apparent situation of the spot, with respect inenon begins; for it cannot pass that right line 1.0 the paper, could not possibly have been altered, without meeting fome line a D d, drawn from a without a real change of the position of these or- point between N and K, and, by intercepting a!! gans. To have this proved, I defired a person to the rays that fall upon the pupil, render it invificurn quickly round, till he became very giddy; ble. in advancing gradually to the line KDE, it then to ftop himfulf, and look fedtaftiy at me. will meet other lines b D f ( Dg, &c. and thereIIe did fo, and I couid plainly fee, that although fore render the points b, c, &c. from N to K fucne thought his eyes were fixed, they were in reality ceslively invisible; and therefore the edge of the noving in their fockets, first toward one side and fixed opaque body CD must seem to swell outthen toward the other."

wards, and cover the whole fpice NK; while GH, 503. M, LE Cat well explains a remarkable de. by its motion, covers MK. When GH is placed Reption, by which a person thall imagine an object at a greater distance from the eye, CD continuing to be on the oppofite Gide of a board, when it is fixed, the space OP to be pailed over in order to not fo, and also inverted and magnified. It is illus. intercept NK is less; and therefore, with an equal Grated by fig 8. pl. CCLVII. in which D represents motion of GH, the apparent swelling of CD muft the eye, and CB a large black board, pierced with be quicker; which is found true by experience. If a small hole. E is a large wbite board, placed be- ML represent a luminous obje&, and REFQ any yond it, and frongly illuminated; and d a pin, or plane exposed to its light, the space FQ will he other small object, held betwixt the eye and the entirely shaded from the rays, and the 1pace FE first board. In these circumstances, the pin Mall will be occupied by a penumbra, gradually darker, be imagined to be at F, on the other side of the from E to F. Let now GH continue fixeri, and board, where it will appear inverted and magni- CD move parallel to the plane EF; and as soon as Fieci; becaufe what is in fact perceived, is the iha- it padles the line LF, it is evident that the shadow dow of the pin upon the retina; and the light that of will seem to swell out wards; and when CD is stopped by the upper part of the pin coming reaches ME, so as to cover with its fhadow the from the lower part of the enlightened board, and space RE, QF, by its extenfion, will cover FE. that which is stopped by the lower part coming This holds true likewise by experiment. from the upper part of the board, the shadow mult s IV. Of the Concave FIGURE of the Sky. necessarily be inverted with respect to the object. sos. This apparent concavity is only an opti

504. There is a curious phenomenon relating to cal deception founded on the incapacity of our ormilion, which some have afcribed to the inflection gans of vifion to take in very large distances. Dr of light, but which mir Melville explains in a Smith in his Complete System of Oprics, hath devery different and very simple manner. When any monstrated, that, if the lurface of the earth was opaque body is held at the distance of 3 or 4 inches perfectly plane, the distance of the visible borizon frc.n the eye, so that a part of some more distant from the eye wou'd scarce exceed the distance of Ju. 1..218 object, such as the window, or the Hame socotimes the height of the eye above the ground, of .canale, naj be seen by rays patling near its suppoling the height of the eye between 's aod 6 Ruge, it another opaque body, nearer to the eye, be feet: beyond this distance all objects would apbrought across from the oppolite tide, the edge of pear in the visible horizon. For, let OP (fig. 13. the ti:A body will fceni to twell outwards and meet 9.CCLIV.) be the height of the eye above the line the latter; and in doing to will intercepi a portion PA drawn upon the ground; and if an objec! AB, of the luminous object that was teen belore. This equal in height to PO, be removed to a distance 2bpetrance he explains in the following manner: På equal to 5000 tiines that height, it will hardiy


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