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quently, the fame object appearing to both eyes to be in the fame place, the mind cannot diftinguish it into two. In answer to an objection to this hypothefis, from objects appearing double when one eye is diftorted, he fays the mind miftakes the pofition of the eye, imagining that it had moved in a manner correfponding to the other, in which cafe the conclufion would have been juft. The power of habit has generally been thought to be fufficient to account for this appearance. Originally, every object making two pictures, one in each eye, is imagined to be double; but by degrees, we find, that when two correfponding parts of the retina are impressed, the object is but one; but if those correfponding parts be changed, by the distortion of one eye, the object must again appear double as at the firft. This feems to be verified by Mr Chefelden; who informs us, that a gentleman, who from a blow on his head had one eye diftorted, found every object to appear double; but by degrees the most familiar ones came to appear fingle, and in time all objects did fo, without any amendment of the diftortion. A cafe fimilar to this is mentioned by Dr Smith.

336. On the other hand, Dr REID is of opinion, that the correfpondence of the centres of the two eyes, on which fingle vifion depends, does not arife from cuftom, but from fome natural conftitution of the eye and of the mind. He makes feveral juft objections to the cafe of Mr Fofter, recited by Dr Smith and others; and thinks that the cafe of the young man couched by Chefelden, who faw fingly with both eyes immediately upon receiving his fight, is decifive in proof of his fuppofition. He also found that three young gentlemen, whom he endeavoured to cure of fquinting, faw objects fingly, as foon as ever they were brought to direct the centres of both their eyes to the fame object, though they had never been ufed to do fo; and he obferves, that there are cafes, in which, notwithstanding the fulleft conviction of an object being fingle, no practice of looking at it will ever make it appear fo, as when it is feen through a multiplying glass.

337. To all these folutions of the difficulty refpecting fingle vifion by both eyes, objections have been lately made which feem infurmountable, by experiments judiciously conceived and accurately conducted. Dr WELLS has fhown, that it is neither by custom alone, nor by any original property of the eyes alone, that objects appear fingle; and having demolished the theories of others, he thus accounts for the phenomenon in his Effay on fimple Vifion: "The visible part of an object being compofed of its vifible diftance and vifible direction, to show how it may appear the fame to both eyes, it will be neceffary to explain in what manner the distance and direction, which are perceived by one eye, may coincide with thofe which are perceived by the other." With respect to visible distance, the author's opinion feems not to differ from that which we have ftated elsewhere, (fee METAPHYSICS, Sec. XII.); and therefore we have to attend only to what he fays of vifible direction. When a fmall object is fo placed with respect to either eye, as to be feen more diftinctly than in any other fituation, our

author fays that it is then in the optic axis, or the axis of that eye. When the two optic axes are directed to a small object not very diftant, they may be conceived to form two fides of a triangle, of which the bafe is the interval between the points of the corners where the axes enter the eyes. This bafe is called the visual bafe; and a line drawn from the middle of it to the point of interfection of the optic axes he calls the common axis. He then proceeds to show, that objects really fituated in the optic axis do not appear to be in that line, but in the common axis.

338. He then proves by experiments, for which, as well as for his arguments, we muft refer to his work, that the objects fituated in the common axis do not appear to be in that line, but in the axis of the eye by which they are not seen; that is, an object fituated in the common axis appears to the right eye in the axis of the left, and vice verfa. His next propofition, proved likewise by experiments, is, that "objects, fituated in any line drawn through the mutual intersection of the optic axes to the visual bafe, do not appear to be in that line, but in another, drawn through the fame interfection, to a point in the visual bafe, diftant half this bafe from the fimilar extremity of the former line towards the left, if the objects be feen by the right eye, but towards the right if feen by the left eye." From these propofitions he thus accounts for fingle vifion by both eyes: "If the queftion be concerning an object at the concourfe of the optic axes, it is feen fingle, because its two fimilar appearances, in regard to fize, fhape, and colour, are seen by both eyes in one and the fame direction, or, if you will, in two directions, which coincide with each other through the whole of their extent. It therefore matters not whether the diftance be truly or falfely estimated; whether the object be thought to touch our eyes, or to be infinitely remote. And hence we have a reafon, which no other theory of visible direction affords, why objects appeared fingle to the young gentleman mentioned by Chefelden, immediately after his being couched, and before he could have learned to judge of diftance by fight. When two fimilar objects are placed in the optic axes, one in each, at equal diftances from the eyes, they will appear in the fame place, and therefore one, for the fame reafon that a truly fingle object, in the concourfe of the optic axes, is feen fingle. It feems only neceffary to determine, whether the dependence of the vifible direction upon the actions of the mufcles of the eyes be established by nature, or by cuftom. But facts are here wanting. As far as they go, however, they serve to prove, that it arifes from an original principle of our conftitution. For Mr Chefelden's patient faw objects fingle, and confequently in the fame directions with both eyes, immediately after he was couched; and perfons affected with fquinting from their earlieft infancy fee objects in the fame directions with the eye they have never been accuftomed to employ, as they do with the other they have conftantly ufed." This work deserves to be ftudied by every optician.

339. Dr JURIN made the following experiments, to determine whether an object seen by both eyes appears brighter than when feen with one only. Bbb 2

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342. A curious deception in vifion, arifing from the ufe of both eyes, was observed and accounted for by Dr SMITH. It is a common obfervation, he fays, that objects feen with both eyes appear more vivid and stronger than they do to a fingle eye; efpecially when both of them are equally good. A perfon not fhort-fighted may foon be convinced of this fact, by looking attentively at objects that are pretty remote, firft with one eye, and then with both. This obfervation gave occafion to the conftruction of the BINOCULAR TELESCOPE, or telescope for both eyes, in the use of which the phenomenon is ftill more ftriking. There is another remarkable phenomenon obfervable with this inftrument. In the foci of the two telescopes there are two equal rings as ufual, which terminate the pictures of the objects there formed, and confequently the vifible area of the objects themselves. Thefe equal rings, by reafon of the equal eye-glaffes, appear equal, and equally diftant, when feen separately by each eye; but when they are feen with both eyes, they appear much larger, and more diftant alfo; and the objects feen through them do alfo appear much larger, though circumfcribed by their united rings, in the fame places as when they were seen separately. The phenomenon of the enlarged circle of the vifible area in the binocular telefcope, may be seen very plainly in looking at diftant objects though a pair of fpectacles, removed from the eyes about 4 or 5 inches, and held fteady at that diftance. The two, innermoft of the four appa rent rings, which hold the glaffes, will then appear in one larger and more diftant ring than the two outermoft, which will hardly be vifible unless the fpectacles be farther removed.

He laid a flip of clean white paper directly before him on a table, and applying the fide of a book close to his right temple, fo as that the book was advanced confiderably more forward than his face, he held it in fuch a manner, as to hide from his right eye that half of the paper which lay to his left hand, while the left half of the paper was feen by both eyes without any impediment. Then looking at the paper with both eyes, he obferved it to be divided, from the top to the bottom, by a dark line, and the part which was feen with one eye only was manifeftly darker than that which was feen with both eyes; and, applying the book to his left temple, he found, by the refult of the experiment, that both his eyes were of equal goodnefs. He then endeavoured to find to what degree this excefs of brightnefs amounted; and comparing it with the appearance of an object illuminated partly by one candle and partly by two, he was surprised to find that an object feen with two eyes is by no means twice as luminous as when it is feen with one only; and after a number of trials, by which he made the proportion lefs and lefs continually, he found that when one paper was illuminated by a candle placed at the distance of three feet, and another paper by the fame candle at the fame distance, and by another candle at the distance of 11 feet, the former feen by both eyes, and the latter with one eye only, appeared to be of equal whiteness; fo that an object feen with both eyes appears brightser than when it is feen with one only, by about a 13th part. He then proceeded to inquire whether an object feen with both eyes appears any thing larger than when feen with one only; but he concluded that it did not, except on account of fome particular circumftances, as in the cafe of the binocular telescope and the concave speculum.

340. M, DU TOUR maintains, that the mind attends to no more than the image made in one eye at a time; and produces feveral curious experiments in favour of this hypothefis, which had also been maintained by Kepler and almost all the firft opticians. But, as M. Buffon obferves, it is a fufficient answer to this hypothefis, how ingeniously foever it may be fupported, that we fee more diftinctly with two eyes than with one; and that when a round object is near us, we plainly fee more of the furface in one cafe than in the other. There are also other facts which clearly prove the contrary of M. du Tour's hypothefis.

341. With respect to fingle vifion with two eyes, Dr HARTLEY obferves, that the optic nerves of men, and fuch other animals as look the fame way with both eyes, unite in the cella turcica, in a ganglion peculiar to themselves; and that the affociations between the fynchronous impreffions on the two retinas must be made fooner and cemented ftronger on this account; alfo that they ought to have a much greater power over one another's images, than in any other part of the body. And thus an impreffion made on the right eye alone, by a fingle object, may propagate itself into the left, and there raise up an image almost equal in vividnefs to itself; and confequently when we fee with one eye only, we may, however, have pic-' tures in both eyes.

343. A curious circumftance, relating to the effect of one eye upon the other, was noticed by M. EPINUS, who obferved, that, when he was look. ing through a hole made in a plate of metal, about the roth part of a line in diameter, with his left eye, both the hole itself appeared larger, and also the field of view seen through it was more extended, whenever he shut his right eye; and both effects were more remarkable when that eye was covered with his hand. He found confiderable difficulty in measuring this augmentation of the apparent diameter of the hole, and of the field of view; but at length he found, that, when the hole was half an inch, and the tablet which he viewed through it was three feet from his eye, if the diameter of the field when both his eyes were open was 1, it became 14 when the other eye was shut, and nearly 2 when his hand was laid upon it. Upon examining this phenomenon, it appeared to depend upon the enlargement of the pupil of one eye when the other is clofed, the physical or anatomical caufe of which he did not pretend to affign; but he observes, that it is wifely appointed by divine Providence, that when one eye fails, the field of view in the other may be extended. That this effect fhould be more fenfible, when the eye is covered with the hand, is owing, he obferves, to the eye-lids not being impervious to the light.. But the enlargement of the pupil does not enlarge the field of view, except in looking through a hole, as in this particular café; and therefore perfons

who

who are blind of one eye can derive no advantage from this circumftance.

344. A great deal has been written by Gaffendi, Le Clerc, Muffchenbroek, and Du Tour, concerning the place to which we refer an object viewed by one or both eyes. But the moft fatisfactory account of this matter, the reader will find in Dr Wells's Effay above quoted.

Of the APPEARANCE of OBJECTS feen through MEDIA of DIFFERENT FORMS.

345. FOR the more eafy apprehenfion of what relates to this fubject, we fhall premise the five following particulars, which either have been already mentioned, or follow from what has been before laid down.

346. I. That as each point of an object, when viewed by the naked eye, appears in its proper place, and as that place is always to be found in the line in which the axis of a pencil of rays flow. ing from it enters the eye, or elfe in the line which Dr WELLS calls the common axis: we hence acquire an habit of judging the point to be fituated in that line: and, because the mind is unacquainted with what refractions the rays fuffer before they enter the eye, therefore, in cafes where they are diverted from their natural course, by paffing through any medium, it judges the point to be in that line produced back, in which the axis of a pencil of rays flowing from it is fituated, the inftant they enter the eye, and not in that it was in before refraction. We fhall therefore, in what follows, fuppofe the apparent place of an object, when feen through a refracting medium, to be fomewhere in that line produced back, in which the axis of a pencil of rays flowing from it proceeds after they have passed through the medium.

347. II. That we are able to judge, though imperfectly, of the distance of an object by the degree of divergency, wherein the rays flowing from the fame point of the object enter the pupil of the eye, in cafes where that divergency is confiderable; but because, in what follows, it will be neceffary to suppose an object, when seen through a medium whereby its apparent diftance is altered, to appear in fome determinate fituation, in those cafes where the divergency of the rays at their entrance into the eye is confiderable, we will fuppofe the object to appear where thofe lines which they defcribe in entering, if produced back, would crofs each other: though it must not be afferted, that this is the precise distance; because the brightness, distinctness, and apparent magnitude of the object, on which its apparent diftance in fome measure depends, will alfo fuffer an alteration by the refraction of the rays in paffing through that medium.

348. III. That we estimate the magnitude of an object by that of the optic angle.

349. IV. That vifion is the brighter, the greater the number of rays is which enter the pupil. And, 350. V. That, in fome cafes, the apparent brightness, diftinctness, and magnitude of an object, are the only means whereby our judgment is determined in eftimating the distance of it.

351. PROP. I. An object placed within a medium terminated by a plane furface on that fide

which is next the eye, if the medium be denfer than that in which the eye is (as we fhall always fuppofe it to be unless where the contrary is expreffed), appears nearer to the furface of the medium than it is. Thus, if A be a point of an object placed within the medium BDCE (fig. 10. Pl. 254.) and Ab Ac be two rays proceeding from thence, these rays paffing out of a denfer into a tive perpendiculars b d, ce, and will enter the eye rarer medium, will be refracted from their refpecat H, fuppofe in the directions bf, g: let then thefe lines be produced back till they meet in F: this will be the apparent place of the point A: and because the refracted rays bf, cg will diverge more than the incident ones Ab, Ac, it will be nearer to the points b and c than the point A; and as the fame is true of each point in the object, the whole will appear to an eye at H, nearer to the furface BC than it is. Hence when one end of a straight stick is put under water, and the ftick is held in an oblique pofition, it appears bent at the furface of the water; viz. because each point that is under water appears nearer the furface, and confequently higher than it is. Hence alfo an object at the bottom of a veffel may be feen when the veffel is filled with water, though it be fo placed with respect to the eye, that it cannot be feen when the veffel is empty. To explain this, let ABCD (fig. 11. Pl. 251.) reprefent a veffel, and let E be an object lying at the bottom of it. This object, when the veffel is empty, will not be feen by an eye at F, because HB, the upper part of the veffel, will obftruct the ray EH; but when it is filled with water to the height GH, the ray EK being refracted at the furface of the water into the line KF, the eye at F fhall fee the object by means of that.

352. In like manner, an object fituated in the horizon appears above its true place, upon account of the refraction of the rays which proceed from it in their paffage through the atmofphere of the earth. For, firft, if the object be fituated beyond the limits of the atmosphere, its rays in entering it will be refracted towards the perpendicular; that is, towards a line drawn from the point where they enter, to the centre of the earth, which is the centre of the atmosphere: and as they pass on, they will be continually refracted the fame way, because they are all along entering a denfer part, the centre of whofe convexity is ftill the fame point; upon which account the line they defcribe will be a curve bending downwards; and therefore none of the rays that come from that object can enter an eye upon the furface of the earth, except what enter the atmofsphere higher than they need to do if they could come in a right line from the object; confequently the object muft appear above its proper place. 2dly, if the object be placed within the atmosphere, the cafe is ftill the fame; for the rays which flow from it must continually enter a denfer medium whofe centre is below the eye; and therefore being refracted towards the centre, that is downwards as before, those which enter the eye must neceffarily proceed as from some point above the object; wherefore the object will appear above its proper place. Hence the fun, moon, and stars, appear above the horizon, when they are just be

low

low it; and higher than they ought to do, when they are above it: likewife diftant hills, trees, &c. feem to be higher than they are. The lower thefe objects are in the horizon, the greater is the obliquity with which the rays which flow from them enter the atmosphere, or pass from the rarer into the denfer parts of it; and therefore they appear to be the more elevated by refraction; upon which account the lower parts of them are apparently more elevated than the other. This makes their upper and under parts feem nearer than they are; as is evident from the fun and moon, which appear of an oval form when they are in the horizon, their horizontal diameters ap. pearing of the same length they would do if the rays fuffered no refraction, while their vertical ones are shortened thereby.

353. PROP. II. An object feen through a medium terminated by plane and parallel furfaces, appears nearer, brighter, and larger, than with the naked eye. For inftance, let AB (Fig. 13. Pl. CCLIII.) be the object, CDEF the medium, and GH the pupil of an eye, which is here drawn large to prevent confufion. And, ift, Let RK, RL, be two rays proceeding from the point R, and entering the denfer medium at K and L; these rays will here by refraction be made to diverge lefs, and to proceed afterwards, fuppofe in the lines Ka, Lb. At a and b, where they pafs out of the denfer medium, they will be as much refracted the contrary way, proceeding in the lines ac, bd, parallel to their first directions. Produce thefe lines back till they meet in e: this will be the apparent place of the point R; and it is evident from the figure, that it must be nearer the eye than that point; and because the fame is true of all other pencils flowing from the object AB, the whole will be feen in the fituation fg, nearer to the eye than the line AB. 2d, As the rays RK, RL, would not have entered the eye, but have paffed by it in the directions Kr, Lt, had they not been refracted in paffing through the medium, the object appears brighter. 3d, The rays Ab, Bi, will be refracted at hand i into the lefs converging bk and il, and at the other surface into kM, IM, parallel to Ah and Bi produced; fo that the extremities of the object will appear in the lines Mk Ml produced, viz. in ƒ and g, and under as large an angle fMg, as the angle AqB under which an eye at q would have feen it had there been no medium in terpofed to refract the rays; and therefore it appears larger to the eye at GH, being feen through the interpofed medium, than otherwife it would have done. But it is here to be observed, that the nearer the point e appears to the eye on account of the refraction of the rays RK, RL, the fhorter is the image ƒg, because it is terminated by the lines Mf and Mg, upon which account the object is made to appear lefs; and therefore the apparent magnitude of an object is not much augmented by being feen through a medium of this form. It is evident from the figure, that the effect of a medium of this form depends wholly upon its thickness; for the distance between the lines Rr and ec, and confequently the diftance between the points e and R, depends upon the length of the line Ka. Again, the diftance between the

lines AM and ƒM depends on the length of the line bk; but both Ku and kh depend on the length of the line bk; but both Ku and kh depend on the diftance between the furfaces CE and DF, and therefore the effect of this medium depends upon its thickness.

354. PROP. III. An object feen through a convex lens, appears larger, brighter, and more diftant, than with the naked eye. To illuftrate this, let AB (fig. 14, pl. 253.) be the object, CD the lens, and EF the eye. 1. From A and B, the extremities of the object, draw the lines AŸr, BXr, croffing each other in the pupil of the eye; the angle ArB comprehended between these lines is the angle under which the object would be feen with the naked eye. But by the interpofition of a lens of this form, whofe property it is to render converging rays more fo, the rays AY and BX will be made to cross each other before they reach the pupil. There the eye at E will not perceive the extremities of the object by means of thefe rays, for they will pafs it without entering, but by fome others, which must fall without the points Y and X, or between them; but if they fall between them, they will be made to concur fooner than they themselves would have done; and therefore, if the extremities of the object could not be feen by them, it will much lefs be seen by these. It remains therefore, that the rays which will enter the eye from the points A and B after refraction, muft fall upon the lens without the points Y and X; let the rays, then, AO and BP be fuch. Thefe after refraction entering the eye at r, the extremities of the object will be feen in the lines rQ rT produced, and under the optic angle Qr V, which is larger than ArB, and therefore the apparent magnitude of the object will be increafed. 2. Let GHI be a pencil of rays flowing from the point G; as it is the property of this lens to render diverging rays lefs diverging, parallel, or converging, it is evident that fome of thofe rays, which would proceed on to F and E, and mifs the eye, were they to fuffer no refraction in paffing through the lens, will now enter it; by which means the object will appear brighter. 3. As to the apparent diftance of the object, that will vary according to the fituation of it with respect to the focus of parallel rays of the lens. 1. Then, let us fuppofe the object placed so much nearer the lens than its focus of parallel rays; that the refracted rays KE and LF, though rendered lefs diverging by paffing through it, may yet have a confiderable degree of divergency, fo that we may be able to form a judgment of the diftance of the object thereby; in this cafe, the object ought to appear where EK FL produced back concur; which, because they diverge less than the rays GH GI, will be beyond G, that is, at a greater diftance from the lens than the object is. But because both the brightness and magnitude of the object will at the fame time be augmented, prejudice will not permit us to judge it quite fo far off as the point where thofe lines meet, but fomewhere between that point and its proper place. 2. Let the object be placed in the focus of parallel rays, then will the rays KE and LF become parallel; and though in this cafe the object would

appear

appear at an immenfe diftance, if that diftance were to be judged of by the direction of the rays KE and LF, yet, upon account of the brightnefs and magnitude of it, we fhall not think it much farther from us than if it were feen by the naked eye. 3. If the object be fituated beyond the focus of parallel rays, as in BA (fig. 5. pl. CCLV.) the rays flowing from thence and falling upon the lens CD, will be collected into their respective foci at a and b, and the intermediate points m, n, &c. and will there form an image of the object AB; and after croffing each other in the feveral points of it, as expreffed in the figure, will pafs on diverging, as from a real object. Now, if an eye be fituated at c, where A c, Bc, rays proceeding from the extreme points of the object, make not a much larger angle Ac B than they would do if there were no lens interpofed, and the rays belonging to the fame pencil do not converge fo much as those which the eye would receive if it were placed nearer to a or b, the object, upon these accounts, appearing very little larger or brighter than with the naked eye, is feen nearly in its proper place; but if the eye recedes a little way towards ab, the object, then appearing both brighter and larger, feems to approach the lens; which is an evident proof of what has been fo often afferted, viz. that we judge of the distance of an object in fome measure by its brightnefs and magnitude; for the rays converge the more the farther the eye recedes from the lens; and therefore, if we judge of the distance of the object by the direction of the rays which flow from it, we ought in this case to conceive it at a greater diftance than when the rays were parallel, or diverged at their entrance into the eye.

355. That the object should seem to approach the lens in this cafe, was a difficulty that exceed ingly puzzled the learned BARROW, and which he pronounces infuperable, and not to be accounted for by any theory we have of vifion. M. MOLINEUX alfo leaves it to the folution of others, as that which will be inexplicable, till a more intimate knowledge of the vifive faculty be obtained by mortals. They imagined, that feeing an object appears farther off, the less the rays diverge which fall upon the eye, if they should proceed parallel to each other, it ought to appear exceeding remote; and if they should converge, it fhould then appear more distant ftill; the reason of this was, because they looked upon the apparent place of an object, as owing only to the direction of the rays wherever it was, and not at all to its apparent magnitude or fplendour. Perhaps it may proceed from our judging of the distance of an object in fome measure by its magnitude, that that deception of fight commonly obferved by travellers may arife; viz. that upon the first appearance of a building larger than usual, as a cathedral church or the like, it generally seems nearer to them than they afterwards find it to be.

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356. PROB. IV. If an object be placed farther from a convex lens than its focus of parallel rays, and the eye be fituated farther from it on the other fide than the place where the rays of the feveral pencils are collected into their respective foci, the object appears inverted, and pendulous is the air, between the eye and the lens. To

explain this, let AB (Fig. 5. Pl. CCLV.) reprefent the object, CD the lens; and let the rays of the pencil ACD be collected in a, and thofe of BCD in b, forming there an inverted image of the object AB, and let the eye be placed in F; it is apparent from the figure, that some of the refracted rays, which pass through each point of the image, will enter the eye as from a real object in that place; and therefore the object AB will appear there, as the propofition afferts. But we are fo little accustomed to see objects in this manner, that it is very difficult to perceive the image with one eye; but if both eyes are fituated in fuch a manner, that rays flowing from each point of the image may enter both, as at G and H, and we direct our optic axes to the image, it is easy to be perceived. If the eye be fituated in a or b, or very near them on either side, the object appears exceedingly confused; viz. if at d, the rays which proceed from the fame point of the object converge fo very much, and if at e, they diverge fo much, that they cannot be collected together upon the retina, but fall upon it as if they were the axes of fo many diftinct pencils coming through every point of the lens; wherefore little more than one fingle point of the object is feen at a time, and that appears all over the lens; from whence nothing but confufion arifes. If the lens be fo large that both eyes may be applied to it, as in h and k, the object will appear double; for it is evident from the figure, that the rays which enter the eye at h from either extremity of the object A or B, do not proceed as from the fame point with that from whence those which enter the other at k feem to flow; the mind therefore is here deceived, and looks upon the object as fituated in two different places, and therefore judges it to be double.

357. PROP. V. An object feen through a concave lens appears nearer, fmaller, and lefs bright, than with the naked eye. Thus, let AB (fig. 17pl. CCLII.) be the object, CD the pupil of an eye, and EF the lens. Now, as it is the property of a lens of this form to render diverging rays more fo, and converging ones lefs fo, the diverging rays GH, GI, proceeding from the point G, will be made to diverge more, and fo to enter the eye as from fome nearer point g; and the rays AH, BI, which converge, will be made to converge lefs, and to enter the eye as from the points a and b ; wherefore the objects will appear in the fituation a gb, lefs and nearer than without the lens. Farther, as the rays which proceed from G are rendered more diverging, fome of them will be made to pass by the pupil of the eye, which otherwife would have entered it, and therefore each point of the object will appear lefs bright.

358. PROP. VI. An object seen through a polygonous glafs, that is, fuch as is terminated by feveral plain furfaces, is multiplied thereby. For inftance, let A (fig. 11. pl. CCLIV.) be an object, and BC a polygonous glafs, terminated by the plain furfaces BD, DE, &c. and let the fituation of the eye F be fuch, that the rays AB, being refracted in paffing through the glafs, may enter it in the direction BF, and the rays AC in the direction CF. Then will the eye, by means of the former, see the object in G, and by the latter in H

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