not large enough to prevent vifion in every part of the base of the nerve, fince the diameter of each of the two vessels occupies no more than th part of it.

319. The common opinion, however, was favoured by the anatomical description of feveral animals by the members of the French academy, and particularly their account of the fea-calf and porcupine; in both of which the optic nerve is inferted in the very axis of the eye, exactly oppofite to the pupil, which was thought to leave no room to doubt, but that in thefe animals the retina is perfectly fenfible to the impreffion of light at the infertion of the nerve. M. DE LA HIRE took part with M. Pecquet, arguing in favour of the retina, from the analogy of the fenfes, in all of which the nerves are the proper feat of fenfation. He however, fuppofed that the choroides receives the impreffions of images, to tranfmit them to the retina. M. PERRAULT alfo took the part of M. Pecquet, and in his works we have several letters that paffed between him and Marriotte upon this fubject.

320. This difpute was revived upon the occa fion of an odd experiment of M. MERY, recorded in the Memoirs of the French Academy for 1704. He plunged a cat in water, and expofing her eye to the ftrong light of the fun, obferved that the pupil was not at all contracted by it; from which he concluded, that the contraction of the iris is not produced by the action of the light, but by fome other circumftance; for he contended, that the eye receives more light in this fituation than in the open air. At the same time he obferved that the retina of the cat's eye was tranfparent, and that he could fee the opaque choroides beyond it; from which he concludes, that the choroides is the substance intended to receive the rays of light, and to be the chief inftrument of vifion. But M. De la Hire replies to this argument of M. Mery, in a memoir for 1709, p. 119; in which he endeavours to shew, that fewer rays enter the eye under water, and that in thofe circumftances it is not fo liable to be affected by them. Befides, it is obvious, that the cat muft have been in great terror in this fituation; and being an animal that has a very great voluntary power over the mufcles of the iris, the might keep her eye open from fear, notwithstanding the action of light upon it, and though it might be very painful to her.

321. M. LE CAT took part with M. Marriotte in this controverfy, it being peculiarly agreeable to his general hypothefis, wherein he differs from Winflow; viz. that the pia mater, of which the choroides is a production, and not the nerves themselves, is the proper inftrument of fenfation. He thought that the change which takes place in the eyes of old people (the choroides growing lefs black with age) favoured his hypothefis, as they do not fee with that diftinctness with which young perfons do. He supposed that the retina anfwers a purpose fimilar to that of the scarf-fkin, covering the papillæ pyramidales, which are the immediate organs of feeling, or that of the porous membrane which covers the glandulous papillæ of the tongue. The retina, he fays, receives the impreffion of

light, moderates it, and prepares it for its proper organ, but is not itself fenfible of it.

322. To thefe arguments in favour of the cho roides, alleged by thofe gentlemen among whom the fubject was firft difcuffed, Dr. PRIESTLEY, in his hiftory, adds the following, among others that had escaped their notice, but which were fuggefted to him by his friend Mr MICHELL. That vifion be diftinét, the pencils of rays which iffue from the feveral points of any object must be collected either accurately, or at least very nearly, to correfponding points in the eye, which can only be done upon fome uniform furface. But the retina being of a confiderable thickness, and the whole of it being uniformly nervous, and at least nearly transparent, prefents no particular furface; fo that, in whatever part of it the pencils be fuppofed to have their foci, the rays belonging to them will be feparated from one another, either before or after they arrive there, and confequently vision will be confufed. It is likewife an argument in favour of the choroides being the organ of vifion, that it is a fubftance which receives a more diftinct impreffion from the rays of light than any other membrane in any part of the animal fyftem, whereas the retina is a fubftance on which the light makes an exceedingly faint impreffion. Befides, the retina is fo fituated as to be expofed to many rays befides thofe which terminate in it, and which, therefore, cannot be fubfervient to vifion, if it be performed there. Now, this is not the cafe with the choroides, which is in no fhape tranfparent, and has no reflecting fubftance beyond it. It is favourable to the feat of vifion being in the choroides, that we can see a reafon for the diverfity of its colour in different animals. In all terreftrial animals, which use their eyes by night, the choroides is either of a bright white, or of fome very vivid colour, which reflects the light very ftrongly. Befides, the choroides in brutes is not in general perfectly white, but a little inclined to blue; and is therefore better adapted to see by the fainter coloured light, which chiefly prevails in the night; and more liable to be ftrongly impreffed by the colours to which they are chiefly exposed. On the other hand, the choroides of birds in general, efpecially eagles, hawks, and other birds of prey, is black; by which means they are able to fee with the greatest distinctness, but only in bright day-light. The owl, however, feeking her food by night, has the choroides white like that of a cat. Laftly, in the eyes of man, which are adapted to various ufes, the choroides is neither fo black as that of birds, nor fo white as that of those animals who make the greatest use of their eyes in the night.

323. As to a third hypothefis, which is that of M. DE LA HIRE, which makes both the retina and the choroides equally neceffary to vision, and fuppofes it to be performed by the impreffion of light on the choroides communicated to the retina, Mr Michell obferves, that the perceptions can hardly be supposed to be fo acute, when the nerves, which are the chief inftruments of fenfation, do not receive the impreffions immediately, but only after they have been communicated to another fubftance. Befides, it must be more natural to

suppose,

fuppofe, that, when the principal impreffion is made upon the choroides, it is communicated to the brain by its own proper nerves, which are abundantly fufficient for the purpefe.

36

324. The dimenfions and precife form of the fpot in the eye in which there is no vifion, were more accurately calculated by Daniel Bernoulli,' in the following manner. He placed a piece of money (fig. 3, Pl. CCLV.) upon the floor; and then fhutting one of his eyes, and making a pen dulum to fwing, fo that the extremity of it might be nearly in the line AO, he obferved at what place C it began to be invifible, and where it again emerged into view at A. Raifing the pendulum higher and lower, he found other points, as H, N, P, G, B, at which it began to be invisible; and others, as M, L, E, A, at which it began to be visible again; and drawing a curve through; them, he found that it was elliptical; and, with refpect to his own eye, the dimenfions of it were as follows; OC was 23, AC 10, BD 3, DH 13, and EG 14; fo that the centre being at E, the greater axis was to the lefs as 8 to 7. From thefe data, the plane on which the figure was drawn being obliquely fituated with respect to the eye, he found, that the place in the eye that corref ponded to it was a circle, the diameter of which was a feventh part of the diameter of the eye, the centre of it being 27 parts of the diameter from the point oppofite to the pupil, a little above the middle. He concludes that, in order that this space in which there is no vifion may be as fmall as poffible, it is neceffary that the nerve should enter the eye perpendicularly, and that both this end, and alfo its entering the eye at a distance from its axis, are gained by the particular manner in which the two optic nerves unite and become separate again, by croffing one another. In favour of one of the obfervations of Mr Michell, concerning the use of the choroides in vifion, Dr Priestley obferves, that Aquapendente mentions the cafe of a perfon at Pifa, who could fee very well in the night, but very little or none in the day-time. This is alfo faid to be the cafe with thofe white people among the blacks of Africa, and the inha bitants of the ifthmus of America, who, from this circumftance, are called moon-eyed. Our author thinks it probable that their choreides is not of a dark colour, as it is in others of the human species; but white or light coloured, as in those animals which have moft occafion for their eyes in the night. See ALBINOS.

325. But the following confiderations are ftill urged, in favour of the retina being the proper feat of vifion. HALLER obferves, that the cho. roides cannot be the univerfal inftrument of vifion, because that fometimes in men and birds, but efpecially in fishes, it is covered internally with a black mucus, through which the rays cannot pe, netrate. He speaks of a fibrous membrane in the retina diftinct from its pulpy fubftance. On these fibres, he conjectures, that the images of objects are painted. M. De la Hire's argument in favour of the retina, from the analogy of the fenfes, is much strengthened by confidering, that the retina is a large nervous apparatus, immediately expofed to the impreffion of light; whereas the choroides receives but a flender fupply of nerves, in common

with the fclerotica, the conjunctiva, and the eyelids; and that its nerves are much lefs exposed to the light than the naked fibres of the optic nerve. Indeed, from anatomical confiderations, one might imagine that any other part of the body was as fenfible of the impreffion of light as the choroides.

326. That the optic nerve is of principal ufe in vifion, is farther probable, from several phenomena attending fome of the difeafes in which the fight is affected. When an amaurofis has affected one eye only, the optic nerve of that eye has been found manifeftly altered from its found state. Dr Priestley was prefent when Mr HEY examined the brain of a young girl, who had been blind of one eye, and saw that the optic nerve belonging to it was confiderably smaller than the other; and he informed him, that, upon cutting into it, he found it to be much harder, and cineritious. As the optic nerve is folely fpent in forming the retina, fo no function of the eye, not immediately fubfervient to vifion, is affected by an amaurofis. On the contrary, those nerves which go to the choroides are found to retain, in this disease, their natural influence. The iris will contract in a recent gutta ferena of one eye if the other remains found, and is fuddenly expofed to a ftrong light. The fclerotis, conjunctiva, and eye-lids, which receive their nerves from the fame branches as the choroides, retain their sensibility in this disorder. The manner in which perfons recover from an amaurofis favours the fuppofition of the feat of vision being in the retina; fince thofe parts which are the most diftant from the infertion of the nerve recover their fenfibility the fooneft, being in thofe places the moft pulpy and fofteft; whereas there is no reason to think that there is any difference in this refpect in the different parts of the choroides. Mr Hey has been repeatedly informed, by perfons labouring under an imperfect amaurofis, or gutta ferena, that they could not, when looking at any object with one eye, fee it for diftinctly when it was placed directly oppofite to the pupil, as when it was fituated fomewhat obliquely. And thofe perfons whom he had known to recover from a perfect amaurofis, firft discovered the objects whofe images fell upon that part of the retina which is at the greateft diftance from the optic nerve.

327. On the whole, if the retina be as tranfparent as it is generally reprefented to be, fo that the termination of the pencils must neceffarily be either upon the choroides, or fome other opaque fubftance interposed between it and the retina, the action and reaction occafioned by the rays of light being at the common furface of this body and the retina, both these mediums (fuppofing them to be equally fenfible to the impreffion of light) may be equally affected; but the retina, being naturally much more fenfible to this kind of impreffion, may be the only inftrument by which the fenfation is conveyed to the brain, though the choroides, or the black fubftance with which it is fometimes lined, may also be neceffary for the purpose of vision. Indeed, when the reflection of the light is made at the common boundary of any two mediums, it is with no propriety that this effect is afcribed to one of them rather than the other; and the ftrongeft reflections are often made back into the denfeft mediums, when they have been conti

gucus

guous to the rareft, or even to a vacuum.

This is not far from the hypothesis of M. de la Hire, and will completely account for the entire defect of vifion at the infertion of the optic nerve.

328. VISION is distinguished into bright and obfeure, diftin&t and confused.—It is said to be bright, when a fufficient number of rays enter the pupil at the fame time; obfcure, when too few. It is distinct when each pencil of rays is collected into a focus exactly upon the retina; confufed, when they meet before they come at it, or when they would pass it before they meet; for, in either of thefe laft cafes, the rays flowing from different parts of the object will fall upon the fame part of the retina, which muft neceffarily render the image confused and indiftinct. Now, that objects may appear with a due brightness, whether more or fewer rays proceed from them, we have a power of contracting or dilating the pupil, by means of the muscular fibres of the iris, in order to take in more or fewer rays as occafion requires. But this power has its limits. In fome animals it is much greater than in others; particularly in fuch as are obliged to seek their food by night as well as by day, as in cats, &c.

329. That the rays may be collected into points exactly upon the retina, that is, that objects may appear diftinct, whether they be nearer or farther off, i. e. whether the rays proceeding from them diverge more or lefs, we have a power of contracting or relaxing the ligamenta ciliaria, and thereby altering the form of the cryftalline humour, and with it the focal diftance of the rays. Thus, when the object we view is far off, and the rays fall upon the pupil with a very fmall degree of divergency, we contract the ligamenta ciliaria, which, being concave towards the vitreous humour, do thereby comprefs it more than otherwife they would do; by which means it is made to prefs harder upon the back fide of the cryftalline humour, which is thereby rendered flatter; and thus the rays proceed farther before they meet in a focus, than otherwise they would have done. Add to this, that we dilate the pupils of our eyes (unlefs in cafes where the light is fo ftrong that it offends the eye), and thereby admit rays into them that are more diverging than those which would otherwise enter. And, when the rays come from an object that is very near, and therefore diverge too much to be collected into their refpective foci upon the retina, by relaxing the ligamenta ciliaria, we give the cryftalline a more convex form, by which means the rays are made to fuffer a proportionably greater degree of refraction in paffing through it. Befides this, by contracting the pupils of our eyes, we exclude the more diverging rays, and admit only fuch as are more eafily refracted into their respective foci. But vifion is not diftinct at all distances, for our power of contract ing and relaxing the ligamenta ciliaria is alfo circumfcribed within certain limits.

330. In thofe eyes where the tunica cornea is very protuberant and convex, the rays of light fuffer a very confiderable refraction at their entrance into the aqueous humour, and are therefore collected into a focus before they fall upon the retina, unless the object be placed very near, fo that the rays which enter the eye may have a VOL. XVI. PART II.

confiderable degree of divergency. People that have fuch eyes are said to be purblind. Now, the nearer an object is to the eye, the greater is the image of it therein, as explained above: thefe people, therefore, can see much faller objects than others, as feeing much nearer ones with the fame diftinctnefs; and their light continues good longer than that of other people, because the tunica cornea of their eyes, as they grow old, becomes plainer, for want of that redundancy of humours with which they were filled before. On the contrary, old men having the cornea of their eyes too flat for want of a fufficient quantity of the aqueous humour to fill them out, if the rays diverge too much before they enter the eye, they cannot be brought to a focus before they reach the retina; on which account those people cannot fee distinctly, unless the object be fituated at a greater diftance from the eye than is required for thofe whofe eyes are of a due form. The latter require the affiftance of convex glaffes to make them fee objects diftinctly; the former of concave ones. For if either the cornea a b c (fig. 4. Pl. CCLV.) or cryftalline humour e, or both, be too flat, as in the eye A, their focus will not be on the retina as at A, where it ought to be, in order to render vifion diftinct, but beyond the eye as at f. This is remedied by placing a convex glass g h before the eye, which makes the rays converge fooner, and imprints the image duly on the retina at d. Again, if either the cornea or cryftalline humours, or both, be too convex, as in the eye B, the rays that enter in from the object C will be converged to a focus in the vitreous humour, as at ƒ; and by diverging from thence to the retina, will form a very confused image thereon; and fo, of course, the obferver will have as confufed a view of the object as if his eye had been too flat. This inconvenience is remedied by placing a concave glass g h before the eye; which glafs, by caufing the rays to diverge between it and the eye, lengthens the focal diftance fo, that if the glafs be properly chofen, the rays will unite at the retina, and form a diftinct image of the object upon it.

331. Such eyes as are of a due convexity, cannot fee any object diftin&tly at lefs diftance than fix inches; and there are numberless objects too fmall to be feen at that distance, because they cannot appear under any fenfible angle.--Concerning the leaft angle under which any object is vifible, there was a debate between Dr HOOKE and HEVELIUS. The former afferted, that no object could well be feen if it fubtended an angle lefs than one minute; and, if the object be round, as a black circular spot upon a white ground, or a white circle upon a black ground, it follows, from an experiment made by Dr SMITH, that this is near the truth; and thence he calculates, that the diameter of the picture of fuch leaft vifible point upon the retina is the 8000th part of an inch; which he therefore calls à fenfible point of the retina. On the other hand, Mr COURTIVRON concluded from his experiments, that the smallest angle of vifion was 40 feconds. According to Dr Jurin, there are cafes in which a much smaller angle than one minute can be difcerned by the eye; and in order to our perceiving the impreffion made

Bbb

by

by any object upon our fenfes, it must either be of a certain degree of force, or of a certain degree of magnitude. For this reafon, a ftar, which appears only as a lucid point through a telescope, fubtending not fo much as an angle of one fecond, is visible to the eye; though a white or black spot, of 25" or 30", is not to be perceived. Alfo a line of the fame breath with the circular spot will be vifible, at fuch a distance as the spot is not to be perceived at; because the quantity of impreflion from the line is much greater than from the fpot; and a longer line is visible at a greater diftance than a fhorter one of the fame breadth. He found by experience, that a filver wire could be feen when it fubtended an angle of 3"; and that a filk thread could be feen when it fubtended an angle of 24".

332. This greater vifibility of a line than of a fpot feems to arife only from the greater quantity of the impreffion; but without the limits of perfect vifion, our author obferves, that another caufe concurs, whereby the difference of vifibility between the spot and the line is rendered much more confiderable. For the impreffion upon the retina made by the line is then not only much greater, but also much stronger, than that of the fpot; because the faint image, or penumbra, of any one point of the line, when the whole is placed beyond the limits of diftinct vision, will fall within the faint image of the next point, and thereby much increase the light that comes from it. In fome cafes, our author found the caufe of indiftinct vifion to be the unfteadiness of the eye; as our being able to see a single black line upon a black ground, and not a white line between two black ones on a white ground. By trying an experiment with two pins of known diameters fet in a window against the fky light, with a space between them equal in breadth to one of the pins, he found that the diftance between the pins could hardly be distinguished, when it fubtended an angle of less than 40', though one of the pins alone could be distinguished when it fubtended a much lefs angle. But though a space between two pins cannot be diftinguished by the eye when it fubtends an angle lefs than 40", it would be a miftake to think that the eye muft neceffarily commit an error of 40" in eftimating the distance between two pins when they are much farther from one another. For if the space between them fubtend an angle of one minute, and each of the pins fubtend an angle of 4", which is greater than the leaft angle the eye can diftinguish, it is manifeft that the eye may judge of the place of each pin within two feconds at the moft; and confequently the error committed in taking the angle between them cannot at the most exceed 4", provided the inftrument be fufficiently exa&t. A black fpot upon a white ground, or a white fpot upon a black ground, he fays, can hardly be perceived by the generality of eyes when it fubtends a lefs angle than one minute. And if two black spots be made upon white paper, with a space between them equal in breadth to one of their diameters, that fpace is not to be diftinguifhed, even within the limits of perfect vifion, under fo fmall an angle as a fingle fpot of the fame fize can be diftinguished. To see the two fpots diftinctly,

therefore, the breadth of the space between them muft fubtend an angle of more than a minute. So that an eye which cannot perceive a fingle obje& under a fmaller angle than one minute, will not perceive the interval between two such objects under a lefs angle than 75 seconds.

333. Without the limits of perfect vifion, the diftance at which a fingle object ceases to be perceivable will be much greater in proportion than the diftance at which a space of equal breadth be tween two fuch objects ceafes to be perceivable. For, without thefe limits, the image of each of the objects will be attended with a penumbra, and the penumbra of the two near objects will take up part of the space between them, and thereby render it lefs perceivable; but the penumbra will add to the breadth of the fingle object, and will thereby make it more perceivable, unless its image be very faint. Upon the fame principles he likewife accounts for the radiation of the ftars, whereby the light seems to project from them different ways at the fame time.

334. Mr MAYER made many experiments to afcertain the smallest angle of vifion in a variety of refpects. He began with obferving at what dif tance a black spot was vifible on white paper; and found, that when it could barely be diftinguished, it fubtended an angle of about 34 feconds. When black lines were difpofed with intervals broader than themselves, they were dif tinguished at a greater diftance than they could be when the objects and the intervals were of an equal breadth. In all these cafes it made no difference whether the objects were placed in the fhade or in the ftrong light of the fun; but when the degrees of light were fmall, their differences had a confiderable effect, though by no means in proportion to the differences of the light. For if an object was illuminated to fuch a degree as to be juft vifible at the diftance of 9 feet, it would be visible at the diftance of 4 feet, though the light was diminished above 160 times. It appeared in the course of thefe experiments, that common day-light is, at a medium, equal to that of twenty-five candles placed one foot from the object.

335. As an image of every' vifible object is painted on the retina of each of our eyes, it thence becomes a natural queftion, Why we do not fee every thing double? It was the opinion of Sir ISAAC NEWTON and others, that objects appear fingle because the two optic nerves unite before they reach the brain. But Dr PORTERFIELD fhows, from the observations of several anatomifts, that the optic nerves do not mix or contound their fubftance being only united by a clofe cohefion: and objects have appeared fingle where the optic nerves were found to be disjoined. Dr BRIGGS fuppofed that fingle vifion was owing to the equal tenfion of the correfponding parts of the optic nerves, whereby they vibrated in a fynchronous manner. But Dr Porterfield fhows that facts by no means favour this hypothefis. To account for this phenomenon, he fuppofes, that by an original law in our nature, we imagine objects to be fituated fomewhere in a right line drawn from the picture of it upon the retina, through the centre of the pupil: confe

quently,