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passing through them, are affected in like manner as in passing Ithrough a convex lens ; and therefore, to understand perfectly thenature of vision, we must first be acquainted with the doctrine oflight and lenses. See Optics. Now as the several pencils of raysflowing from the distinct points in the surface of an object placedbefore a lens are collected into r.o many points at a certain dis-tance, on the other side of the lens, and form an image'there, whenreceived upon white paper, so pencils of rays proceeding from anobject placed before the eye, at a proper distance from it, amibeing retracted in passing through the humours of it, are collectedinto their respective foci upon the retina, where they form a repre-sentation of that object; and by their impulses upon the tendernerves of the retina, an idea of the object is excited in the mind.See Refraction , and Optics. Vision is distinguished into brightand obscure, distinct and confused. It is said to be bright, whena sufficient number of rays enter the pupil at the same time ; andobscure, when too few ; it is called distinct, when each pencil ofrays is collected into a focus, exactly upon the retina; and con-fused, when they meet before they come at it, or when theywould pass it before they meet. Now since parallel rays only havetheir focus upon, or meet in, the retina, they alone can paintthere a distinct image of an object. If, therefore, the objectbe so near, that the rays from any particular point come diverg-ing to the pupil, they will necessarily require a greater focal dis-tance than the retina, or bottom of the eye ; whence the rays notbeing united upon the retina, that point cannot lie 1 here distinctlyrepresented, but will appear confused : thus, A15, All, (PlateCLXXX. fig. I,) are two parallel ray: falling upon the pupil ofthe eye; and CB, CB, two other rays, which, though really di-verging, yet, by reason of the remoteness of the point C, whence* they proceed, will at the entrance of the eye be so nearly coinci-dent with the parallel rays, as to have nearly the same focal pointon the retina, so that the point C will there be distinctly repre-sented by c. But if any other point E, be viewed very near theeye, so that the angles EBA, which they contain, with the parallelrays, be very considerable, they will, after refraction, tend to-wards the point f, in the axis of the eye produced, and upon theretina will represent only a circular indistinct area ; like that at e,whose breadth is equal to ab, the distance of the rays upon theretina. The same point at D, will not be quite so much dilated ;as the rays DB, DB, have a less degree of divergence. It isfound, by experience, that the nearest limit of distinct vision isabout six inches from the eye ; for if a book be held nearer to theeye than that, the letters and lines will immediately become con-fused and indistinct. Now this cause of indistinct vision may bein some measure remedied, by lessening the pupils, which we na-turally do in looking at near objects, by contracting the annularfibres of the uvea; and artificially, by looking through a smallhole made with a pin in a card, &c. for then a small print may he Iread much nearer than otherwise: the reason is plain, for the less Ithe diameter of the aperture or pupil 15B, the less will the raysdiverge in coming from D or E, or the more nearly will they co-incide with parallel rays. Besides the contraction of the pupil,nature has furnished the eye with a faculty of adapting the confor-mation of the several parts to the respective positions of objects,as they are nigh or more remote; for this purpose, the cornea isof an elastic jielding substance, and the crystalline is inclosedwith a little water in its capsula, that by the contraction and re-laxation of the ciliary ligament, the convexity of both the surfacesof the capsula may be a little altered, and perhaps the position ofthe crystalline ; by which means the distance from the retina maybe fitted and adjusted to nigli objects, so as to have their imagesvery distinctly formed upon the retina. Nigh objects only havebeen mentioned (by which are meant sucli as are near the limit ofdistinct vision, as between six and a hundred inches distance) be-cause objects more remote require scarcely any change of the con-formation of the eye; the focal distance in them varying so verylitlle. Tims, suppose all the refraction of the eye were equivalentto that of a double and equally convex lens, whose radius r = 1inch; if then the object were 10 inches distant, or d, = 10, we

dr

should have lire focal distance s =-= 0.11111: and

if another object be distant 100 inches, then d= 100, and/=

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0.10101. The difference between these two

focal distances is 0 0101, viz. the hundredth part of an inch, whichtlie eye can easily provide against. If we go beyond this, sup-

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pose to an object 1000 inches distant, we have f—~2 -—

0.1001001, which is only the thousandth part of an inch less thanthe former, and is therefore inconsiderable. Now rays proceedingfrom any point more than sjx inches distant from the eye, will,when they enter the pupil, be very nearly coincident with parallelrays; and therefore, to a sound eye, distinct visidn cannot beeffected at less than six or eight inches distance, as may be foundby any one who will make the experiment. Six inches, therefore,constitutes the limit of distinct vision for near objects. We shallnow consider the limit for remote objects: for objects may ap-pear indistinct and confused, by being removed too far from theeye, as well as when they are too near it. And in this case wefind objects will appear distinct so long as their parts are separateand distinct in the image formed on the retina; and those partswill be separate so long as the axes of the pencils of rays, whichpaint them, are separate at their incidence on the retina; that is,so long as the angle they contain is not less than one-tenth of a de-gree : for it is found, by experience, that objects and their partsbecome indistinct, when the angle they subtend at the pupil oftlie eye is less than that quantity. Thus, suppose OB, (fig. 2,) tohe a circle of one-tenth of an inch in diameter, it will appear dis-tinct with its central spot, till you recede to the distance of sixfeet from it, and then it becomes confused ; and if it be one-fifth of an inch, it will begin to be confused at twelve feet distance,and so on ; in which cases the angle subtended at the eye, viz.OAB, is about oue-tenth of a degree, or six minutes. And thusall objects, as they are bigger, appear distinct at a greater dis-tance; a small print will become confused at a less distance thana larger; and in a map of England, the names of places in smallletters become first indistinct, where those in capital are veryplain and legible ; at a great distance these become confused,while tlie several counties appear well defined to a much greaterdistance ; these also at last become so indistinct as not to beknown one from another, when at the same time the whole islandpreserves its form very distinctly, to a very great distance; whichmay be so far increased, that it also, at last, will appear but a confus-ed and unmeaning spot. We have seen the causes of indistinct vi-sion in the objects, and shall now inquire what may produce thesame in the eye itself. And first it is to be observed, that there isa proper degree of convexity in the cornea KPL, (fig. 3,) andcrystalline ST, for converging parallel rays to a focus on the bot-tom of tlie eye, in a sound state: hence every distant object OB,will have its image IM, accurately depicted on tlie retina, and bythat means produce distinct vision. But if the cornea KPL,(fig. 4,) or crystalline ST, or both, should chance to be a littlemore convex than jusi, it will cause tlie pencil of rays oCo, whichcomes to tlie pupil oo, from anv point C in tlie object OB, tounite in a focus before they arrive at the retina in the bottom ofthe eye; the image IM, of tlie object OB, will be formed in thebody of the vitreous humour, and will therefore be very confusedand indistinct on the retina at im. A person having such an eye,is called myops, in allusion to tlie eye of a mouse, by reason of itsgreat convexity. To remedy this defect of the eye, a concavelens EF, is applied before it ; for by this means the rays C a, Cb,which fall diverging on the lens, will, after reflection through it,he made to proceed still more diverging, viz. in the directions ur,br, (instead of ao, bo,) as if they catnefrom the point c, instead ofC. Hence it follows, that since the rays are made to fall withgreater divergence upon tlie eye, they will require a greater focaldistance to be united in the axis ; and, consequently, the focusmay be made to fall very nicely on the retina, by using a lens EF,of a proper degree of concavity ; so that distinct vision will beeffected in tlie same manner as in an eye of a just conformation.Since tlie point c, is nearer to tin: eye than the point C, the appa-rent place of objects seen through a concave lens is nearer thantlie true place; or the object will appear at OB, instead of OB 5and also, since converging rays O a, B b, proceed less convergingafter refraction than before, the object appears under a less an-

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