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contend with, as in the telescope. His obfervation taught him, that, in the eye, the evil was cured by combining together lenfes compofed of different fubftances, i. e. of fubftances, which poffeffed different refracting powers. Our artist borrowed from thence his hint ; and produced a correction of the defect by imitating, in glaffes made from different materials, the effects of the different humours through which the rays of light pass before they reach the bottom of the eye. Could this be in the eye without purpose, which fuggefted to the optician the only effectual means of attaining that purpose ?

But further; there are other points, not fo much perhaps of strict resemblance between the two, as of fuperiority of the eye over the telescope; yet, of a fuperiority, which being founded in the laws that regulate both, may furnish topics of fair and juft comparison. Two things were wanted to the eye, which were not wanted, at leaft in the fame degree, to the telescope; and thefe were, the adaptation of the organ, firft, to different degrees of light; and, fecondly, to the vaft diversity of distance at which objects are viewed by the naked eye, viz. from a few inches to as many

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miles. These difficulties prefent not themfelves to the maker of the telescope. wants all the light he can get; and he never directs his inftrument to objects near at hand. In the eye, both these cafes were to be provided for; and for the purpofe of providing for them a fubtile and appropriate mechanism is introduced.

I. In order to exclude excess of light, when it is exceffive, and to render objects visible under obfcurer degrees of it, when no more. can be had; the hole or aperture in the eye, through which the light enters, is so formed, as to contract or dilate itself for the purpose of admitting a greater or less number of rays at the fame time. The chamber of the eye is a camera obfcura, which, when the light is too fmall, can enlarge its opening; when too ftrong, can again contract it; and that without any other affiftance than that of its own exquifite machinery. It is further also, in the human subject, to be observed, that this hole in the eye, which we call the pupil, under all its different dimenfions, retains its exact circular shape. This is a structure extremely artificial, Let an artist only try to execute the fame. He will find that his threads and ftrings must be difpofed

difpofed with great confideration and contrivance, to make a circle, which shall continually change its diameter, yet preferve its form. This is done in the eye by an application of fibres, i. e. of strings, fimilar, in their pofition and action, to what an artist would and must employ, if he had the fame piece of workmanship to perform.

II. The fecond difficulty which has been ftated, was the fuiting of the fame organ to the perception of objects that lie near at hand, within a few inches, we will fuppofe, of the eye, and of objects which were placed at a confiderable diftance from it, that, for example, of as many furlongs (I fpeak in both cafes of the diftance at which diftinct vifion can be exercised). Now, this, according to the principles of optics, that is, according to the laws by which the tranfmiffion of light is regulated, (and these laws are fixed,) could not be done, without the organ itself undergoing an alteration, and receiving an adjustment, that might correfpond with the exigency of the cafe, that is to fay, with the different inclination to one another under which the rays of light reached it. Rays iffuing from points placed at a small distance from the eye, and which confequently

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muft enter the eye in a spreading or diverging order, cannot, by the fame optical inftrument in the fame ftate, be brought to a point, i. e. be made to form an image, in the fame place with rays proceeding from objects fituated at a much greater diftance, and which rays arrive at the eye in directions nearly, and phyfically speaking, parallel. It requires a rounder lense to do it. The point of concourse behind the lenfe muft fall critically upon the retina, or the vifion is confufed; yet, this point, by the immutable properties of light, is carried further back, when the rays proceed from a near object, than when they are sent from one that is remote. A perfon, who was using an optical inftrument, would manage this matter by changing, as the occafion required, his lenfe or his telescope; or by adjusting the distance of his glaffes with his hand or his screw but how is it to be managed in the eye? What the alteration was, or in what part of the eye it took place, or by what means it was effected (for, if the known laws which govern the refraction of light be maintained, fome alteration in the state of the organ there must be), had long formed a fubject of enquiry and conjecture. The change, though fufficient

fufficient for the purpose, is fo minute as to clude ordinary obfervation. Some very late discoveries, deduced from a laborious and moft accurate inspection of the structure and operation of the organ, feem at length to have ascertained the mechanical alteration which the parts of the eye undergo. It is found, that by the action of certain muscles, called the ftraight muícles, and which action is the most advantageous that could be imagined for the purpose, it is found, I fay, that, whenever the eye is directed to a near object, three changes are produced in it at the same time, all severally contributing to the adjustment required. The cornea, or outermoft coat of the eye, is rendered more round and prominent; the cryftalline lenfe underneath is pushed forwards; and the axis of vision, as the depth of the eye is called, is elongated. These changes in the eye vary its power over the rays of light in fuch a manner and degree as to produce exactly the effect which is wanted, viz. the formation of an image upon the retina, whether the rays come to the eye in a state of divergency, which is the cafe when the object is near to the eye, or come parallel to one another, which is the cafe when the object is placed at a distance.

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