the telescope, that the one is a perceiving organ, the other an unperceiving inftrument. The fact is, that they are both instruments. And, as to the mechanism, at least as to mechanism being employed, and even as to the kind of it, this circumftance varies not the analogy at all. For obferve, what the conftitution of the eye is. It is neceffary, in order to produce distinct vision, that an image or picture of the object be formed at the bottom of the eye. Whence this neceffity arifes, or how the picture is connected with the fenfation, or contributes to it, it may be difficult, nay we will confefs, if you please, impoffible for us to fearch out. But the prefent question is not concerned in the enquiry. It may be true, that, in this, and in other instances, we trace mechanical contrivance a certain way; and that then we come to fomething which is not me. chanical, or which is infcrutable. But this affects not the certainty of our investigation, as far as we have gone. The difference between an animal and an automatic ftatue, confifts in this, that, in the animal, we trace the mechanism to a certain point, and then we are ftopped; either the mechanifm becoming too fubtile for our discernment, or something

elle befide the known laws of mechanifin taking place; whereas, in the automaton, for the comparatively few motions of which it is capable, we trace the mechanism throughout, But, up to the limit, the reasoning is as clear and certain in the one cafe as the other. In the example before us, it is a matter of certainty, because it is a matter which experience and obfervation demonstrate, that the formation of an image at the bottom of the eye is neceffary to perfect vifion. The image itself can be fhewn. Whatever affects the diftinctness of the image, affects the diftin&nels of the vilion. The formation then of fuch an image being neceffary (no matter how), to the sense of fight, and to the exercife of that sense, the apparatus by which it is formed is conftru&ed and put together, not only with infinitely more art, but upon the self-fame principles of art, as in the telescope or tl camera obfcura. The perception arifing from the image may be laid out of the queftion: for the production of the image, thefe are inftruments of the fame kind. The end is the fame; the means are the fame. The purpofe in both is alike; the contriva ce for accomplishing that purpose is in both alike. The lenfes of the telescope,

and

and the humours of the eye bear a complete resemblance to one another, in their figure, their pofition, and in their power over the rays of light, viz. in bringing each pencil to a point at the right distance from the lense; namely, in the eye, at the exact place where the membrane is spread to receive it. How is it poffible, under circumftances of fuch close affinity, and under the operation of equal evidence, to exclude contrivance from the one, yet to acknowledge the proof of contrivance having been employed, as the plaineft and clearest of all propofitions in the other?

The resemblance between the two cafes is ftill more accurate, and obtains in more points than we have yet represented, or than we are, on the first view of the fubject, aware of. In dioptric telescopes there is an imperfection of this nature. Pencils of light, in paffing through glafs lenfes, are separated into different colours, thereby tingeing the object, especially the edges of it, as if it were viewed through a prifm. To correct this inconvenience had been long a defideratum in the art. At laft it caine into the mind of a fagacious optician, to enquire how this matter was managed in the eye; in which there was exactly the fame difficulty to contend

C 4

contend with, as in the telescope. His obfervation taught him, that, in the eye, the evil was cured by combining together lenses compofed of different substances, i. e. of substances 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 suggested to the optician the only effectual means of attaining that purpose?

But further; there are other points, not fo ruch perhaps of ftrict 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 least in the fame degree, to the telescope; and these were, the adaptation of the organ, first, 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 miles.

6

miles. These difficulties prefent not themfelves to the maker of the telescope. He wants all the light he can get; and he never directs his inftrument to objects near at hand. In the eye, both thefe cafes were to be provided for; and for the purpose 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 strong, can again contract it; and that without any other affiftance than that of its own exquifite machinery. It is further alfo, in the human fubject, 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 ftructure extremely artificial. Let an artist only try to execute the fame. He will find that his threads and ftrings must be difpofed