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Application of the Argument

This is atheism: for every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature; with the difference, on the side of nature, of being greater and more, and that in a degree which exceeds all computation. I mean, that the contrivances of nature surpass the contrivances of art, in the complexity, subtilty, and curiosity of the mechanism; and still more, if possible, do they go beyond them in number and variety: yet less evidently contrivances, not less evidently accommodated to their end, or suited to their office, than are the most perfect productions of human ingenuity.

I know no better method of introducing so large a subject, than that of comparing a single thing with a single thing; an eye, for example, with a telescope. As far as the examination of the instrument goes, there is precisely the same proof that the eye was made for vision, as there is that the telescope was made for assisting it. They are made upon the same principles; both being adjusted to the laws by which the transmission and refraction of rays of light are regulated. I speak not of the origin of the laws themselves; but such laws being fixed, the construction, in both cases, is adapted to them. For instance; these laws require, in order to produce the same effect, that the rays of light, in passing from water into the eye, should be refracted by a more convex surface than when it passes out of air into the eye. Accordingly we find, that the eye of a fish, in that part of it called the crystalline lens, is much rounder than the eye of terrestrial animals. [Plate II. fig. 1.] What plainer manifestation of design can there be than this difference? What could a mathematical instrument-maker have done more, to show his knowledge of his principle, his application of that knowledge, his suiting of his means to his end; I will not say to display the compass or excellence of his skill and art, for in these all comparison is indecorous, but to testify counsel, choice, consideration, purpose?

To some it may appear a difference sufficient to destroy all similitude between the eye and the telescope, that the one is a perceiving organ, the other an unperceiving instrument. 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 circumstance varies not the analogy at all. For, observe what constitution of the eye is. [Plate II. fig. 2.] It is necessary, in order to produce distinct vision, that an image or picture of the object be formed at the bottom of the eye. Whence this necessity arises, or how the picture is connected with the sensation, or contributes to it, it may be difficult, nay we will confess, if you please, impossible for us to search out. But the present question is not concerned in the inquiry. It may be true, that, in this, and in other instances, we trace mechanical contrivance a certain way; and that then we come to something which is not mechanical, or which is inscrutable. But this affects not the certainty of our investigation, as far as we have gone. The difference between an animal and an automatic statue, consists in this,—that, in the animal, we trace the mechanism to a certain point, and then we are stopped; either the mechanism becoming too subtile for our discernment, or something else beside the known laws of mechanism 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 case as in the other. In the example before us, it is a matter of certainty, because it is a matter which experience and observation demonstrate, that the formation of an image at the bottom of the eye is necessary to perfect vision. The image itself can be shown. Whatever affects the distinctness of the image, affects the distinctness of the vision. The formation then of such an image being necessary (no matter how) to the sense of sight, and to the exercise of that sense, the apparatus by which it is formed is constructed and put together, not only with infinitely more are, but upon the selfsame principle of art, as in the telescope or the camera obscure. The perception arising from the image may be laid out of the question; for the production of the image, these are instruments of the same kind. The end is the same; the means are the same. The purpose in both is alike1. The lenses of the telescope, [Plate II. fig. 3, 4.] and the humours of the eye, bear a complete resemblance to one another, in their figure, their position, and in their power over the rays of light, viz. in bringing each pencil to a point at the right distance from the lens; namely, in the eye, at the exact place where the membrane is spread to receive it. How is it possible, under circumstances of such 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 plainest and clearest of all propositions, in the other?

The resemblance between the two cases is still more accurate, and obtains in more points than we have yet represented, or than we are, on the first view of the subject, aware of. In dioptric telescopes there is an imperfection of this nature. Pencils of light, in passing through glass lenses, are separated into different colors, thereby tinging the object, especially the edges of it, as if it were viewed through a prism. To correct this inconvenience had been long a desideratum in the art. At last it came into the mind of a sagacious optician, to inquire how this matter was managed in the eye; in which there was exactly the same difficulty to contend with as in the telescope. His observation taught him, that, in the eye, the evil was cured by combining lenses composed of different substances, i.e. of substances which possessed different refracting powers. Our artist borrowed thence his hint, and produced a correction of the defect by imitating, in glasses 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 purpose2?

But farther; there are other points, not so much perhaps of strict resemblance between the two as of superiority of the eye over the telescope, which being found in the laws that regulate both, may furnish topics of fair and just comparison. Two things were wanted, to the eye, which were not wanted (at least in the same degree) to the telescope: and these were the adaptation of the organ, first, to different degrees of light; and, secondly, to the vast diversity of distance at which objects are viewed by the naked eye, viz. from a few inches to as many miles. These difficulties present not themselves to the maker of the telescope. He wants all the light he can get; and he never directs his instrument to objects near at hand. In the eye, both these cases were to be provided for; and for the purpose of providing for them a subtile and appropriate mechanism is introduced:—

I. In order to exclude excess of light, when it is excessive, and to render objects visible under obscurer 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 same time. The chamber of the eye is a camera obscura3, which, when the light is too small, can enlarge its opening; when too strong, can again contract it; and that without any other assistance than that of its own exquisite machinery.  It is farther also, in the human subject, to be observed, that this hole in the eye, which we call the pupil, under all its different dimensions, retain its exact circular shape.  This is a structure extremely artificial. Let an artist only try to execute the same; he will find that his threads and strings must be disposed with great consideration and contrivance to make a circle, which shall continually change its diameter, yet preserve its form. This is done in the eye by an application of fibres, i.e. of strings, similar, in their position and action, to what an artist would and must employ, if he had the same piece of workmanship to perform4. [Plate II. Fig. 5 & 6.]

1The comparison with the lens of the telescope is not perfectly exact for the crystalline lens is a substance composed of concentric layers, of unequal density, the hardness of which increases from the surface to the centres and hence possesses a more refractive power than any artificial lens.  Mr. Ramsden supposes that this texture tends to correct the aberration occasioned by the spherical form of the cornea, and the focus of each oblique pencil of rays falls accurately on the concave surface of the retina—Paxton.
2“It does not appear that the hint of this discovery was taken by Mr. Dollond from the structure of the eye, as supposed by our author, but was obtained in a different manner. This circumstance does not however lessen the force of the reasoning. The principle thus applied in the construction of achromatic telescopes, has been since carried still farther, and in its new application, illustrates more strongly, if possible, the point so well insisted on by Dr. Paley, namely, the resemblance between the eye and our optical instruments. In the best achromatic telescopes, composed of the different kinds of glass, according to the discovery of Mr. Dollond, white or luminous objects are not shown perfectly free from color, their edges being tinged on one side with a claret colored, and on the other with a greenish fringe. This remaining imperfection has been got rid of by the combination of solid and fluid lenses in the object and eye-glasses of telescopes. For this beautiful discovery science is indebted to Dr. Blair of Edinburgh, who found that by placing a concave lens of muriatic acid with a metallic solution, between two convex lenses of glass, a combined lens was formed which refracted rays with perfect regularity and equality. A lens like this has been used with great advantage. The most important point is, however, to consider this improvement in tits application to the argument, and it will be seen how much nearer this construction brings the telescope to the eye. In Dollond’s telescope there is a combination of  solid lenses of different substances.—In Blair’s, a combination of fluid and solid; which is exactly the case in the human eye. The only difference is, that in the eye there is a solid lens between two fluid ones; and in the telescope a fluid between two solid. The combination is closely similar, and the final cause in both probably the same, namely, to correct the unequal refraction of light.”-See Edinburgh Journal of Science, No. viii. p. 212: and Library of Useful Knowledge, No 1& 12. [Ed.
3As the rays of light flowing from all the points of an object through the pupil of the eye, by the refraction of the lens and humours of the eye, form an exact representation at the bottom of the  eye on the retina; so the camera obscura, by means of a lens refracting the rays, exhibits a picture of the scene before it on the opposite wall.—Paxton.
4Some eminent anatomists have doubted the muscularity of the iris and have given very different explanations of its motions, attributing the contraction and dilation either to the varied impulse of the blood in its vessels, or to its own vita propria. The enlightened physiologist Magendie affirms, that the latest researches upon the anatomy of the iris proves its muscular structure, and that it is composed of two layers of fibres, the external, Plate II. (Fig. 5.) radiated, which dilate the pupil, the other fibres appear to be supported by a species of ring, which each of the radiated fibres contribute to form, and in which they slide during the alternate contractions and relaxations of the pupil.—Paxton.

There is a curious circumstance in the way in which light produces the contraction of the opening of the iris, which strengthens very much the argument derived from design manifested in its structure and adaptation to its purpose. The object of the iris, it is to be observed, has reference to the quantity of light to be admitted upon the retina or expansion of the optic nerve. It is the state of the retina then which regulates the motions of the iris,  and it is the action of the light on the retina which causes those motions and not its action upon the iris itself. This has been shown by a very delicate experiment. If a ray of light be accurately thrown in such a direction, that it shall fall upon the circle of the iris itself, and not pass through its aperture, no contraction of the aperture takes place; but if it be so thrown as to pass through the aperture, and fall upon the retina without touching the iris at all, still a contraction of the iris immediately takes place. So that light upon the iris alone occasions no contraction, although it is the part which really contracts when the same light falls upon a distant part. The design here is too obvious to need being enlarged upon. How could the iris acquire the power of contracting when light falls on another membrane, for the protection of that membrane? although it does not contract when the light falls upon itself alone?—[Ed.

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