selves, not to the manner in which they are excited. When we have been endowed with new faculties, faculties capable of dissecting and of analyzing the mind, of understanding the constitution and the fabric of spirit, then, and then only, will it cease to be a waste of time and of talent to engage in the pursuit of such subtilties. It must be much more useful and satisfactory to confine our inquiries to those operations which can be made the subject of observation, and intelligibly explain the mode in which matter acts on matter, in order to produce those effects, the existence of which we know, but for the connexion of which with their cause we can look alone to the fiat of creative wisdom.

As we proceed to those organs formed for more extensive and more complicated application, greater difficulties are supposed to present themselves; and when arrived at the seat of vision, at that delicate instrument which enables man to traverse the heavens, and to distinguish and appreciate the minute and countless varieties which fill up the ample space over which it expatiates, the philosophers seem to imagine that something vastly more complicated must be found in its constitution; and, equally unable to find any marks of such intricate machinery in the eye, or to believe that the wondrous effects produced by its action can be produced by causes simple as those which produce sensation in the more limited organs, the inquiry has been given up as hopeless.

"The

A discovery interesting in itself, and highly esteemed by all late writers on the subject, and which indeed is referred to as of itself affording a demonstration of the theory of sight, has in my opinion been one of the chief causes of our general ignorance. sagacious Kepler," says Dr. Reid, "first made the noble discovery that distinct but inverted pictures of visible objects are formed upon the retina by the rays of light coming from the object," the rays, after being refracted by the cornea and crystalline, meeting in one point of the retina, and there "painting the colour of that point of the object from which they come.

Having thus assumed the fact of the existence of a picture on the retina, it was concluded that this picture must be instrumental in communicating the information received. An insuperable difficulty, however, immediately occurred, for no one could even conjecture how these pictures could in any degree produce the effect. Our celebrated philosopher already quoted, Dr. Reid, after noticing some of the conjectures upon the subject, and their unsoundness, expresses himself in several passages as follows: "Nor is there any probability," says he, "that the mind perceives the pictures on the retina. These pictures are no more objects of our perception than the brain is, or the optic nerve." Since the picture upon the retina, therefore, is neither itself seen by the mind, nor produces any impression upon the brain or sensorium which is seen by the

* Inquiry into the Human Mind, 4th edit. p. 256.

mind, nor makes any impression upon the mind that resembles the object, it may be still asked, how this picture upon the retina causes vision.* "In answer," Dr. Reid observes, "we must resolve this solely into a law of our constitution. We may form such pictures, by means of optical glasses, upon the hand, or upon any other part of the body, but they are not felt, nor do they produce any thing like vision. A picture on the retina is as little felt as one upon the hand, but it produces vision, for no other reason, that we know, but because it is destined by the wisdom of nature to this purpose."t This is rather a sovereign mode of disposing of the difficulty, and applicable only to first principles; but as there could be found no trace, either in the structure of the parts, or in the analogy of their functions, to explain the province of these pictures, there remained no alternative. "It is evident," says the Doctor, "that the pictures upon the retina are by the laws of nature a mean of vision, but in what way they accomplish their end we are totally ignorant." §

Inquiry into the Human Mind, 4th edit. p. 257. + P. 262.

† P. 254. § I should have thought it hardly possible that, to any one who had listened to these quotations, it could have remained doubtful whether Dr. Reid meant, by the pictures which he considered to be a mean of vision, the assemblage of rays of light as they pass through the retina, or reflected pictures, according to the ordinary meaning of the term, not passing through, but painted on, that portion of the nerve. As, however, some members of the Royal Society of Edinburgh did, when the paper was read, express a decided opinion that Dr. Reid had given precisely the same view of the subject that I have done, it seems necessary, in defence against such respectable opponents, to prove somewhat more at large what really was Dr. Reid's opinion. In doing this, it is unnecessary to enter into any disputation about the application of the term picture to a determinate assemblage of rays, not reflected, but passing through a transparent body. I am of opinion that such application of the term is improper; but that is of little moment; for as to the theory of pictures, as explained by Dr. Reid, there can be no doubt that the pictures they refer to are reflected pictures, and therefore with theirs at least my views can have no accordance. In one of the quotations inserted in this paper Dr. Reid says we may form such pictures by means of optical glasses on the hand. Can any one doubt that the Doctor here means a common reflected picture, not passing through the hand, but painted on its surface. The uniform expression of a picture painted on the retina affords proof equally conclusive. Had he referred to the assemblage of rays passing through the retina, he never would have called these a picture on the retina, but a picture in it, or passing through it. In what other sense than that of a reflected picture can we understand the following language: "Of all the organs of sense, the eye only, as far as we can discover, forms any kind of image of its object, and the images formed by the eye are not in the brain, but only in the bottom of the eye." (P. 256.) "By what law of nature is a picture upon the retina the means or occasion of my seeing an external object of the same figure and colour in a contrary position, and in a certain direction from the eye." (P. 260.) "Perhaps some readers will imagine that it is easier to conceive a law of nature by which we shall always see objects in the place in which they are, and in their true position, without having recourse to images on the retina. To this I answer, that nothing can be a law of nature which is contrary to fact." Not to multiply quotations too much, I only add one more a "We conclude that our seeing an object in that particular direction in which we do see it is not owing to any law of nature by which we are made to see it in the direction of the rays, either before their refraction in the eye, or after, but to a law of our nature by which we see the object in the direction of the right line that passeth from the picture of the object upon the retina to the centre of the eye.”

It certainly requires some hardihood to attack these formidable pictures; but I feel bold enough to denounce them as mere phantoms. I deny that any picture is painted on the human retina; and even in those animals whose nocturnal pursuits, requiring peculiar organization, have obtained for them a structure which will produce a picture (not on the retina, however, but on the choroides): it will, I trust, be made evident that the picture in these cases is but a passive accompaniment, not an active instrument in the production of sight. Nothing is to me more surprising than the universal and unhesitating credence that has been given to a supposition, which has never, and never could be, verified; but which, on the contrary, is in direct opposition to facts demonstrated, and inrolled amongst the fundamental principles of science. Dr. Reid, who seems to have felt doubtings upon the subject, states, with truth, "No man ever saw the pictures in his own eye, nor indeed the pictures in the eye of another, until it was taken out of the head and duly prepared." It was this passage which excited my scepticism; and I hesitated not to reject the hypothetical fact altogether, when I found that the Doctor ought not to have made any exception in his statement, no man having ever seen, or being able to see these pictures in the eye of another, more than in his own; for the preparation alluded to, and which does produce the capability of showing a picture, changes the subject of it from an eye to something else, to an instrument as different from an eye as a window is from a reflecting mirror.

What then is the evidence of the existence of these pictures? It is this: that if an eye be taken, and the sclerotic and choroid coats being removed, a piece of white paper, or any white substance, fitted to reflect the rays of light, be substituted for the retina, or placed behind it, then the picture of any object held opposite to the pupil will be seen distinctly painted on the paper. This is the demonstration on which the whole hypothesis is founded; and on examination it will appear palpably deficient in every quality of evidence to prove the conclusions drawn from it.

What is a picture? It is the reflection of the rays of light under

(P. 269.) I trust it is now manifest (for in the last passage the picture and the assemblage of rays are contrasted) that Dr. Reid's theory does introduce real reflected pictures painted on the retina. Indeed, so completely did Dr. Paley adopt this idea, that he compares the eye to a reflecting telescope, the images of the objects being painted in the same manner on both.

He

I have to make one remark more. Even were it still maintained that Dr. Reid means pictures passing through the retina; and were that point yielded, little would be gained; for certainly Dr. Reid acknowledges that he is altogether unac quainted with the mode by which these pictures become a mean of vision. has not ventured a conjecture on the subject, but resolves it, as the only resource, into a law of nature. It therefore remains to be shown how a theory which denies the existence of any pictures painted on the retina, and which offers an explana. tion (be it good or bad) of the mode in which the rays affect the optic nerve so as to afford the varied perceptions of dimension, figure, and colour, can be so iden tified with Dr. Reid's opinion, for he has no theory, as to differ only by a change of terms..

such an arrangement as to correspond exactly with the different parts of the body which the picture is intended to represent. The essential properties, therefore, of an instrument for reflecting pictures must be, first, that the rays emanating from the original shall be collected, so that they may impinge on the reflector in accurate, distinct, and corresponding figures and colours; and, secondly, that when they thus reach the focus where the image is to be painted, there be there a proper reflector to send all the rays back again, to produce in the spectator the perceptions of a picture. The first of these qualifications, that for refracting and concentrating the rays, the natural and the prepared eye both possess. These are the functions of the aqueous and the crystalline humours; and about the nature or extent of these functions there is no dispute. With regard to the second qualification, however, there is a most material difference between the natural eye and the eye prepared for this demonstration. In the natural eye there is no reflector. The retina itself is nearly, if not quite, transparent. In proof of this we have only to remember that the black appearance which distinguishes the pupil proceeds from the choroides which lies behind the retina; and that the retina is so transparent as not even to raise a cloud on the intensity of the black mantle behind it. But if the retina be really so transparent, it can be no reflector. There is the same difference between the retina and the white paper that distinguishes a piece of common glass from a steel speculum. What is demonstrated by the one can afford no conclusion as to the other. They are, in fact, in opposition to each other. A speculum in a telescope will show the picture of an object within the field of the instrument; but substitute for it a piece of transparent glass, and the picture vanishes; the rays, instead of being reflected, are transmitted. Let it not be said that, even in this case, the picture would be formed in the glass, though it would not be visible; for a picture does not exist by the mere assemblage of those rays, which, if reflected, would exhibit a picture. It is the reflection, and not the assemblage, which calls the picture into being; for if the rays be not reflected, but they pass on to the eye, then we see, not a picture, but the body itself. It is evident, then, that the retina of the natural eye

It may perhaps be objected that, although the retina be sufficiently transparent fully to bear out the argument against the pictures, it is not so transparent as here represented. Even when the eye is taken warm from the head of an animal, the transparency is found to be imperfect. In this case I beg to refer the decision to the eye in the living body, where unquestionably the transparency of the retina is so great as to render it invisible. Even in cases of hydrocephalus, when the pupil is so distended as to admit a considerable portion of light, and the retina remains invisible; and, according to my judgment, a more perfect transparency can scarcely be conceived than that which prevents us from perceiving, notwithstanding all the different membranes and liquors which intervene, that the black coating of the choroides is not situated on the external surface of the eye. I apprehend that the transparency of the eye diminishes much on the extinction of the vital energy; and, therefore, that the removal of it even from a living body would not afford a fair testimony, or such as could weigh at all against the evideuce afforded by the living eye itself.

does not reflect a picture in the manner in which it is reflected by the paper in the prepared state; and if there be demonstration in the case, it is a demonstration that the retina cannot possibly reflect a picture at all.

But it has been said by Dr. Priestley and others that, though no picture is formed on the retina, it is formed on the coat behind itthe choroides. A dilemma immediately occurred on this assumption sufficient to have startled the most hardy philosophers. By thus transferring the picture from the retina to the choroides, the optic nerve was discarded as an instrument of vision; for the retina is in fact the optic nerve. The transparency of that membrane, however, afforded too palpable an obstacle to its being considered capable of reflecting; and therefore these philosophers conceived that, as a picture must be found somewhere, and it could not be formed on the retina, it might be formed behind it. The difficulty, however, was equally insurmountable; for, to wave the manifest absurdity of excluding the optic nerve from the theory of vision, it is evident that the choroides is as incapable as the retina of reflecting objects so as to form a picture. The choroides at this part in many animals, as in man, is black-a pure black. Now why is it black? Is it not because it does not reflect the rays of light? Although it is black, or rather dark, before it absorbs any rays, it is not because it is dark that it does absorb them, but it is because it does absorb them that we perceive that it is black. Whatever, therefore, may be thought of the reflecting powers of the cat, the owl, and other animals which seek their prey in comparative darkness, clear it is that, with regard to the human eye, no picture can be formed either on the retina or choroides. By the one all the rays of light are transmitted; by the other, they are all absorbed.

Liberated from the trammels of this paralizing phantom, let us compare the perceptions which the eye is fitted to produce with the perceptions produced by the action of the kindred organs.

With regard to the anatomy of these parts, it is unnecessary for me to say much. The structure of the eye is generally known, and can scarcely be mistaken. Suffice it to observe that, after passing through the ball of the eye, and being collected, and concentrated in their passage, the rays of light impinge on the retina, which is the extremity of the optic nerve, reticulated on a thin membrane, and which forms a transparent screen, through which these rays are transmitted to the choroides, where they are all immediately absorbed. Our present inquiry is limited to the operation of the rays on the retina; for, as they afford no perception of vision previously to reaching that point, and are all absorbed by the choroides immediately on passing through it, the single question is, how can these rays affect the retina so as to produce vision?

Vision is composed of two things: the perception of figure or dimension, and the perception of colour. These are so distinct and unconnected with each other, that they ought to be reckoned different senses. There is certainly much less distinctive difference be