and we

as incapable of distinction as the sounds were, see the object single. But when they have not the same visible place, they are perfectly distinguishable, and we see the object double. We see the object single only, when the eyes are directed in one particular manner; while there are many other ways of directing them within the sphere of our power, by which we see the object double.

Dr. Smith justly attributes to custom that well-known fallacy in feeling, whereby a button pressed with two opposite sides of two contiguous fingers laid across, is felt double. I agree with him, that the cause of this appearance is, that those opposite sides of the fingers have never been used to feel the same object, but two different objects, at the same time. And I beg leave to add, that, as custom produces this phænomenon, so a contrary custom destroys it; for, if a man frequently accustoms himself to feel the button with his fingers across, it will at last be felt single; as I have found by experience.

It may be taken for a general rule, that things which are produced by custom, may be undone or changed by disuse, or by a contrary custom. On the other hand, it is a strong argument, that an effect is not owing to custom, but to the constitution of nature, when a contrary custom, long continued, is found neither to change nor weaken it. I take this to be the best rule by which we can determine the question presently under consideration. I shall, therefore, mention two facts brought by Dr. Smith, to prove that the corresponding points of the retinæ have been changed by custom; and then I shall mention some facts tending to prove, that there are corresponding points of the retina of the eyes originally, and that custom produces no change in them.

"One fact is related upon the authority of Martin Folkes, Esq., who was informed by Dr. Hepburn of Lynn, that the Rev. Mr. Foster of Clinchwharton, in that neighbourhood, having been blind for some years of a

gutta serena, was restored to sight by salivation; and that, upon his first beginning to see, all objects appeared to him double; but afterwards, the two appearances approaching by degrees, he came at last to see single, and as distinctly as he did before he was blind."

Upon this case, I observe, First, That it does not prove any change of the corresponding points of the eyes, unless we suppose, what is not affirmed, that Mr. Foster directed his eyes to the object at first, when he saw double, with the same accuracy, and in the same manner, that he did afterwards, when he saw single. Secondly, If we should suppose this, no account can be given, why at first the two appearances should be seen at one certain angular distance rather than another; or why this angular distance should gradually decrease, until at last the appearances coincided. How could this effect be produced by custom? But, Thirdly, Every circumstance of this case may be accounted for on the supposition that Mr. Foster had corresponding points in the retina of his eyes from the time he began to see, and that custom made no change with regard to them. We need only further suppose, what is common in such cases, that, by some years' blindness, he had lost the habit of directing his eyes accurately to an object, and that he gradually recovered this habit when he came to see.

The second fact mentioned by Dr. Smith, is taken from Mr. Cheselden's " Anatomy," and is this:-"A gentleman who, from a blow on the head, had one eye distorted, found every object appear double; but, by degrees, the most familiar ones became single; and, in time, all objects became so without any amendment of the distortion."

I observe here, that it is not said that the two appearances gradually approached, and at last united, without any amendment of the distortion. This would indeed have been a decisive proof of a change in the corresponding points of the retina, and yet of such a change as

could not be accounted for from custom. Bu. this is not said; and, if it had been observed, a circumstance so remarkable would have been mentioned by Mr. Cheselden, as it was in the other case by Dr. Hepburn. We may, therefore, take it for granted, that one of the appearances vanished by degrees, without approaching to the other. And this I conceive might happen several ways. First, The sight of the distorted eye might gradually decay by the hurt; so the appearances presented by that eye would gradually vanish. Secondly, A small and unperceived change in the manner of directing the eyes, might occasion his not seeing the object with the distorted eye, as appears from § 15, Art. 10. Thirdly, By acquiring the habit of directing one and the same eye always to the object, the faint and oblique appearance presented by the other eye, might be so little attended to when it became familiar, as not to be perceived. One of these causes, or more of them concurring, might produce the effect mentioned, without any change of the corresponding points of the eyes.

For these reasons, the facts mentioned by Dr. Smith, although curious, seem not to be decisive.

The following facts ought to be put in the opposite scale. First, in the famous case of the young gentleman couched by Mr. Cheselden, after having had cataracts on both eyes until he was [above] thirteen years of age, it appears that he saw objects single from the time he began to see with both eyes. Mr. Cheselden's words are, "And now, being lately couched of his other eye, he says, that objects, at first, appeared large to this eye, but not so large as they did at first to the other; and, looking upon the same object with both eyes, he thought it looked about twice as large as with the first couched eye only, but not double, that we can anywise discover.'

Secondly, The three young gentlemen mentioned in the last section, who had squinted, as far as I know, from infancy, as soon as they learned to direct both eyes

In these four cases, it ap

to an object, saw it single. pears evident that the centres of the retina corresponded originally, and before custom could produce any such effect; for Mr. Cheselden's young gentleman had never been accustomed to see at all before he was couched; and the other three had never been accustomed to direct the axes of both eyes to the object.

Thirdly, from the facts recited in § 13, it appears, that, from the time we are capable of observing the phanomena of single and double vision, custom makes no change in them.

I have amused myself with such observations for more than thirty years; and in every case wherein I saw the object double at first, I see it so to this day, notwithstanding the constant experience of its being single. In other cases, where I know there are two objects, there appears only one, after thousands of experiments.

Let a man look at a familiar object through a polyhedron, or multiplying-glass, every hour of his life, the number of visible appearances will be the same at last as at first; nor does any number of experiments, or length of time, make the least change.

Effects produced by habit, must vary according as the acts by which the habit is acquired are more or less frequent; but the phænomena of single and double vision are so invariable and uniform in all men, are so exactly regulated by mathematical rules, that I think we have good reason to conclude that they are not the effect of custom, but of fixed and immutable laws of nature.

Section XVIII.

OF DR. PORTERFIELD'S ACCOUNT OF SINGLE AND DOUBLE VISION.

Bishop Berkeley and Dr. Smith seem to attribute too much to custom in vision, Dr. Porterfield too little.

This ingenious writer thinks, that, by an original law

of our nature, antecedent to custom and experience, we perceive visible objects in their true place, not only as to their direction, but likewise as to their distance from the eye; and, therefore, he accounts for our seeing objects single, with two eyes, in this manner. Having the faculty of perceiving the object with each eye in its true place, we must perceive it with both eyes in the same place; and, consequently, must perceive it sin gle.

He is aware that this principle, although it accounts for our seeing objects single with two eyes, yet does not at all account for our seeing objects double; and, whereas other writers on this subject take it to be a sufficient cause for double vision that we have two eyes, and only find it difficult to assign a cause for single vision, on the contrary, Dr. Porterfield's principle throws all the difficulty on the other side.

Therefore, in order to account for the phænomena of double vision, he advances another principle, without signifying whether he conceives it to be an original law of our nature, or the effect of custom. It is, That our natural perception of the distance of objects from the eye, is not extended to all the objects that fall within the field of vision, but limited to that which we directly look at ; and that the circumjacent objects, whatever be their real distance, are seen at the same distance with the object we look at; as if they were all in the surface of a sphere, whereof the eye is the centre.

Thus, single vision is accounted for by our seeing the true distance of an object which we look at; and double vision, by a false appearance of distance in objects which we do not directly look at.

We agree with this learned and ingenious author, that it is by a natural and original principle that we see visible objects in a certain direction from the eye, and honour him as the author of this discovery: but we cannot assent to either of those principles by which he ex