plains single and double vision-for the following rea

sons:

1. Our having a natural and original perception of the distance of objects from the eye, appears contrary to a well-attested fact: for the young gentleman couched by Mr. Cheselden imagined, at first, that whatever he saw touched his eye, as what he felt touched his hand.

2. The perception we have of the distance of objects from the eye, whether it be from nature or custom, is not so accurate and determined as is necessary to produce single vision. A mistake of the twentieth or thirtieth part of the distance of a small object, such as a pin, ought, according to Dr. Porterfield's hypothesis, to make it appear double. Very few can judge of the distance of a visible object with such accuracy. Yet we never find double vision produced by mistaking the distance of the object. There are many cases in vision, even with the naked eye, wherein we mistake the distance of an object by one half or more: why do we see such objects single? When I move my spectacles from my eyes toward a small object, two or three feet distant, the object seems to approach, so as to be seen at last at about half its real distance; but it is seen single at that apparent distance, as well as when we see it with the naked eye at its real distance. And when we look at an object with a binocular telescope, properly fitted to the eyes, we see it single, while it appears fifteen or twenty times nearer than it is. There are then few cases wherein the distance of an object from the eye is seen so accurately as is necessary for single vision, upon this hypothesis: this seems to be a conclusive argument against the account given of single vision. We find, likewise, that false judgments or fallacious appearances of the distance of an object, do not produce double vision: this seems to be a conclusive argument against the account given of double vision.

3. The perception we have of the linear distance of objects seems to be wholly the effect of experience,

This, I think, hath been proved by Bishop Berkeley and by Dr. Smith; and when we come to point out the means of judging of distance by sight, it will appear that they are all furnished by experience.

4. Supposing that, by a law of our nature, the distance of objects from the eye were perceived most accurately, as well as their direction, it will not follow that we must see the object single. Let us consider what means such a law of nature would furnish for resolving the question, Whether the objects of the two eyes are in one and the same place, and consequently are not two, but one?

Suppose then, two right lines, one drawn from the centre of one eye to its object, the other drawn, in like manner, from the centre of the other eye to its object. This law of nature gives us the direction or position of each of these right lines, and the length of each; and this is all that it gives. These are geometrical data, and we may learn from geometry what is determined by their means. Is it, then, determined by these data, Whether the two right lines terminate in one and the same point, or not? No, truly. order to determine this, we

In

We must know whether the

must have three other data. two right lines are in one plane; we must know what angle they make; and we must know the distance be tween the centres of the eyes. And when these things are known, we must apply the rules of trigonometry, before we can resolve the question, Whether the objects of the two eyes are in one and the same place; and, consequently, whether they are two or one?

5. That false appearance of distance into which double vision is resolved, cannot be the effect of custom, for constant experience contradicts it. Neither hath it the features of a law of nature, because it does not answer any good purpose, nor, indeed, any purpose at all, but to deceive us. But why should we seek for arguments, in a question concerning what appears to us, or does not appear? The question is, At what distance do the ob

jects now in my eye appear? Do they all appear at one distance, as if placed in the concave surface of a sphere, the eye being in the centre? Every man, surely, may know this with certainty; and, if he will but give attention to the testimony of his eyes, needs not ask a philosopher how visible objects appear to him. Now, it is very true, that, if I look up to a star in the heavens, the other stars that appear at the same time, do appear in this manner: yet this phænomenon does not favour Dr. Porterfield's hypothesis; for the stars and heavenly bodies do not appear at their true distances when we look directly to them, any more than when they are seen obliquely : and if this phænomenon be an argument for Dr. Porterfield's second principle, it must destroy the first.!

The true cause of this phænomenon will be given afterwards; therefore, setting it aside for the present, let us put another case. I sit in my room, and direct my eyes to the door, which appears to be about sixteen feet distant at the same time, I see many other objects faintly and obliquely-the floor, floor-cloth, the table which I write upon, papers, standish, candle, &c. Now, do all these objects appear at the same distance of sixteen feet? Upon the closest attention I find they do not.

Section XIX.

OF DR BRIGGS'S THEORY AND SIR ISAAC NEWTON'S CONJECTURE ON THIS SUBJECT.

I am afraid the reader, as well as the writer, is already tired of the subject of single and double vision. The multitude of theories advanced by authors of great name, and the multitude of facts, observed without sufficient skill in optics, or related without attention to the most material and decisive circumstances, have equally contributed to perplex it.

In order to bring it to some issue, I have, in the 13th section, given a more full and regular deduction than had been given heretofore, of the phænomena of single and double vision, in those whose sight is perfect; and have traced them up to one general principle, which appears to be a law of vision in human eyes that are perfect and in their natural state.

In the 14th section, I have made it appear, that this law of vision, although excellently adapted to the fabric of human eyes, cannot answer the purposes of vision in some other animals; and therefore, very probably is not common to all animals. The purpose of the 15th and 16th sections is, to inquire, Whether there be any deviation from this law of vision in those who squint?— a question which is of real importance in the medical art, as well as in the philosophy of vision; but which, after all that hath been observed and written on the subject, seems not to be ripe for a determination, for want of proper observations. Those who have had skill to make proper observations, have wanted opportunities; and those who have had opportunities, have wanted skill or attention. I have therefore thought it worth while to give a distinct account of the observations necessary for the determination of this question, and what conclusions may be drawn from the facts observed. I have likewise collected, and set in one view, the most conclusive facts that have occurred in authors, or have fallen under my own observation.

It must be confessed that these facts, when applied to the question in hand, make a very poor figure; and the gentlemen of the medical faculty are called upon, for the honour of their profession, and for the benefit of mankind, to add to them.

All the medical, and all the optical writers upon the strabismus that I have met with, except Dr. Jurin, either affirm, or take it for granted, that squinting persons see the object with both eyes, and yet see it single. Dr.

Jurin affirms that squinting persons never see the object with both eyes; and that, if they did, they would see it double. If the common opinion be true, the cure of a squint would be as pernicious to the sight of the patient, as the causing of a permanent squint would be to one who naturally had no squint; and therefore, no physician ought to attempt such a cure, no patient ought to submit to it. But, if Dr. Jurin's opinion be true, most young people that squint may cure themselves, by taking some pains; and may not only remove the deformity, but, at the same time, improve their sight. If the common opinion be true, the centres, and other points of the two retinæ, in squinting persons, do not correspond, as in other men, and Nature, in them, deviates from her common rule. But, if Dr. Jurin's opinion be true, there is reason to think that the same general law of vision, which we have found in perfect human eyes, extends also to those which squint.

It is impossible to determine, by reasoning, which of these opinions is true; or whether one may not be found true in same patients, and the other in others. Here, experience and observation are our only guides; and a deduction of instances is the only rational argument. It might, therefore, have been expected, that the patrons of the contrary opinions should have given instances in support of them that are clear and indisputable; but I have not found one such instance on either side of the question, in all the authors I have met with. I have given three instances from my own observation, in confirmation of Dr. Jurin's opinion, which admit of no doubt; and one which leans rather to the other opinion, but is dubious. And here I must leave the matter to further observation.

In the 17th section, I have endeavoured to shew that the correspondence and [or] sympathy of certain points. of the two retina, into which we have resolved all the phænomena of single and double vision, is not, as Dr.