cles, whatever they be, by which the conformation of the eyes is varied according to the distance of objects.

It ought, however, to be observed, that, although it appears to be by natural instinct that both eyes are always turned the same way, there is still some latitude left for custom.

What we have said of the parallel motion of the eyes, is not to be understood so strictly as if nature directed us to keep their axes always precisely and mathematically parallel to each other. Indeed, although they are always nearly parallel, they hardly ever are exactly so. When we look at an object, the axes of the eyes meet in that object and, therefore, make an angle, which is always small, but will be greater or less, according as the object is nearer or more remote. Nature hath very wisely left us the power of varying the parallelism of our eyes a little, so that we can direct them to the same point, whether remote or near. This, no doubt, is learned by custom and accordingly we see, that it is a long time before children get this habit in perfection.

This power of varying the parallelism of the eyes is naturally no more than is sufficient for the purpose intended by it; but by much practice and straining, it may be increased. Accordingly, we see, that some have acquired the power of distorting their eyes into unnatural directions, as others have acquired the power of distorting their bodies into unnatural postures.

Those who have lost the sight of an eye, commonly lose what they had got by custom, in the direction of their eyes, but retain what they had by nature; that is, although their eyes turn and move always together, yet, when they look upon an object, the blind eye will often have a very small deviation from it; which is not perceived by a slight observer, but may be discerned by one accustomed to make exact observations in these matters.

Section XI.

OF OUR SEEING OBJECTS ERECT BY INVERTED IMAGES.

Another phænomenon which hath perplexed philosophers, is our seeing objects erect, when it is well known that their images or pictures upon the tunica retina of the eye are inverted.

The sagacious Kepler 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 same great philosopher demonstrated, from the principles of optics, how these pictures are formed-to wit, That the rays coming from any one point of the object, and falling upon the various parts of the pupil, are, by the cornea and crystalline, refracted so as to meet again in one point of the retina, and there paint the colour of that point of the object from which they come. As the rays from different points of the object cross each other before they come to the retina, the picture they form must be inverted; the upper part of the object being painted upon the lower part of the retina, the right side of the object upon the left of the retina, and so of the other parts.

This philosopher thought that we see objects erect by means of these inverted pictures, for this reason, that, as the rays from different points of the object cross each other before they fall upon the retina, we conclude that the impulse which we feel upon the lower part of the retina comes from above, and that the impulse which we feel upon the higher part comes from below.

Des Cartes afterwards gave the same solution of this phænomenon, and illustrated it by the judgment which we form of the position of objects which we feel with our arms crossed, or with two sticks that cross each other.

But we cannot acquiesce in this solution. First, Be

cause it supposes our seeing things erect, to be a deduction of reason, drawn from certain premises: whereas it seems to be an immediate perception. And, secondly, Because the premises from which all mankind are supposed to draw this conclusion, never entered into the minds of the far greater part, but are absolutely unknown to them. We have no feeling or perception of the pictures upon the retina, and as little surely of the position of them. In order to see objects erect, according to the principles of Kepler or Des Cartes, we must previously know that the rays of light come from the object to the eye in straight lines; we must know that the rays from different points of the object cross one another before they form the pictures upon the retina; and, lastly, we must know that these pictures are really inverted. Now, although all these things are true, and known to philosophers, yet they are absolutely unknown to the far greatest part of mankind: nor is it possible that they who are absolutely ignorant of them, should reason from them, and build conclusions upon them. Since, there

fore, visible objects appear erect to the ignorant as well as to the learned, this cannot be a conclusion drawn from premises which never entered into the minds of the ignorant. We have indeed had occasion to observe many instances of conclusions drawn, either by means of original principles, or by habit, from premises which pass through the mind very quickly, and which are never made the objects of reflection; but surely no man will conceive it possible to draw conclusions from premises which never entered into the mind at all.

Bishop Berkeley having justly rejected this solution, gives one founded upon his own principles; wherein he is followed by the judicious Dr. Smith, in his "Optics; " and this we shall next explain and examine.

That ingenious writer conceives the ideas of sight to be altogether unlike those of touch. And, since the notions we have of an object by these different senses

have no similitude, we can learn only by experience how one sense will be affected, by what, in a certain manner, affects the other. Figure, position, and even number, in tangible objects, are ideas of touch; and, although there is no similitude between these and the ideas of sight, yet we learn by experience, that a triangle affects the sight in such a manner, and that a square affects it in such another manner-hence we judge that which af fects it in the first manner, to be a triangle, and that which affects it in the second, to be a square. In the same way, finding, from experience, that an object in an erect position affects the eye in one manner, and the same object in an inverted position affects it in another, we learn to judge, by the manner in which the eye is affected, whether the object is erect or inverted. In a word, visible ideas, according to this author, are signs of the tangible; and the mind passeth from the sign to the thing signified, not by means of any similitude between the one and the other, nor by any natural principle, but by having found them constantly conjoined in experience, as the sounds of a language are with the things they signify: so that, if the images upon the retina had been always erect, they would have shewn the objects erect, in the manner as they do now that they are inverted—nay, if the visible idea which we now have from an inverted object, had been associated from the beginning with the erect position of that object, it would have signified an erect position, as readily as it now signifies an inverted one. And, if the visible appearance of two shillings had been found connected from the beginning with the tangible idea of one shilling, that appearance would as naturally and readily have signified the unity of the object as now it signifies its duplicity.

This opinion is, undoubtedly, very ingenious and, if it is just, serves to resolve not only the phænomenon now under consideration, but likewise that which we

shall next consider-our seeing objects single with two

eyes.

It is evident that, in this solution, it is supposed that we do not originally, and previous to acquired habits, see things either erect or inverted, of one figure or another, single or double; but learn, from experience, to judge of their tangible position, figure, and number, by certain visible signs.

Indeed, it must be acknowledged to be extremely difficult to distinguish the immediate and natural objects of sight, from the conclusions which we have been accustomed from infancy to draw from them. Bishop Berkeley was the first that attempted to distinguish the one from the other, and to trace out the boundary that divides them. And if, in doing so, he hath gone a little to the right hand or to the left, this might be expected in a subject altogether new, and of the greatest subtilty. The nature of vision hath received great light from this distinction; and many phænomena in optics, which before appeared altogether unaccountable, have been clearly and distinctly resolved by it. It is natural, and almost unavoidable, to one who hath made an important discovery in philosophy, to carry it a little beyond its sphere, and to apply it to the resolution of phænomena which do not fall within its province. Even the great Newton, when he had discovered the universal law of gravitation, and observed how many of the phænomena of nature depend upon this, and other laws of attraction and repulsion, could not help expressing his conjecture, that all the phænomena of the material world depend upon attracting and repelling forces in the particles of matter. And I suspect that the ingenious Bishop of Cloyne, having found so many phænomena of vision reducible to the constant association of the ideas of sight and touch, carried this principle a little beyond its just limits.

In order to judge as well as we can whether it is so, let us suppose such a blind man as Dr. Saunderson,