picture being formed by the central rays, and the righthand picture by the rays which pass on the right of the eye's centre. It is farther to be observed, that the object which appears on the right, is not that which is seen through the hole on the right, but that which is seen through the hole on the left; and, in like manner, the left-hand object is seen through the hole on the right, as is easily proved by covering the holes successively : so that, whatever is the direction of the rays which form the right-hand and left-hand pictures, still the right-hand picture shews a left-hand object, and the left-hand picture shews a right-hand object.

Experiment 4. It is easy to see how the two last experiments may be varied, by placing the object beyond the farthest limit of distinct vision. In order to make this experiment, I looked at a candle at the distance of ten feet, and put the eye of my spectacles behind the card, that the rays from the same point of the object might meet and cross each other, before they reached the retina. In this case, as in the former, the candle was seen triple through the three pin-holes; but the candle on the right was seen through the hole on the right; and, on the contrary, the left-hand candle was seen through the hole on the left. In this experiment it is evident, from the principles of optics, that the rays forming the several pictures on the retina cross each other a little before they reach the retina; and, therefore, the left-hand picture is formed by the rays which pass through the hole on the right: so that the position of the pictures is contrary to that of the holes by which they are formed; and, therefore, is also contrary to that of their objects—as we have found it to be in the former experiments.

These experiments exhibit several uncommon phanomena, that regard the apparent place, and the direction of visible objects from the eye; phænomena that seem to be most contrary to the common rules of vision.

When we look at the same time through three holes that are in a right line, and at certain distances from each other, we expect that the objects seen through them should really be, and should appear to be, at a distance from each other. Yet, by the first experiment, we may, through three such holes, see the same object, and the same point of that object; and through all the three it appears in the same individual place and direction.

When the rays of light come from the object in right lines to the eye, without any reflection, inflection, or refraction, we expect that the object should appear in its real and proper direction from the eye; and so it commonly does. But in the second, third, and fourth experiments, we see the object in a direction which is not its true and real direction from the eye, although the rays come from the object to the eye, without any inflection, reflection, or refraction.

When both the object and the eye are fixed without the least motion, and the medium unchanged, we expect that the object should appear to rest, and keep the same place. Yet, in the second and fourth experiments, when both the eye and the object are at rest, and the medium unchanged, we make the object appear to move upwards or downwards, or in any direction we please.

When we look, at the same time and with the same eye, through holes that stand in a line from right to left, we expect that the object seen through the left-hand hole should appear on the left, and the object seen through the right-hand hole should appear on the right. Yet, in the third experiment, we find the direct contrary.

Although many instances occur in seeing the same object double with two eyes, we always expect that it should appear single when seen only by one eye. Yet, in the second and fourth experiments, we have instances wherein the same object may appear double, triple, or quadruple to one eye, without the help of a polyhedron or multiplying glass.

All these extraordinary phænomena, regarding the direction of visible objects from the eye, as well as those that are common and ordinary, lead us to that law of nature which I have mentioned, and are the necessary consequences of it. And, as there is no probability that we shall ever be able to give a reason why pictures upon the retina make us see external objects, any more than pictures upon the hand or upon the cheek; or, that we shall ever be able to give a reason, why we see the object in the direction of a line passing from its picture through the centre of the eye, rather than in any other direction—I am, therefore, apt to look upon this law as a primary law of our constitution.

To prevent being misunderstood, I beg the reader to observe, that I do not mean to affirm that the picture upon the retina will make us see an object in the direction mentioned, or in any direction, unless the optic nerve, and the other more immediate instruments of vision, be sound, and perform their function. We know not well what is the office of the optic nerve, nor in what manner it performs that office; but that it hath some part in the faculty of seeing, seems to be certain; because, in an amaurosis, which is believed to be a disorder of the optic nerve, the pictures on the retina are clear and distinct, and yet there is no vision.

We know still less of the use and function of the choroid membrane; but it seems likewise to be necessary to vision for it is well known, that pictures upon that part of the retina where it is not covered by the choroid-I mean at the entrance of the optic nerveproduce no vision any more than a picture upon the hand. We acknowledge, therefore, that the retina is not the last and most immediate instrument of the mind in vision. There are other material organs, whose operation is necessary to seeing, even after the pictures upon the retina are formed. If ever we come to know the structure and use of the choroid membrane, the optic

nerve, and the brain, and what impressions are made upon them by means of the pictures on the retina, some more links of the chain may be brought within our view, and a more general law of vision discovered; but, while we know so little of the nature and office of these more immediate instruments of vision, it seems to be impossible to trace its laws beyond the pictures upon the retina.

Neither do I pretend to say, that there may not be diseases of the eye, or accidents, which may occasion our seeing objects in a direction somewhat different from that mentioned above. I shall beg leave to mention one instance of this kind that concerns myself.

In May, 1761, being occupied in making an exact meridian, in order to observe the transit of Venus, I rashly directed to the sun, by my right eye, the cross hairs of a small telescope. I had often done the like in my younger days with impunity; but I suffered by it at last, which I mention as a warning to others.

I soon observed a remarkable dimness in that eye; and for many weeks, when I was in the dark, or shut my eyes, there appeared before the right eye a lucid spot, which trembled much like the image of the sun seen by reflection from water. This appearance grew fainter, and less frequent, by degrees; so that now there are seldom any remains of it. But some other very sensible effects of this hurt still remain. For, First, The sight of the right eye continues to be more dim than that of the left. Secondly, The nearest limit of distinct vision is more remote in the right eye than in the other; although, before the time mentioned, they were equal in both these respects, as I had found by many trials. But, thirdly, what I chiefly intended to mention is, That a straight line, in some circumstances, appears to the right eye to have a curvature in it. Thus, when I look upon a music book, and, shutting my left eye, direct the right to a point of the middle line of the five which compose

the staff of music, the middle line appears dim, indeed, at the point to which the eye is directed, but straight; at the same time, the two lines above it, and the two below it, appear to be bent outwards, and to be more distant from each other and from the middle line, than at other parts of the staff, to which the eye is not directed. Fourthly, Although I have repeated this experiment times innumerable, within these sixteen months, I do not find that custom and experience takes away this appearance of curvature in straight lines. Lastly, This appearance of curvature is perceptible when I look with the right eye only, but not when I look with both eyes; yet I see better with both eyes together, than even with the left eye alone.

I have related this fact minutely as it is, without regard to any hypothesis; because I think such uncommon facts deserve to be recorded. I shall leave it to others

to conjecture the cause of this appearance. To me it seems most probable, that a small part of the retina toward the centre is shrunk, and that thereby the contiguous parts are drawn nearer to the centre, and to one another, than they were before; and that objects whose images fall on these parts, appear at that distance from each other which corresponds, not to the interval of the parts in their present preternatural contraction, but to their interval in their natural and sound state.

Section XIII.

OF SEEING OBJECTS SINGLE WITH TWO EYES.

Another phænomenon of vision which deserves attention, is our seeing objects single with two eyes. There are two pictures of the object, one on each retina, and each picture by itself makes us see an object in a certain direction from the eye; yet both together commonly