double vision, that she saw no object as distinctly as before every thing appearing to be dimly perceived, in comparison with that state of vision to which she had been accustomed. This dimness of vision was a natural consequence of seeing objects double, as the real object in this instance, would be perceived only with one eye, while the other would be directed to that which is apparent only. The same effect as this is produced in the very common experiment of placing a lighted candle before you, and pressing one side of the eye with the finger, so as to push that eye out of its state of parallelism with the other; for in this case, two candles will appear to be in view. This single experiment, simple as it is, shows, that in order to see objects single with two eyes, the images of them must be painted upon corresponding parts of the retina; for when by forcing one of the eyes out of its natural position, the images are made to fall upon parts of the retina which do not correspond, we seem to see objects double. For this reason, it would seem tolerably certain, that persons who squint, must originally see objects double, as it is evident that from some detect in the conformation of the organ, they are deprived of the power of moving their eyes so as to preserve their axes parallel, and of consequence the images painted on the retina must be in parts which do not correspond. That they will soon become insensible of this double vision, and appear to see objects like other people, would seem to be also a natural result from the force of habit. As the object, towards which they directly turn one of their eyes, will always appear much brighter than the apparent figure or ima, e presented to the other, they would instinctively turn the attention of the mind solely to it, and all perception of the false object would soon pass unnoticed. Thus when we direct our view to any thing in nature, we always have within our sphere of vision many other objects besides the one immediateiy occup, ing our attention, although from the habit of directing the view of the mind to that one only all the rest of them are unnoticed. The same doctrine is confirmed by a fact related by Dr. Smith in his opticks. “ If two lighted candles of equal height be viewed at the distance of two or three feet from the eyes, so that the picture of the right hand candle, on the left retina, shall correspond to that of the left hand candle, on the right retina, only one image will be produced by these two corresponding pictures. But the two pictures which do not correspond, viz. that of the right hand candle, on the right retina, and that of the left hand candle, on the left retina, will each produce its proper image.

With these principles in our possession, I conceive the explanation will be easy, of the fact mentioned by Cheselden. “A gentleman, he tells us, 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.” In this case, may he not by habit have ceased to pay any attention to the false appearances of objects, and have confined his view solely to the real objects themselves? We can learn to do this as easily with the eye, as with the sense of hearing, we can attend solely to some sounds which interest us, and lose all perception of the numberless other noises which at the same moment, may be assailing the ear. A similar explanation may be given of another case referred to by Dr. Smith in a note, upon article 137, of his opticks. “ The Rev Mr. Foster, having been blind for some years, of a gutta serena, was restored to sight by salivation. 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 be. fore he was blind.” After Mr. Foster, by salivation, had been relieved from the obstruction which prevented his seeing, the double appearance of objects might have been occasioned at first by some irregular action in the muscles of the eye, or in the nerves leading to the brain, which ceased as soon as he was restored to sound and perfect health. Upon the whole we think that upon the true inductive plan of reasoning, in which all our conclusions are made to rest upon experience and observation, we have sufficient ground upon which to establish the following principles; that we are originally so constituted by our Creator as to see objects single with both eyes; that in order to this purpose, the muscles and membranes of the eyes are so adjusted, as to enable us to move them in concert with each other; that images of objects are formed upon corresponding parts of the retina; and that a similar action upon the optick nerves leading to the brain must be produced, and that those actions must be made to mingle and coalesce in their progress to the sensorium. . These are doctrines which appear to be sufficiently proved by experience. It must be left to the practitioners of medicine to determine in each case the remedies, which will be most likely to effect a cure. In the case of double vision arising out of debility, the system must be strengthened and its tone restored, and in that of squinting every expedient should be resorted to, consistent with the delicacy of the case, to assist the muscles in performing their regular functions of dilatation and contraction, so as to communicate to the eyes their appointed movements.

Mr. De la Hire supposes, as Mr. Jurin represents his opinion, that in the generality of mankind, that part of the retina, which is seated in and about the axis of the eye, is of a more delicate sense and perception, than what the rest of that coat is endowed with; and therefore, that we direct both axes to the same object, not only for the sake of direct vision, whereby the image of the object may be more distinctly and perfectly painted upon the retina, but likewise, and indeed, chiefly, in order to receive the picture upon that part of the retina, which can best and most accurately perceive it. But in persons who squint, he conceives the most


sensible part of the retina of one eye, not to be placed in the axis, but at some distance from it on the one side, or on the other; and that, therefore, in the eye so unusually framed, not the axis but this more sensible part of the retina is turned toward the object, on which the axis of the other eye is fixed; and consequently both axes are not directed to the same point!" If this theory were true, which Dr. Smith by an appeal to fact and experience has demonstrated to be false, would not persons who squint always see objects dlouble? I apprehend they would, since in this case, images of any object would not be formed upon corresponding points of the retina, and yet each one would be as bright and distinct as the other, and of course the object would be as clearly seen by one eye as the other. After confuting by unanswerable arguments the opinion of Mr. De le. Hire, Mr. Jurin, I conceive, falls into mistakes himself. “Nor is squinting occasioned,” says he, “ by any detect in the muscles of the distorted eye. For when the other is shut, this eye is moved by the action of its muscles, in all possible directions, as freely as that of any other person.” This argument by no means proves the point at which it aims. For might not the muscles be so formed as to move both eyes very freely in their sockets, and yet be so imperfectly adjusted during the formation of the fætus in the womb as to pull irregularly, and occasion that distortion which is called squinting? Ropes might easily be adjusted so as to pull two globes freely at the same time in their sockets, and yet not act in concert with each other. Neither is his second proof any more conclusive. “ Neither is it (squinting) owing," he continues, “ to the want of correspondence in the muscles of both eyes, such as to hinder them from being both moved the same way at the same time. For when both eyes are open, and the undistorted eye is moved either upwards or downwards, or to the right or left, the other always accompanies it, and is turned the same way at the same instant of

time.” Surely the muscles might be so arranged as to pull the eyes the same way at the same time; and yet, if from any defect in their original formation, they could not contract and dilate alike, they might draw one of the eyes out of its state of parallelism with the other, and thus occasion that phenomenon which is called squinting?

It is very possible that squinting may, in some instances, be produced in the manner maintained by Mr. Jurin. He thinks, that in persons of this description, when they look at any object, the pupil of the distorted eye is drawn close to the nose, and the distance between the pupils lessened, and this lesser distance between the two pupils continues the same in all oblique directions of the eye: so that the two axes are never pointed at the same object, though the muscles do so far act in concert with each other, as to move both the eyes the same way at the same instant of time.

If I am not mistaken, squinting is not always occasioned by the pupils coming nearer to each other, than they do in those persons not affected with this blemish, as the same defective appearance in the eyes would be exhibited in case the pupils were always too remote from each other. But even supposing that they do, do we not find at the same time, that the eyes do not move in concert, and that the axis of the deformed one always approaches tou near, or recedes too far from the perfect one?

We concur in the concluding observations of Mr. Jurin under this article. “ This vicious habit,” says he, “ of squinting, may be contracted by a child, if he is often laid in his cradle in such a position, as to be able to see either the light or any other remarkable object with one eye only. And when by this means he is brought to squint, and is afterwards confirmed and settled in the practice of doing so, I apprehend it will be in vain to attempt a cure by his wearing tubes or shells, with small holes in them to look through. Do what you will of this kind, he will continue to see

« ForrigeFortsett »