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means of those rays of light, that pass in straight lines from those different parts to the eye. By the straight lines that pass from the upper part of any thing to the eye, we perceive the upper part of it, and so of the lower. Of consequence, in vision, whatever may be the position of the image upon the retina, whether it be inverted or erect, we must perceive it as it exists, or nearly as it exists in nature, unless the sight be deceived, as we know frequently happens. This, perhaps, is as far as we are able to penetrate into the mysteries of nature. Des Cartes, however, has undertaken to proceed a step farther in this matter, and explain in what manner, by means of an inverted image at the bottom of the eye, we may perceive an erect object. His account of the phenomenon is the following. He asserts, that every part of the retina is connected by the fine capillaments of the optick nerves to corresponding parts of the brain; and that when the rays of light from any part of an object fall on the retina, by means of these fine capillaments of the optick nerves, an effect is produced upon the corresponding parts of the brain, by which means the mind perceives every thing in the direction of those rays of light. Hence it is, that although the rays of light that pass from the upper part of an object, and those from the lower part cross each other as they make their way through the humours and lenses of the eye, and fall upon opposite parts of the retina, those that are emitted or rather reflected from the upper part of the object going to the lowest on the retina, and the contrary; yet, in as much as the effect is produced upon

the ponding parts of the brain, we perceive the upper part of the object by means of the rays that fall upon the lower part of the retina, and the lower part of the object by means of rays that fall upon the upper part of the retina, as we feel the upper and lower part of any thing by means of two sticks that cross each other, and are held in both hands at the same time; the hand that is below enabling us to perceive by means of it the higher part of the object, and that which is above enabling us to perceive by means of another stick crossing the first, the lower part. This account may serve well enough as an illustration of the operations of one sense, by tracing an analogy between them and the operations of another, but furnishes no solution of the phenomenon of our seeing erect objects by inverted images. We should think it no satisfactory explanation of the manner in which, by the sight, we are able to hold converse with objects at a distance, to say that by means of rays of light transmitted in straight lines to the eye, we perceive them, as we perceive remote objects by means of a stick held in our hands by which we touch them, or in the language of Mr. Addison in his Spectator, that the sight is only a more delicate and diffusive kind of touch, presenting to us at a single glance the greatest distances ard magnitudes in the universe. This is a beautiful similitude with which to please the fancy in the productions of the poet or fine writer, but in the rigid disquisitions of philosophy should be repudiated. How we are able to feel objects in immediate contact with us, by means of the organs of the body, we know no more than how we are able, by means of rays of light intromitted into the eye, to discern remote ones. To allege, therefore, as is done by Des Cartes and others, that we perceive the upper and lower parts of objects by means of rays crossing each other in the eyes, as we are able to perceive the upper lower parts of objects by means of sticks crossing each other, is not to advance a single step in our philosophical inquiries, or in the development of the properties and laws of nature. All that can ever be known to the most diligent and persevering inquirer on this point are, evidently, the following facts: that the rays of light which are emitted from the upper part of objects are transmitted to the lower part of the retina, and those which are emitted from the lower part of objects to the higher part of the retina, and that of

corres

and

consequence these rays cross each other: that such is the connection of that delicate membrane, the tunica retina, with the fine capillaments of the optick nerves leading to the brain, that any action produced by the rays of light upon the lower part of it gives us a perception of the upper part of the object, and any action upon the upper part of it, in like manner, a perception of the lower. This is a law of our constitution, and when we have resolved it into that law, we have probably advanced to the utmost limits of human knowledge in the matter. All that we are able to demonstrate from fact and observation, is, that there are certain parts of the retina which must be acted upon by the rays of light in order to present objects to us in one situation, and other parts which must be acted upon in order to present them in ano. ther. To illustrate this observation by a familiar example. While looking at a candle, I press the ball of one eye out of its usual position; so as to cause its axis no longer to be directed towards the candle. In this experiment, I no longer see a single candle, but the real candle, together with one more faint in its appearance. In this case, with the eye which has been updisturbed, I see the real candle, and with the eye that has been pressed, such a candle, as, if there was another real one in that direction, would be so faint and dim as to produce no greater impression upon the organ than is now produced by the one before me, casting its rays obliquely upon the bottom of the eye. Now what is the reason that by only turning my eye round in its socket, I am made to see, not only the real candle with one eve, but the image of it with the other? Evidently, because upon the eye which is turned aside, the same effect is produced, as if a real candle of the figure and appearance of the image, were presented; that is to say the tunica retina, and the nervous coat connected with the brain, together with the brain itself, are acted upon in the same manner as they would be if a candle of that appearance were exhibited to the eye. When one part, therefore, of the tunica retina is operated upon, it is calculated to present an object to us in one situation; when another is acted upon in another, and this upper or under, on the right or left side. Now, again, let us put the candle in our view, and press our eye as before, and we perceive two candles, a real one which is brighter, and its image which is more faint. Let us now suppose a luminous object extending from the candle to its image. It is evident that the rays which come from the right hand of this luminous object, fall upon the left hand of the retina, and those which come from the left upon the right hand of the retina. Supposing the real candle before to have been upon our right, by directing our eyes to the right side of the luminous object, we shall sce it appear as luminous as the real candle did before, while the part of the same object upon the left appears more faint as the image before mentioned did. These facts show without any room for doubt, that when those parts of the tunica retina that lie upon the right hand are operated upon by the rays of light coming from any thing, they make that object appear upon the left, and vice versa. The same reasoning will apply to the upper and under parts of objects. All the conclusion, therefore, to which we can come upon this subject, is, that it is the established law of our constitution, that rays passing from the upper parts of bodies, and falling upon the lower parts of the reti. na, present to our perception those upper parts, while those which come from the lower impinge upon the upper regions of the retina, and cause us to see the lower; and if from any cause the effect can be produced upon the upper or lower parts of the retina, the corresponding portion of the object will appear to be exhibited although it should not exist in reality.

CHAPTER II.

The same Subject Continued.

The next questions to be solved on this subject are, how come we to see any object singly with two eyes? Do we see objects single or double originally with both eyes? Is our seeing an object single with both eyes an original or acquired perception?

It must be admitted to be oftentimes an extremely difficult task, to distinguish our acquired from our original perceptions. In many cases our acquired perceptions become so familiarly connected with our primitive ones, that it is difficult, even in imagination, to separate them from each other. Every experiment which has been hitherto made in this matter, leads to the conclusion, that nature has endowed us with the power of seeing objects single with both eyes, immedi. ately and without effort. In every case in which the blind were restored to sight by Cheselden, he could never discover that any one among them saw objects double. Chil. dren, as soon as they begin to see, move both eyes in concert, when any thing is presented before them, which seems to indicate that both eyes are employed at the same time in the contemplation of any thing. Children, indeed, as well as those who are suddenly restored to sight, find a difficulty in directing their eyes to different objects; but they discover no difficulty in causing them to act together. All the phenomena, therefore, lead to the conclusion, that nature has communicated to us the power of effecting a simultaneous motion of both eyes, in order more successfully to accomplish the purposes

of vision; and this simultaneous movement of

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