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Of the Eye, and the Manner of Vision.
CHARLES. I do not understand what you meant when you said, the optic nerve served to convey to the brain the sensations produced on the retina.
Tutor. Nor do I pretend to tell you in what manner the image of any object painted on the retina of the eye is calculated to convey to the mind an idea of that object : but I wish to show you, that the images of the various objects which you see are painted on the retina. Here is a bullock's eye, from the back part of which I cut away the three
coats, but so as to leave the vitreous humour perfect: I will now put against the vitreous humour a piece of white paper, and hold the eye towards the window ; what do you see?
James. The figure of the window is drawn upon the paper; but it is inverted.
Tutor. Open the window, and you will see the trees in the garden drawn upon it in the same inverted state, or any other bright object that is presented to it.
Charles. Does the paper, in this instance, represent the innermost coat called the retina ?
Tutor. It does '; and I have made use of paper because it is easily seen through, whereas the retina is opake ; transparency would be of no advantage to it. The retina, by means of the optic nerve, is conveyed to the brain, or, in other words, the optic nerve is an extension of the retina.
James. And does it carry the news of every object that is painted on the retina ?
Tutor. So it should seem ; for we have
an idea of whatever is drawn upon it. I direct my eyes to you, and the image of your person is painted on the retina of my eye, and I say I see you. So of any thing else.
Charles. You said the rays of light proceeding from external objects were refracted in passing through the different humours of
Tutor. They are, and converged to a point, or there would be no distinct picture drawn on the retina, and of course no distinct idea conveyed to the mind. I will show you what I mean by a figure, taking an arrow again as an illustration.
As every point of an object A B C (Plate iv. Fig. 27.) sends out rays in all directions, some rays from each point on the side next the eye, will fall upon the cornea between x y, and by pasqing through the humours of the eye they will be converged and brought to as many points on the retina, and will form on it a distinct inverted picture c b a of the object.
James. This is done in the same manner as you showed us by means of a double convex lens.
Tutor. All three of the humours have some effect in refracting the rays of light, but the crystalline is the most powerful, and that is a complete double convex lens : and you see the rays from A are brought to a point at a; those from B will be converged at b, and those from c at c, and, of course, the intermediate ones between A and B, B and c will be formed between a and b, and b and c. Hence the object becomes visible by means of the image of it being drawn on the retina.
Charles. Since the image is inverted on the retina, how is it that we see things in the proper position ?
Tutor. This is a proper question, but one that is not very readily answered. It is well known that the sense of touch or feeling very much assists the sense of sight; some paiptings are so exquisitely finished, and so much resemble sculpture, that the eve is completely deceived, we then naturally extend the hand to aid the sense of seeing Children who have to learn the use of all their senses, make use of their hands in every thing; they see nothing which they do not wish to handle, and therefore it is not improbable, that by the sense of the touch, they learn, unawares, to rectify that of seeing. The image of a chair, or table, or other object, is painted in an inverted position on the retina; they feel and handle it, and find it erect; the same result perpetually recurs, so that, at length, long before they can reason on the subject, or even describe their feelings by speech, the inverted image gives them an idea of an erect object.