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Charles. You said the rays of light proceeding from external objects were refracted in passing through the different humours of the eye.

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 2 y, and by passing 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 cb 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

Hence the object becomes visible by means of the image of it being drawn on the retina.

Charles. Since the image is in

C.

verted 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 an-. swered. It is well known that the sense of touch or feeling very much assists the sense of sight; some paintings are so exquisitely finished, and so much resemble sculpture, that the eye 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 objeet.

Charles. I can easily conceive that this would be the case with common objects, such as are seen every day and hour. Bụt will there be no. difficulty in supposing that the same must happen with regard to any thing which I had never seen before? I never saw ships sailing on the sea till within this month; but when I first saw them, they did not appear to me in an inverted position.

Tutor. But you have seen water and land before, and they appear to you, by habit and experience, to be lowermost, though they are painted on the eye in a different position : and the bottom of the ship is next the water, and consequently, as you refer the water to the bottom, so you must the hull of the ship which is con nected with it. In the same manner all the parts of a distant prospect are right with respect to each other; and therefore, though there may be a hundred objects in the landscape entirely new to you, yet as they all bear a relation to one another, and to the earth on which they are, you refer them, by experience, to an erect position.

James. How is it that in so small a space as the retina of the eye, the

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