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 в 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 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 object.

Charles. I can easily conceive that this would be the case with common objects, such as are seen every day and hour. But 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 with

in 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 connected 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 experi ence, to an erect position.

James. How is it that in so small a space as the retina of the eye, the images of so many objects can be formed?

Tutor. Dr. Paley* tells us, "The pros pect from Hampstead Hill is compressed into the compass of a sixpence, yet circumstantially represented. A stage coach, travelling at its ordinary rate, for half an hour, passes in the eye only over the twelfth part of an inch, yet the change of place is distinctly perceived throughout its whole progress." Now what he asserts we all know is true: go to the window, and look steadily at the prospect before you, and see how many objects you can discern without moving your eye.

James. I can see a great number very distinctly indeed, besides which I can discern others, on both sides, which are not clearly defined.

Charles. I have another difficulty; we have two eyes, on both of which the images of objects are painted, how is it that we do not see every object double?

See Paley's Natural Theology, p. 35, seventh edition, or p. 13, in the Analysis of that work by the Author of these Dialogues.

Tutor. When an object is seen distinctly with both eyes, the axes of them are directed to it, and the object appears single; for the optic nerves are so framed, that the correspondent parts, in both eyes, lead to the same place in the brain, and excite but one sensation. But if the axes of both eyes are not directed to the object, that object seems double.

James. How does that appear?

Tutor. Look at your brother, while I push your right eye out of its place towards the left.

James. I see two brothers, the one receding to the left hand of the other.

Tutor. The reason is this; by pushing the eye out of its natural place, the pictures in the two eyes do not fall upon correspondent parts of the retina, and therefore the sensations from each eye are excited in different parts of the brain.