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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 experie 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 prospect 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 Nuthor of these Dialogues.
Tutor. When an object is seen distinctly with both eyes,
of them are direct. ed 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 ex. cited in different parts of the brain,
Of Spectacles, and of their Uses.
CHARLES. Why do people wear spectacles ?
Tutor. To assist the sight, which may be defective from various causes. Some eyes are too flat, others are too convex: in some the humours lose a part of their transparency, and on that account, a deal of light that enters the eye is stopt and lost in the passage, and every object appears dim. The eye, without light, would be a useless inachine. Spectacles are intended to collect the light, or to bring it to a proper degree of convergency
Charles. Are spectacle-glasses always convex?
Tutor. No: they are convex when the eyes are too ilat; but if the eyes are already very convex, then concave glasses are used. You know the properties of a convex glass ?
James. Yes ; it is to make the rays of light converge sooner than they would with out.
Tutor. Suppose then a person is unable to see objects distinctly, owing to the cornea CD (Plate iv. Fig. 28.,) or to the crystalline a b, or both, being too flat. The focus of rays proceeding from any object, x, will not be on the retina, where it ought to be, but at z beyond it.
Charles. How can it be beyond the eye?
Tutor. It would be beyond it, if there were any thing to receive it; as it is, the rays flowing from x, will not unite at d, so as to render vision distinct. To remedy this, a convex glass min is placed between the object and the eye, by means of which