Page images
PDF
EPUB

66 The

images of so many objects can be formed?

Tutor. Dr. Paley* tells us, 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 prospeçt before you, and see how many objects you can discern without moving your eye.

James. ; I can see a great number

• See Paley's Natural Theology, p. 35, 7th edit. or p. 13, in the Analysis of that work by the Author of these Dialogues.

[ocr errors]

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?

Tutor. When an object is seen distinctly with both eyes, the axes of 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

[blocks in formation]

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.

CONVERSATION XVII.

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 eners the eye is stopt and lost in the passage, and every object appears dim. The eye, without light, would be a useless machine. Spectacles are intended to collect the light, or to bring it to a proper degree of convergency.

Charles. . Are spectacle-glasses al

ways convex?

Tutor. No: they are convex when the eyes are too flat; 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 without.

Tutor. Suppose then a person is unable to see objects distinctly, owing to the cornea c D (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

« PreviousContinue »