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 m n is placed between the object and the eye, by means of which the rays are brought to a focus sooner, and the image is formed at d.

James. Now I see the reason why people are obliged, sometimes, to make trial of many pairs of spectacles before they get those that will suit them. They cannot tell exactly what degree of convexity is necessary to bring the focus just to the retina.

Tutor.

That is right; for the

shape of the eye may vary as much as that of their countenance; of course, a pair of spectacles that might suit you, would not be adapted to another, whose eyes should require a similar aid.-What is the property of concave glasses?

Charles. They cause the rays of light to diverge.

Tutor. Then for very round and globular eyes, these will be useful, because if the cornea c D, or crystalline a b (Plate IV. Fig. 29.) be too convex, the rays flowing from a will unite into a focus before they arrive at the retina, as at z.

Charles. If the sight then depend on sensations produced on the retina, such a person will not see the object at all, because the image of it does not reach the retina.

Tor. True: but at the rays cross one another, and pass on to the retina, where they will produce some sensations, but not those of distinct vision, because they are not brought to a focus there. To remedy this, the concave glass m n is interposed between the object and the eye, which causes the rays coming to the eye to diverge; and being more divergent when they enter the eye, it requires a very convex cornea or crystalline to bring them to a focus at the retina.

James. I have seen old people, when examining an object, hold it a good distance from their eyes.

Tutor. Because their eyes being too flat, the focus is thrown beyond the eye, and therefore they hold the

object at a distance to bring the focus (Fig. 28.) to the retina.

Charles. Very short-sighted people bring objects close their eyes.

Tutor. Yes, I once knew a young man who was apt, in looking at his paper, to rub out with his nose what he had written with his pen. In this case, bringing the object near the eye produces a similar effect to that produced by concave glasses: because the nearer the object is brought to the eye, the greater is the angle under which it is seen; that is, the extreme rays, and, of course, all the others, are made more divergent.

James. I do not understand this. Tutor. Well, let E be the eye, (Plate IV. Fig. 30.) and the object ab seen atz, and also at r, double the

distance; will not the same object appear under different angles to an eye so situated?

James. Yes, certainly a b will be larger than c E d, and will include it. ⠀

Tutor. Then the object being brought very near the eye, has the same effect as magnifying the object, or of causing the rays to diverge; that is, though a b and c d are of the same lengths, yet a b being nearest to will appear the largest. Charles. You say the eyes of old people become flat by age, is that the natural progress?

the eye,

Tutor. It is; and therefore people who are very short-sighted while young, 'will probably see well when they grow old.

James. That is an advantage denied to common eyes.