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 ☛ 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 per son will not see the object at all, because the image of it does not reach the retina. Tutor. True: but at z 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 be tween 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 there eyes being too flat, the focus is thrown beyond the eye, and therefore they hold the object at a distance to bring the focus z (Fig. 28.) to the retain. Charles. Very short-sighted people bring objects close to their eyes. Tutor. Yes, I once knew a young man who was apt, in looking at his paper, to rub 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 of concave glasses? - property 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 r 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 per son will not see the object at all, because the image of it does not reach the retina. Tutor. True: but at z 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 be+ tween 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 there eyes being too flat, the focus is thrown beyond the eye, and therefore they hold the object at a distance to bring the focus z (Fig. 28.) to the retain, Charles. Very short-sighted people bring objects close to 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 a b seen at z, and also at x, double the distance; will not the same object appear under different angles to an eye so situated? James. Yes, certainly a E b will be lar ger 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 of the same lengths, yet a b being nearest to the eye, will appear the largest. are Charles. You say the eyes of old people become flat by age, is th at the natural progress? |