Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained

by Mrs. Marcet; Thomas P. Jones

Caroline. I suppose that a contrary remedy must be applied to this defect; that is to say, a convex lens, L M, [fig. 2], to make up for the deficiency of convexity of the crystalline humour, O P. For the convex lens would bring the rays nearer together, so that they would fall, either less divergent, or parallel, on the crystalline humour; and, by being sooner converged to a focus, would fall on the retina.

Mrs. B. Very well, Caroline. This is the reason why elderly people, the humours of whose eyes are decayed by age, are under the necessity of using convex spectacles. And when deprived of that resource, they hold the object at a distance from their eyes, as in [fig. 3], in order to bring the focus more forward.

Caroline. I have often been surprised, when my grandfather reads without his spectacles, to see him hold the book at a considerable distance from his eyes. But I now understand the cause; the more distant the object is from the crystalline lens, the nearer to it, will the image be formed.

Emily. I comprehend the nature of these two opposite defects very well; but I cannot now conceive, how any sight can be perfect: for, if the crystalline humour is of a proper degree of convexity, to bring the image of distant objects to a focus on the retina, it will not represent near objects distinctly; and if, on the contrary, it is adapted to give a clear image of near objects, it will produce a very imperfect one, of distant objects.

Mrs. B. Your observation is very good, Emily; and it is true, that every person would be subject to one of these two defects, if we had it not in our power to adapt the eye, to the distance of the object; it is believed that this is accomplished, by our having a command over the crystalline lens, so as to project it towards, or draw it back from the object, as circumstances require, by means of the two muscles, to which the crystalline humour is attached; so that the focus of the rays, constantly falls on the retina, and an image is formed equally distinct, either of distant objects, or of those which are near.

Caroline. In the eyes of fishes, which are the only eyes I have ever seen separate from the head, the cornea does not protrude, in that part of the eye which is exposed to view.

Mrs. B. The cornea of the eye of a fish is not more convex than the rest of the ball of the eye; but to supply this deficiency, their crystalline humour is spherical, and refracts the rays so much, that it does not require the assistance of the cornea to bring them to a focus on the retina.

Emily. Pray, what is the reason that we cannot see an object distinctly, if we place it very near to the eye?

Mrs. B. Because the rays fall on the crystalline humour, too divergent to be refracted to a focus on the retina; the confusion, therefore, arising from viewing an object too near the eye, is similar to that which proceeds from a flattened crystalline humour; the rays reach the retina before they are collected to a focus, ([fig. 4.]) If it were not for this imperfection, we should be able to see and distinguish the parts of objects, which, from their minuteness, are now invisible to us; for, could we place them very near the eye, the image on the retina would be so much magnified, as to render them visible.

Emily. And could there be no contrivance, to convey the rays of objects viewed, close to the eye, so that they should be refracted to a focus on the retina?