_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?
_Mrs. B._ The microscope is constructed for this purpose. The single microscope (fig. 5.) consists simply of a convex lens, commonly called a magnifying glass; in the focus of which the object is placed, and through which it is viewed: by this means, you are enabled to place your eye very near to the object, for the lens A B, by diminishing the divergence of the rays, before they enter the pupil C, makes them fall parallel on the crystalline humour D, by which they are refracted to a focus on the retina, at R R.