Page:Popular Science Monthly Volume 1.djvu/484

470 a wide slit, it affords a view over a greater part of the eyeball, and we think it bigger, simply because we see more of it. In the same manner, our judgment is deluded by the different degrees of prominence of the eye. A staring or protruding eye impresses us as being larger, although it is only pressed forward; while in advanced age, or in consuming sickness, the sunken eye is thought smaller.

If the eye really is larger, then the distance between the cornea and the lens will be greater; and if the effects of the refracted light remain the same in the latter, the image will no longer be projected on the cornea, but in front of it. And this is what really takes place in that wide-spread malady called short-sightedness. Here we have especially to note that the mean axis of the eye is too long. There are others called far-sighted eyes, whose visual axis is too short, the image for such eyes falling behind the retina. In order to reestablish the conditions of keen vision in both cases, the effects of refraction must be diminished for the short-sighted, by diverging or concave glasses; for the far-sighted, by collective or convex glasses. Those conditions have nothing to do with the want of the power of adjustment of focus. If you correct the defective construction of the short-sighted eye with a concave glass, and that of a far-sighted with a convex one, the lens—its mobility being preserved—can with their aid accommodate itself to near and distant objects, which neither an old man, deprived altogether of the power of adjustment, nor an individual who has been operated on for the cataract, is capable of doing.

Let us now pursue the analysis of the model in the same order as at the outset. First, then, fold back the cornea with the anterior section of the sclerotica; the margin of the eye to the front is now interrupted by the pupil, and is for the rest formed by the tissue of the iris, and the anterior section of the choroid. The space (at present wanting) in front of the iris-curtain was filled with aqueous humor, which you are to suppose has escaped. Let us now remove the posterior half of the tissue of the visual nerve; the whole eye will then be closed up by the passage of the choroid and iris, which now meets with no break, except anteriorly from the pupil, and posteriorly from the entrance of the optic nerve. If, now, as with the sclerotica, we fold back the anterior division of the choroid along with the iris, in doing which we have an opportunity of convincing ourselves of the true nature of the pupil, and that it is indeed an opening, we then come upon the hard lens lying behind. Taking this away, as also the gelatinous vitreous humor, we at length remove the posterior section of the choroid, and have nothing left but the optic nerve and the retina. Thus we have again arrived at the starting-point of our reflections; and it only now remains for me to bring before you the retina.

Before, however, entering on this division of my subject, let me call your attention to a few fundamental processes in the act of seeing. First, the picture or image on the retina is perfectly sharp only at one