it is plain that the central portion of each wave-front will be retarded more than the marginal part, having a greater thickness to pass through, so that the central part will lag back; and, when the wave-front emerges, its form will have become concave, instead of convex; and as, with the
particular form of lens that we have assumed used, its form will be spherical, each wave will run to a center or focus, and create there a great agitation.
Now, the same thing exactly will happen if the vibrating molecule is removed to an indefinitely great distance, as for instance to one of the stars: in this case the wave-fronts will be sensibly plane, on account of the distance of the center of curvature, just as the surface of water standing in a pail is sensibly plane, although the center of its curvature is only four thousand miles distant.
It is found experimentally, or it can be demonstrated mathematically, that the vibrating molecule, the center of the lens, and the focus of the emerging concave wave-fronts, lie in a straight line; with this fact distinctly in mind, it is clear that a second vibrating molecule, say, situated in another star, in nearly the same direction from the earth as the first, will also form a second center of agitation or focus, exactly behind the center of the lens, as viewed from that star; and so on from any number of vibrating molecules, each and every one producing a different center of agitation, exactly behind the center of the lens as viewed from them, of course within reasonable limits on each side of the direction of the axis of the lens.
We are now in a position to understand clearly the reason why we are able to see distinctly the forms of distant objects.
Diagram 4 represents the lens of the eye, with plane wave-fronts of light, from two different vibrating molecules, situated in different stars, entering it, and running to a focus or center of intense vibration behind it. The short lines at the back of the eye represent the so-called rods of the retina; when one only of these rods receives