ency of light to move in straight rays. After the emerging waves have run to a focus, they diverge again from this focus as a new center, with spherical fronts, and in exactly the opposite direction to that from which they arrived, just as if the light emerging from all parts of the lens was propagated through and beyond the focus in straight lines; hence the marginal portion of the converging and diverging wave-fronts on each side of the focus will form two cones, turned in diametrically opposite directions, their common apex being the common center of the spherical wave-fronts, viz., the focus of the lens.
It is now evidently a simple matter to place a second lens at such a distance behind the focus of the first lens that it will transform the spherical wave-fronts diverging from this focus into plane wave-fronts, parallel to those entering the first lens; and, because these waves emerging from the second lens have plane fronts, they must, if they are allowed to enter the eye, come to a focus on the retina, and cause the eye to see a point of light, for precisely the same reason that it would see that point if the two lenses were removed, and the direct light from the vibrating molecule were allowed to enter it.
This is the principle of the refracting telescope; the first lens represents the object-glass, and the second lens the eye-piece.
The Diagram 6 represents the object-glass, the eye-piece, and the eye, in their proper relative positions; also the light-waves from an infinitely
distant vibrating molecule entering the object-glass, emerging from it with spherical wave-fronts, which converge to a point of great agitation or focus, whence they diverge with spherical fronts, until, by passing through the eye-piece, they are converted into plane wave-fronts; thence, entering the eye, they come to a focus on the retina.
The diameter of the pupil of the eye being one fifth of an inch, the eye-piece must be of such a focal length that it can be placed so near the focus of the object-glass that the diameter of the emerging cylinder of plane wave-fronts shall not exceed one fifth of an inch, else the cylinder of light entering the object-glass will not be reduced in