An admirable example of temporary long-sightedness is familiar to every diver, though he may be unconscious of its cause. Suppose that into very clear water of some twelve feet in depth, a white object, say a common jam-pot, is thrown, it can be clearly discerned from the shore, unaltered in shape or size. But, when the diver searches for it, he sees at first only something white, large, undefined, and wavering, and only finds it resume its proportions as he approaches it. This phenomenon is due to the pressure of the water upon the eyeball, which flattens it, and so throws the focus too far back for a clear image. Nowadays this defect is remedied by the use of very convex spectacles, so convex, indeed, that, if worn in the air, they would render the wearer incapable of seeing anything at more than an inch or so away from him. But, when worn in the water, they only supply the deficiency of the compressed eyeball, and so restore the focus to its proper position. * * * * * THOSE who suffer from short-sightedness can see with great distinctness objects which are close at hand, but those at a little distance seem to have no particular outline, and appear as if they were viewed through a fog, thus causing a constant and almost painful strain on the eyes. The cause of this defect is the too great convexity of the lens, which therefore throws its focus short of the required spot. The means of remedy are exactly opposite to those which are used for long-sighted persons, a concave lens being placed in front of the eye, so as to throw the focus farther back, and relieve the organ from the strain. Although we have not yet invented a machine that can alter the focus at will, we may take a hint from Nature. We have already seen how the pressure of water upon the front of the eye lessens its convexity, and makes it long-sighted. Consequently, if we could apply pressure round it, we could make it more convex, and so neutralise the weight of the water. There is a wonderful piece of machinery in Nature which really does perform this office, the eye, at the will of its owner, becoming either telescopic or microscopic. This quality is very desirable in birds, especially those which are predacious and of rapid flight, as they might either fail to see their prey at a distance, or might dash themselves against some obstacle when they were close upon it. The eye of the Owl affords a beautiful example of machinery which produces this effect, and the means which are used may be understood by inspecting the accompanying illustration.