M. Prévost, who published an account of his own case, in the "Annales de Chimie et de Physique," 1832, thought that the diplopia under which he laboured might arise from a fracture, bruise, or partial flattening of the lens, or from a separation of its laminæ. Professor Airy and Mr. Babbage both experience the inconvenience of diplopia; in the case of the latter, both eyes are defective, but he can obviate the defect by looking through a small hole in a card, or through a convex lens. "Professor Airy finds that his eye refracts the rays to a nearer focus in the vertical than in the horizontal plane, and he has ingeniously contrived to remedy it by the use of a double concave lens, one surface of which is spherical and the other cylindrical. The spherical surface is to correct the general effect of a too convex cornea; the cylindrical is to converge or diverge those rays at right angles to the axis; while the parallelism of those which impinge upon it in the plane of its axis is unaffected. Thus the focus of the spherical surface will remain unaltered in one plane, but in the other it will be changed to that of a lens, formed by it and a spherical surface of equal curvature with the cylinder. With the aid of a glass of this description, professor Airy could read the smallest print at a considerable distance, equally as well with the defective eye as the sound one. He found that vision was most distinct when the glass was pretty close to the eye, and the cylindrical surface turned from it. With these precautions, he observes, I find that the eye which I once feared would become quite useless, can be used in almost every respect as well as the other."