The earliest magnifying lens of which we have any knowledge was one rudely made of rock-crystal, which Mr. Layard found, among a number of glass bowls, in the north-west palace of Nimroud; but no similar lens has been found or described to induce us to believe that the microscope, either single or compound, was invented and used as an instrument previous to the commencement of the seventeenth century. In the beginning of the first century, however, Seneca alludes to the magnifying power of a glass globe filled with water; but as he only states that it made small and indistinct letters appear larger and more distinct, we cannot consider such a casual remark as the invention of the single microscope, though it might have led the observer to try the effect of smaller globes, and thus obtain magnifying powers sufficient to discover phenomena otherwise invisible. Lenses of glass were undoubtedly in existence at the time of Pliny; but at that period, and for many centuries afterwards, they appear to have been used only as burning or as reading glasses; and no attempt seems to have been made to form them of so small a size as to entitle them to be regarded even as the precursors of the single microscope.--_North-British Review_, No. 50. The _rock-crystal lens_ found at Nineveh was examined by Sir David Brewster. It was not entirely circular in its aperture. Its general form was that of a plano-convex lens, the plane side having been formed of one of the original faces of the six-sided crystal quartz, as Sir David ascertained by its action on polarised light: this was badly polished and scratched. The convex face of the lens had not been ground in a dish-shaped tool, in the manner in which lenses are now formed, but was shaped on a lapidary’s wheel, or in some such manner. Hence it was unequally thick; but its extreme thickness was 2/10ths of an inch, its focal length being 4½ inches. It had twelve remains of cavities, which had originally contained liquids or condensed gases. Sir David has assigned reasons why this could not be looked upon as an ornament, but a true optical lens. In the same ruins were found some decomposed glass. HOW TO MAKE THE FISH-EYE MICROSCOPE. Very good microscopes may be made with the crystalline lenses of fish, birds, and quadrupeds. As the lens of fishes is spherical or spheroidal, it is absolutely necessary, previous to its use, to determine its optical axis and the axis of vision of the eye from which it is taken, and place the lens in such a manner that its axis is a continuation of the axis of our own eye. In no other direction but this is the albumen of which the lens consists symmetrically disposed in laminæ of equal density round a given line, which is the axis of the lens; and in no other direction does the gradation of density, by which the spherical aberration is corrected, preserve a proper relation to the axis of vision.