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(12.) The relative places of stars in the heavens, as seen in the field of view of a telescope, are marked by fine lines of wire placed before the eye-glass, and which cross each other at right angles. The stars appearing in the telescope as mere lucid points without sensible magnitude, it is necessary that the wires which mark their places should have a corresponding tenuity. But these wires being magnified by the eye-glass would have an apparent thickness, which would render them inapplicable to this purpose, unless their real dimensions were of a most uncommon degree of minuteness. To obtain wire for this purpose, Dr. Wollaston invented the following process:—A piece of fine platinum wire, ab, is extended along the axis of a cylindrical mould, AB, fig.1. Into this mould, at A, molten silver is poured. Since the heat necessary for the fusion of platinum is much greater than that which retains silver in the liquid form, the wire ab remains solid, while the mould AB is filled with the silver. When the metal has become solid by being cooled, and has been removed from the mould, a cylindrical bar of silver is obtained, having a platinum wire in its axis. This bar is then wire-drawn, by forcing it successively through holes C, D, E, F, G, H, diminishing in magnitude, the first hole being a little less than the wire at the beginning of the process. By these means the platinum ab is wire-drawn at the same time and in the same proportion with the silver, so that whatever be the original proportion of the thickness of the wire ab to that of the mould AB, the same will be the proportion of the platinum wire to the whole at the several thicknesses C, D, &c. If we suppose the mould AB to be ten times the thickness of the wire ab, then the silver wire, throughout the whole process, will be ten times the thickness of the platinum wire which it includes within it. The silver wire may be drawn to a thickness not exceeding the 300th of an inch. The platinum will thus not exceed the 3000th of an inch. The wire is then dipped in nitric acid, which dissolves the silver, but leaves the platinum solid. By this method Dr. Wollaston succeeded in obtaining wire, the diameter of which did not exceed the 18000th of an inch. A quantity of this wire, equal in bulk to a common die used in games of chance, would extend from Paris to Rome.