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until breakfast-time. Two good hours' work ought to produce some effect." The lower disc, now become convex, was wetted and lightly touched over with number five emery, which seemed soft enough for anything; the well-advanced mirror was turned over upon it, fitting now very closely, and with the sweet morning air floating in from the pine-woods, and the birds singing all around, the monotonous task went on with its intermissions till Uncle Richard gave the final wash off, and said--"Breakfast!" They were so far advanced now that Tom was as eager to recommence as his uncle, and by that evening so much progress had been made that the setting sun was made to shine in upon it, to be reflected back in a bright spot on the wall without the aid of water; while two evenings later, when the great round glass was stood all dry the polish upon it was limpid, and seemed to be as pure as could be. There was not the faintest scratch visible, and Tom cried in triumph-- "There, now it is done! Oh, uncle, it is grand!" "Grand enough so far, my boy. We have succeeded almost beyond my expectations; but that is only the first stage." "First--stage?" faltered Tom, looking at his uncle aghast. "Yes, boy; we have succeeded in making a beautiful spherical concave mirror, which could be of no use whatever for my purpose." "Then why did we make it?" cried Tom. "For practice?" "No, boy; because it is the step towards making an ellipse, or, as they call it when shaped for a reflecting telescope, a parabola. You know what an ellipse is?" "Gooseberry," said Tom bluntly. "Gooseberry-shaped," said his uncle. "Well then, what is a parabola?" "One of those things we used to learn about in geometry." "Good. Well, to-morrow we must begin polishing, or rather I must, to turn our glass from a spherical-curved mirror into a parabola." "You'll let me help, uncle?" "As much as I can, my boy; but the amount I have to polish off, in what is called figuring, is so small that it requires the most delicate of treatment, and first of all we have to prepare a small polisher to work by hand." This was formed of lead in the course of the next day--a nearly flat but slightly convex disc, with a handle upon its back, and when made perfectly smooth it was covered with hot pitch, which, as it cooled, was made to take the exact curve of the nearly finished mirror, by being pressed upon it, the pitch yielding sufficiently for the pur
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