Page:Popular Science Monthly Volume 24.djvu/97

Rh ten thicknesses of paper will form a tube sufficiently thick and firm for our purpose; but only three or four layers should be laid at one time, and, when these are dry, three or four more may be added, and so on, until the requisite thickness is attained. When thoroughly dry, which will be in three or four days, you will have a stiff, straight, and light tube, the ends of which must be neatly and squarely cut off with a sharp knife, so as to leave it, when finished, exactly twenty-eight inches long. With a bit of sponge tied on the end of a stick, and some common or India ink, black the whole inside of the tube, and set it aside, on end, until the other parts are ready.

Next form a tube on the smaller roller, with only four or five thicknesses of paper, fifteen inches in length. When this is dry, proceed to form a third tube, over this second one as a roller, using six or seven thicknesses of paper in its formation. This last is to be used as a draw-tube for focusing with, and must be cut neatly and squarely off at the ends to a length of fifteen inches. A portion of the inner tube on which this was formed will be required for the eye-piece, directions for making which I shall give further on. Blacken the insides of both tubes, and set them aside, on end.

One more tube is required, viz., that in which the draw-tube shall slide. It needs to be only six inches long, but, in order to smooth working, should be lined inside with fine cloth or cotton-velvet. Procure, therefore, a piece of black broadcloth, six inches long, and of sufficient width to fit easily and accurately around the draw-tube. Then, using the latter as a roller, first neatly fit the cloth thereon as a first layer; next paste or gum the back of the cloth, and, with this for the innermost layer, form a short tube, six inches long, with paper and paste, as before directed, using here not more than six thicknesses. The draw-tube will now be found to move easily and smoothly back and forth in this cloth-lined sheath; but, for fear that the gum or paste should have penetrated the cloth lining, and should stick the tube and its sheath together, it will be safer to draw them apart before drying, and thus save needless trouble and annoyance.

On comparing the external diameter of this sheath with the interior diameter of the large tube first made, it will be found that some packing is required, to hold the former steadily and concentrically within the latter. Take, therefore, some three-quarters inch strips of brown paper, and, having pasted them, wind around the sheath at each end, to form rings or collars of equal thickness, and large enough to fit snugly within the main tube. The appearance of the sheath when completed will be as shown in Fig. 1, where a a' are the collars just described.

Now take the compound object-glass, consisting of a double-convex crown-glass lens, A (Fig. 2), and a plano-convex flint-glass lens, B. They will come from the optician's shop separate, but loosely fitted into each other. Be careful to see that their several surfaces are bright