Fig. 1.
The stand must be firm, so as not to vibrate when any one passes along the floor of the room, and it must have a vertical and lateral motion connected with it. [Fig. 1] shows what our instrument will be like when finished.
The first thing to be made is the tube; this must be thirty-nine inches long and two inches in diameter inside. Get a wooden roller four feet long and two inches wide. A piece of curtain rod will do. Now mix some strong glue. If you have not a glue-pot, mix it in a jar placed in a saucepan of water. Get some sheets of stout brown paper and well damp them. Take a strip of brown paper, that has not been damped, thirty-nine inches long and seven inches wide. Rub the roller all over well with powdered chalk and put this dry paper strip round it to form a case, lapping and gluing the edges together, but being very careful not to let any glue touch the roller. Now take your damped paper and rub it all over on both sides with hot glue, and roll it on the roller; roll it tightly and rub the glue well in, and rub each layer of paper well in to the under one, so that when dry it will form a mass of paper and glue. Put on enough paper to form a casing a quarter of an inch thick. When you have done papering set the whole on one side to get quite dry and hard. While this is drying we can be making the eyepiece and stand. For the eyepiece we shall want a piece of brass tube four inches long and large enough for the larger of the two lenses that form the eyepiece to go inside; that will be a little more than an inch in diameter inside. Get your lenses before getting the tube. This tube can be bought at the ironmonger’s.
Now for the lenses. Go to Messrs. Dollond and ask for a two-inch simple object-glass, forty inches focus. This will be one shilling and sixpence. For the eyepiece ask for two plano-convex lenses—one of one-inch focus, the other of two-inch focus. These will be three shillings and sixpence the pair. The object glass is to be double convex. Now, having got your lenses, we will fix them in the tube.
Fig. 2.
Cut a piece of cardboard three-quarters of an inch wide, and long enough to go all round inside the tube tightly, and not to lap. Push this in to form a lining at one end, and forming a shelf out of the thickness. This shelf is to be about two inches from the end. Now turn the other end of the tube up and drop the larger of the two lenses—which is called the field lens—on to the shelf with the rounded side downwards. Now push in on the top of it another cardboard lining three-quarters of an inch wide. Push this lining quite down on to the flat side of the lens to keep it firm. On to the shelf formed by this lining place a disc of cardboard the size of the inside of the tube, and with a hole cut in the centre half an inch in diameter. This hole must be cut quite clean. On to this disc push in a cardboard lining one inch wide to keep all firm. Now cut two discs of cardboard, one the exact size of the inside of the tube, with a hole in the centre a trifle smaller in diameter than the small lens, which is called the eye-lens; the other a quarter of an inch smaller, and having a hole in the centre the exact size of the eye-lens. Glue these two discs together (as in [Fig. 2]), being careful to get them concentric. When this is dry push the eye-lens into the ledge formed, the flat side downwards, and put the cardboard discs on to the lining in the tube, the rounded side of the glass inside the tube. Fasten the disc and lens in place with a narrow strip of cardboard, going all round just inside the tube.
