Page:Popular Science Monthly Volume 15.djvu/830

 finished piece of mechanism with adjusting screw, etc., is necessary; but the beginner may content himself with an arrangement of much humbler pretensions. I have used a small plate of silver in which an opening (about three eighths of an inch long and one twentieth of an inch wide) had been cut, the width of the opening being reducible at pleasure by means of a little door or shutter sliding smoothly in grooves, and each opposing edge being faced with a strip of watch-spring. Fitted closely into the end of the tube is a piece of cork or wood having in its center a hole larger than the aperture in the slit plate; over this hole the slit plate is fastened, care being taken to stop up any accidental holes or crevices in the cork (or wood, as the case may be), in order to prevent the entry of extraneous light into the tube.

Primitive as this contrivance may appear, I have seen by its means, when using two flint-glass prisms, the D line of sodium double, and beautifully distinct. A slit, such as I have described, need not cost more than one tenth of the price charged for a slit by the optician.

Fig. 2 shows a slit of this kind open. B is the sliding shutter, and A is a piece soldered on the slit plate to bring the surface up level with B.

Concerning the observing telescope, perhaps Proctor's remarks about a finder for an astronomical telescope may be repeated: "It will be easy for the student to construct one for himself, and will be a useful Fig. 2. exercise in optics." But in case the student may not want to take the trouble, he will be glad to remember that an ordinary pocket telescope or spy-glass may be purchased ready made for a dollar or two, and by removing the erecting lenses a small astronomical telescope may be produced which, as the magnifying power required is small, will answer every purpose of the beginner.

The stand may be made in a variety of styles, from the unpretending box on legs, with holes in the sides for the collimator and telescope, to the highly finished tripod of the most aristocratic looking instrument. The following is one way of making a cheap and at the same time serviceable stand: Procure two disks of seasoned walnut or mahogany or any other hard wood, one about a foot in diameter by three eighths of an inch in thickness, and the other six inches by one quarter of an inch; also get two strips of wood about eight inches long and an inch and a half wide.

Make the larger disk into a table by screwing on three feet—metal hooks such as are used for hanging up clothes make excellent feet; then make a hole in the center of the large disk, and a corresponding hole in one end of each of the strips. Pass a screw downward through the two strips and through the hole in the disk, and let there be a thumb-nut on the screw so that it may be tightened underneath.