TELESCOPE WHICH A BOY CAN MAKE.
First, obtain two lenses; the larger having a long and the smaller a short focus.
A powerful telescope, having a large field of vision, requires a lens at least two inches in diameter, with a focus of from two to three feet for the larger glass; and another lens of from one-half to one inch in diameter, and with a focus of one inch, for the smaller end. Having your lenses, the next important step is to make your tubes; this is done by bending a piece of pasteboard a foot long by seven inches wide in the shape of a tube, whose diameter shall be about one-sixteenth of an inch larger than that of your lens. Glue the edges firmly together, and tie a piece of tape around to insure their keeping in place. Make two tubes of this size and one rather smaller, that its ends may fit in the other two. Lap these ends together, and paste or glue them in place (see cut). Joining these sections together is simply to insure a proper length of tube. If a piece of pasteboard can be found large enough to make a tube three feet long, it will look much neater than the one described above. Take a narrow strip of pasteboard and glue it around the inside of the tube, half an inch from one end; put the large lens in its place, and press it against the edge of this band. Now take another strip, three-eighths of an inch wide, and paste around the inside between the lens and the end of the tube. By this means the glass is kept in place, it being held by the edges of the pasteboard on either side.
Another and smaller tube, five or six inches in length, and of a size just sufficient to slide easily in the other end of the long tube should now be made. Around the inside of one end paste a band of pasteboard, as in the larger section, but much nearer the edge. When this is dry, paste still another strip inside this one, making a wide edge for the lens to rest against. As this tube is of much greater diameter than the glass, inclose the latter between two disks of cardboard of the same size as the opening in the tube, and each having a round hole cut in its center for the eye to look through. Cover the inner side with paste, and press it against the edges of the strips. Finally, cover the whole thing with some dark-colored paper, pasting it carefully over the surface, and your telescope is completed.
This instrument will present everything in an inverted position, but if the lenses are carefully adjusted, objects at a long distance can be very plainly seen, and a boy can derive a great amount of solid comfort, not only while constructing, but from its subsequent use.
To find the focal distance of a lens, if for any reason the optician does not give it, hold it in the sun, and observe at what distance from itself it makes the smallest point of light. That, if measured, will be its focal distance. The long tube should be from two to three inches shorter than the focal distance of the larger lens.
—————◀▶—————