Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

SOUND 261 suggest themselves; and Miller of Innsbruck has constructed a machine in which two discs are rotated, partially overlap- ping each other, each with a white circle cut excentrically in black varnish, the idea being essentially the same. The principle of the harmonograph has also been applied to trace the figures by two pendulums on a smoked glass in the stage of the lantern, and such an instrument can be obtained of any optician. Coming back to vibrating mirrors and a pencil of light, Professor Dolbear describes an arrangement he has adapted to a horizontal whirling-table, in which two friction-wheels rolling on opposite edges of the disc, by cranks fitted to them, actuate two mirrors mounted on rocking-pivots, over opposite sides of the re- volving table. Mr. G. M. Hopkins, of a , New York, uses two mirrors, each of which is mounted as at M on the middle part of two parallel strained wires, A B, in the manner first used for other purposes by Professor 0. Rood. Two such wires will impart to a mirror good vibration, if properly strained, and the period may be varied by attaching another wire, a b, with loads at the ends w w (fig. 138) to the back of the mirrors, so that it can be varied in angle (as c d), such an alteration altering the period of vibration. One pair of wires is of course strained vertically and the other pair horizontally, in parallel planes. Such an apparatus is easily constructed; but it is very difficult to get exact ratios with it, and difficult to arrange for the pencil oi light to ' clear' both pairs of wires. But by far the best, most effective, and most comprehensive