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 



