SOUND 263 



which form the base of the apparatus ; after which Mr. 

 Neilson succeeded in bringing the speaking-boxes into small 

 and compact form. 1 The wind is supplied to these chests 

 (two divisions of one box) by a Y-tube with a stopcock on 

 each fork, the trunk of the Y being connected with the 

 bellows. Each short rubber tube is furnished with a pinch- 

 cock. The reeds are severally mounted upon identical 

 bevelled wooden slides, so that any note slides into dovetails, 

 and forms for the time the front of its box ; and each is 

 mounted with a mirror of silvered glass, f-inch diameter, 

 attached to its free end by a small pillar of cork. After being 

 fitted with mirrors (which load them), the reeds are fairly 

 tuned excessive accuracy is not required (see g hereafter). 

 The reed-boxes are adjustable round vertical axes coincident 

 with the vertical diameters of the mirrors, whether the boxes 

 are in the horizontal or perpendicular position. One retains 

 a perpendicular position; the other can either be similarly 

 placed, or fixed in a horizontal position rectangularly to it, 

 being clamped in either by a screw. 



As regards the action of the apparatus, if it be confined 

 solely to Lissajous' figures, the pencil of light might be 

 reflected direct to the screen from the second mirror, as from 

 a pair of forks. But being desirous of projecting open 

 * scrolls ' also, after Tyndall's method, and especially in the 

 case of ' beats,' I adopted the arrangement shown in plan in 

 fig. 140, Omitting all details of focussing, the pencil of light 

 from the lantern L is reflected from the mirror on the first 

 reed-box, E, to that on the second, E E, and is thence reflected 



1 Dr. Mann's arrangement was quite different. He used much larger 

 boxes or speaking-chambers, with open apertures for the supply of wind. The 

 supply-tube came direct from the bellows, with a nozzle at the end contracted 

 to about one-third the size of the hole in the reed-box, and ending with a free 

 space of half an inch between this nozzle and the hole in the box. Had I 

 found his arrangement described earlier, I should probably have adopted both 

 it, and the ingenious inventor's conclusion that it was indispensable ; as it is 

 I prefer (perhaps naturally) the smaller boxes as more easily adjusted, and the 

 closed supply as more certain and using less wind. 



