262 OPTICAL PROJECTION 



apparatus is one of reeds mounted with mirrors, as shown in 

 fig. 139, which I was able to arrange for Messrs. Newton 

 & Co. with the help of Mr. G. Neilson in the speaking part 

 of the apparatus, it being particularly difficult to make 

 any range of reed-notes vibrate on a closed box of conve- 



PlG. 139. Lissajous' Apparatus of Reeds 



nient size. 1 I at last partially overcame the difficulty 

 by supplying the boxes with wind through short rubber 

 tubes of large diameter, from large intermediate wind-chests 



1 I feel bound to state that the idea of this construction was not mine. It 

 was suggested to me entirely by seeing such a reed apparatus named, in a 

 catalogue of the apparatus to be sold of Mr. Thomas Harrison of Manchester. 

 Mr. Harrison having left England, I could learn nothing whatever about the 

 arrangement, except that it had been constructed by Dr. Mann, to whom there- 

 fore the first design of such is due. Long after, when my own was completed, 

 and much time and experiment de novo had overcome the chief difficulty by 

 quite different means, I succeeded in discovering a description of the ap- 

 paratus of Dr. James Dixon Mann in Proc. Manchester Lit. and Phil. Soc. 

 xvii. 91. It was not, however, capable of exhibiting beats, or other scroll 

 figures ; one reed-box being fixed horizontally. 



