BY COLOURED MEDIA. 489 



binations of the nature above mentioned, in which seve- 

 ral different notes shall be transmitted, while the inter- 

 mediate one, finding no unisons, or near approaches to 

 unison in the systems established, shall be extinguished ; 

 so by analogy we may perceive how any number of 

 bright and dark lines may be produced in a spectrum 

 unequally absorbed. 



(14.) The case last put is entirely analogous in its 

 principle to that of a phenomenon which is described in 

 my Treatise on Sound, and of which, at the time of the 

 publication of that Treatise, I believed myself to have 

 been the first and only observer, though I have recently 

 learned to rectify that impression, and have great plea- 

 sure in referring the experiment, which is a remarkably 

 easy and striking one, to Mr Wheatstone, the author of 

 so many other ingenious and instructive experiments in 

 this department of physics. If a tuning fork be held 

 over the open end of a pipe pitched in unison with it, 

 the pipe will speak by resonance. (If the fork be disked, 

 and the aperture of the pipe be nearly covered by the 

 disk, the tone brought out is one of a clearness and 

 purity quite remarkable.) Now both Mr Wheatstone 

 and myself have observed that if two forks, purposely 

 pitched out of unison with each other, so as to yield the 

 beats of imperfect concords, be at once held over the 

 orifice, the pipe will, at one and the same moment, yield 

 both the notes, and will utter loud beats, being actually 

 out of unison with itself. In proportion, however, as 

 the pitch of one or other fork deviates from that to which 

 the length of the pipe corresponds, and which the pipe 



