of Light by Coloured Media. 409 



be extinguished ; so by analogy we may perceive how any 

 number of bright and dark lines may be produced in a spec- 

 trum unequally absorbed. 



The case last put is entirely analogous in its principle to 

 that of a phenomenon which is described in my Essay on 

 Sound*, and of which, at the time of the publication of that 

 Essay, I believed myself to have been the first and only ob- 

 server, though I have recently learned to rectify that impres- 

 sion, and have great pleasure 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 ori- 

 fice, 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 alone would utter, the re- 

 sonance of its tone is feeble, and beyond a certain interval be- 

 comes inaudible. 



The dynamical principle on which these and similar phae- 

 nomena depend is that of " forced vibrations," as it is stated in 

 the Essay on Sound above referred to, or, more generally, in 

 a more recent publication, (Cab. Cyclop., volume on Astrono- 

 my,) in terms as follow : " If one part of any system, con- 

 nected either by material ties or by the mutual attractions of 

 its members, be continually maintained by any cause, whether 

 inherent in the constitution of the system or external to it, in 

 a state of regular periodic motion, that motion will be propa- 

 gated throughout the whole system, and will give rise in every 

 member of it, and in every part of each member, to periodic 

 movements, executed in equal periods with that to which they 

 owe their origin, though not necessarily synchronous with them 

 in their maxima and minima." The general demonstration 

 of this as a dynamical theorem is given in the Essay on Sound 

 already referred to, and its applicability to the transmission of 

 light through material bodies is indicated in a note thereto ap- 

 pended. 



The mode, then, in which we may conceive the transmission 



* Encyclopedia Metropolitana, 2nd Div. vol. ii. p. 790. 

 Third Series. Vol. 3. No. 18. Dec. 1833. 3 G 



