406 Sir John F. W. Herschel on the Absorption 



ever, in opposite phases, and will therefore destroy each other 

 at their point of reunion, and in every point of their subsequent 

 course along the pipe D E ; so that on applying the ear at E 

 no sound should be heard, or at best a very feeble one, arising 

 from some slight inequality in the intensities wherewith the 

 undulations arrive by the longer and shorter pipe, — a differ- 

 ence which may be made to disappear, by giving the longer a 

 trifle larger area for its section *. 



Suppose now that the pipe instead of being cylindrical were 

 square, and that the whole surface of one side of a chamber 

 were occupied with the orifices A of such pipes, leaving only 

 such intervals as might be necessary to give room for their 

 due support, and for their subdivision according to the con- 

 dition above explained; and suppose, further, that the other 

 ends (E) of all the reunited pipes opened out, in like manner, 

 into another chamber, at some considerable distance from the 

 first, and separated from it by masonry or some material, 

 filling in all the intervals between the pipes, so as to be com- 

 pletely impervious to sound. Things being so disposed, let the 

 whole scale be sounded, or a concert of music performed in 

 the first chamber, then will every note, except that one to 

 which the pipes are thus rendered impervious, be transmitted. 

 The scale, therefore, so transmitted, will be deficient by that 

 note, which has been, to use the language of photologists, 

 absorbed in its passage. If several such chambers were dis- 

 posed in succession, communicating by compound pipes, 

 rendered impervious (or wituned, as we may term it,) to so 

 many different notes, all these would be wanting in the scale 

 on its arrival in the last chamber ; thus imitating a spectrum 

 in which several rays have been absorbed in their passage 

 through a coloured medium. 



In my Essay on Light, Art. 505, I have suggested, as a 

 possible origin of the fixed lines in the solar spectrum, and 

 (pari ratione) of the deficient or less bright spaces in the 

 spectra of various flames, that the same indisposition in the 

 molecules of an absorbent body to permit the passage of a par- 

 ticular coloured ray through them, may constitute an obstacle, 

 in limine^ to the production of that ray from them. The follow- 

 ing easy experiment will explain my meaning. Take two 



* I ought to observe, that I have not made the experiment described in 

 the text, nor am I aware that it has ever been made ; but it is easy to see that 

 it ought to succeed, and would furnish an apt enough illustration of the 

 principle of interference. Instead of a pipe, inclosing air, a canal of water 

 might be used, in which waves of a certain breadth, excited by some me- 

 chanical contrivance at one end, would not be propagated beyond the point 

 of reunion, D, of the two canals into which the main channel, A B, was di- 

 vided. 



