4^4 ON THE ABSORPTION OF LIGHT 



shall (like A B) bisect the angle between the branches. 

 The branches, however, are of unequal length, the one 

 BCD being longer than the other, by a quantity equal 

 to half the length of the undulation or pulse of the musi- 

 cal note in question. It is evident, then, that if that 

 note be sounded at A, each pulse will subdivide itself at 

 B , and the divided portions will run on along the two 

 branches with equal intensities till they reunite at D d. 

 They will arrive there, however, in opposite phases, and 

 will therefore counteract each other at their point of re- 

 union, 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 difference which may be made to dis- 

 appear, by giving the longer a trifle larger area for its 

 section.* 



(10.) Suppose now that the pipes instead of being 

 cylindrical were square, and that the whole surface of 

 one side of a chamber were occupied with the on 

 A of such pipes, leaving only such intervals as might be 

 necessary to give room for their due support, and for 



* I ought to observe, that I have not made the experiment de- 

 scribed 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 mijjht be used, in which waves 

 of a certain breadth, excited by some mechanical contrivance at one 

 end, would not be propagated beyond the point of reunion, D, of 

 the tu-r can^Js into which the main channel, A B, was divided. 



