Sound through Nc 



Slits. 



157 



the end of the hearing-tube ; but it is otherwise with the latter, 

 whose excitation is due, not to variable pressure, but to 

 variable motion. If, as usual, the flame burns vertically, it 

 is insensitive to sounds arriving in a vertical direction, 

 and even in the horizontal plane the sensitiveness is of a 

 semi-circular character, vanishing in two opposite azimuths*. 

 It may thus easily happen in the above experiment that the 

 flame may be insensitive to reflected sounds, which are 

 nevertheless capable of influencing the ear. 



The difficulty of accepting the explanation by reflected 

 waves is in great degree that of understanding how they can 

 be powerful enough in comparison with the direct sound at 

 such short distances from the source. In forming a judgment 

 we must bear in mind that it is amplitudes that are com- 

 pounded and not intensities, so that as in the theory of 

 Newton's rings seen by transmission, a sound which would be 

 inaudible by itself may be competent to cause a very per- 

 ceptible variation in the loudness of another. 



In order to obtain further evidence a modification was 

 introduced in ihe terminal of the hearing-tube. Thus if this 

 be a T-piece (fig. 1) of such dimensions that the head of the 

 T measures half a wave-length, discrimination will be made 



Ficr 1. 



between different directions somewhat as in the case of the 

 flame. If the head be parallel to a wave-front, the two 

 openings co-operate ; but on the other hand if the head be 

 perpendicular to the wave-front, the phases at the ends are 

 opposed and nothing is propagated to the ear. This is the 

 reciprocal of the effect noticed on a former occasion | that 

 open organ-pipes emit little sound in the direction of their 

 length. The T-piece may be used to eliminate a reflected 

 sound travelling at right angles to the one which it is desired 

 to isolate. 



For my purpose a more symmetrical arrangement was 

 preferable which should treat similarly sounds arriving in all 



* < Nature,' xxxviii. p. 208 (1888) ; 'Scientific Papers/ iii. p. 24. 

 f Phil. Mag. vi. p. 304 (1903). 



