156 Lord Rayleigh on the Passage of 



transfer of the apparatus to the outside proved indeed to be 

 no remedy. With the slit facing upwards, observations along 

 the vertical line passing through it indicated alternations 

 nearly as marked as before. Although it is impossible to 

 avoid obstacles altogether — the observer himself and the 

 wind-bag would be the principal ones — the result seemed 

 unfavourable to the reflexion theory. 



At this stage I thought that the limitation of the disk upon 

 which the slit was mounted — its diameter was about 9 inches 

 — might be a source of complication, and the apparatus was 

 modified so as to permit the disk to form part of the general 

 floor of the laboratory. For this purpose a smaller containing 

 jar was necessary and the opportunity, was taken to replace the 

 bird-call by a small whistle or open organ-pipe whose pitch 

 lay more within the capacity of my own ears. Preliminary 

 experiments with the flame and movable reflector showed 

 that (as had been expected from the pitch f Y and dimensions 

 of the pipe) the complete wave-length was 2^ inches, giving 

 1J inch periods between nodes or between loops. 



Experiments with the apparatus thus mounted exonerated 

 the disk, for alternations of a marked kind were still recog- 

 nized commencing at a few inches' distance from the slit, 

 which might now be regarded as situated in an infinite 

 impermeable plane. The character of these alternations was 

 peculiar. Sometimes they occurred in a period of 1J inch, 

 as if due to a reflector in front which might perhaps be the 

 ceiling of the room. But as a rule the more conspicuous 

 feature was a period of about 2J inches, and this sometimes 

 manifested itself in a condition of great purity. 



Many times the thought occurred that I had misestimated 

 the octave, and that the fundamental wave-length was really 

 double what had been supposed, but this suggestion could 

 not be maintained. After all, if reflexions are admitted, the 

 period of the alternations is not limited to the half wave- 

 length. Even when stationary waves are formed truly in 

 one dimension^ i. e. in parallel planes, it is possible by crossing 

 them obliquely to encounter periods which may exceed the 

 half wave-length in any proportion. The whole wave-length 

 would in fact be a reasonable average value. In particular, 

 the whole wave-length would be the period due to the inter- 

 ference of the direct sound with one arriving from a distant 

 reflector in a perpendicular direction. 



An origin of this kind would afford an explanation of a 

 several times suspected difference of behaviour between the 

 ear and the sensitive flame. As regards the former, it makes 

 but little difference in which direction a sound arrives at 



