120 



Lord Rayleigh on the Application of [June 15^ 



proved by a subsidiary experiment that the root of the flame, where it 

 issues from the burner, is the seat of sensitiveness. With this arrange- 

 ment the effect of a cardboard or glass screen interposed between the 

 reed and the fl.ame was found to be different, according as the screen 

 was close to the flame or close to the reed. In the former case the flame 

 indicated the action of sound, but in the latter remained uninfluenced. 

 Since the motion of the screen is plainly equivalent to an interchange of 

 the reed and flame, there is to all appearance a failure in the law of 

 reciprocity. 



At first sight this experiment is difficult to reconcile with theoretical 

 conclusions. It is true that the conditions under which reciprocity is to 

 be expected are not very perfectly realized, since the flame ought not to 

 be moved from one position to the other. Although the seat of sensi- 

 tiveness may be limited to the root of the flame, the tall column of 

 highly heated gas might not be without effect ; and in fact it appeared to 

 me possible that the response of the flame, when close to the screen, 

 might be due to the conduction of sound downwards along it. Not feeling 

 satisfied, however, with this explanation, I determined to repeat the ex- 

 periment, and wrote to Prof. Tyndall, asking to be allowed to see the 

 apparatus. In reply he very kindly proposed to arrange a repetition of 

 the experiment at the Royal Institution for my benefit, an offer which I 

 gladly accepted. 



The effect itself was perfectly distinct, and, as it soon appeared, was 

 not to be explained in the manner just suggested, since the response of 

 the flame when close to the screen continued, even when the upper part 

 of the heated column was protected from the direct action of the source 

 by additional screens interposed. I was more than ever puzzled until 

 Mr. Cottrell showed me another experiment in which, I believe, the key 

 of the difficulty is to be found. 



"When the axis of the tube containing the reed is directed towards the 

 flame, situated at a moderate distance, there is a distinct and immediate 

 response ; but when the axis is turned away from the flame through a 

 comparatively small angle, the effect ceases, although the distance is the 

 same as before, and there are no obstacles interposed. If now a card- 

 board screen is held in the prolongation of the axis of the reed, and at 

 such an angle as to reflect the vibrations in the direction of the flame, 

 the effect is again produced with the same apparent force as at first. 



These results prove conclusively that the reed does not behave as the 

 simple source of theory, even approximately. When the screen is close 

 (about 2 inches distant) the more powerful vibrations issuing along the 

 axis of the instrument impinge directly upon the screen, are reflected 

 back, and take no further part in the experiment. The only vibrations 

 which have a chance of reaching the flame, after diffraction round the 

 screen, are the comparatively feeble ones which issue nearly at right 

 angles with the axis. On the other hand, when the screen is close to 



