Dr. R. Konig on Manometric Flames. Ill 



produced ; for it is known that, there being a difference of half 

 a vibration-period between two equal sounds in unison, while 

 the fundamental and the odd overtones are destroyed, all the 

 even overtones in both sounds vibrate without difference of phase 

 and strengthen one another. The flame also makes the octave 

 recognizable in the rotating mirror, since we see a series of low 

 wide flame-pictures, of which each single one is forked. It is 

 well in this experiment to employ a rather stronger air-pressure, 

 in order to increase the intensity of the octave in the sound of 

 the pipes. 



As this prominence of the octave at the interference of the 

 fundamentals of two sounds is demonstrated particularly well by 

 means of the double siren of Helmholtz, I represented the phe- 

 nomenon in this case also by the flames. To this end I provided 

 each of the two sounding-chests over the turning plates with a 

 tube, which permitted its interior space to be placed in direct 

 communication with the tube leading to the capsule. This tube 

 was of india-rubber, thus retaining the power of movement within 

 certain limits for the upper wind-chest of the siren, so as to be 

 able by its different positions to produce the interference or to 

 withdraw it. Invariably, if we approach the siren-chest to the 

 interference-place, we see the great vibrations of the funda- 

 mental gradually disappear, and the short forked flame take 

 their place as the picture of the octave. 



A particular apparatus, which I construct for the observation 

 of interference-phenomena of the most various kinds, is founded 

 on the method first employed by Herschel, and after him by 

 many natural philosophers. This is to produce interference by 

 permitting the waves from the same source to traverse two 

 courses differing in length by half a wave, and then to reunite 

 them. It consists of a tube that between its ends branches into 

 two arms, one of which can by drawing out be lengthened at 

 will (fig. 13). If we wish a complete interference, we must in- 

 troduce a simple note into the tube, which is joined to a reso- 

 nator, before which we sound the proper tuning-fork. If we now 

 lengthen the one arm until the difference of length of the two 

 is equal to half the wave-length of the note of the tuning-fork, 

 the waves coming from the two arms are mutually destroyed at 

 the other end of the tube ; and if we fix this into a small cavity, 

 over which a manometric capsule is placed, we see, on drawing- 

 out one of the arms of the tube, how the at first deeply serrated 

 flame- series in the rotating mirror gradually transforms itself 

 into a simple band of light, until the difference of a half wave- 

 length is attained., 



But the interference can be shown still more beautifully by 

 another arrangement. Instead of causing the arms united to a 



