Barrett — Note on Combination-Tones. 367 



once remarked that the note given by the flame was exactly two 

 octaves below that given by the higher fork, i.e., a note corre- 

 sponding to 128 vibrations per second. This is what might have 

 been expected, as the difference-tone of the two forks is 512 - 384 

 = 128. Selecting a Konig resonator corresponding to this low 

 note, the difference-tone arising from the flame could be distinctly 

 heard throughout the room. 1 Having briefly referred to this 

 experiment in my public lecture, I intended to pursue the matter 

 further, hoping that by this means some further light might be 

 thrown on the vexed question of the objective reality of combina- 

 tion-tones arising from distinct and independent primaries. 



Mr. Belas having, at my suggestion, taken up the photographic 

 study of water- jets under the influence of sound, succeeded in 

 obtaining photographic evidence of a sensitive water-jet responding 

 first to the same respective primaries, viz., the 512 and 384 tuning- 

 forks, and then to the difference-tone of these primaries when 

 both forks were sounded simultaneously. Moreover, the jet 

 showed the difference-tone even when the primaries were extremely 

 feeble. A similar result, Mr. Belas found, could be heard when 

 the water-jet impinged on a stretched membrane. In the fore- 

 going Paper Mr. Belas describes these experiments, accompanied 

 with admirable instantaneous photographs. 



The interest of these experiments consists in the fact that they 

 appear to indicate that the difference-tone has an objective exis- 

 tence outside the ear. As this has long been a disputed point, the 

 matter is worth further consideration. In 1740 Sorge first noticed 

 these difference-tones, subsequently called Tartini's tones. In 



1 Since writing the foregoing, I have found that, in March, 1900, a German physicist, 

 N. Schmidt, had already made a very similar observation, a brief abstract of which, 

 taken from a German scientific periodical, is published in " Science Abstracts " (vol. 

 iii., p. 700). In this case, however, the primaries were much higher in pitch and 

 more intense, arising from two Galton's whistles, and, as in my observation, the sen- 

 sitive flame itself gave forth the difference-tone. Audible difference- tones may arise 

 from primaries whose pitch is so high as to be separately inaudible : this has been 

 shown by Konig and Mayer. On this Lord Eayleigh remarks (" Theory of Sound," vol. 

 ii., p. 462), " The passage of an inaudible beat into an audible difference-tone seems to 

 be more easily explicable upon the basis of Helmholtz's theory" — that is, supposing 

 the inaudible sounds were due to very vigorous disturbances of the air. Whether this 

 is so or not, it could be tested by the sensitive flame. For my own part, I think the 

 result and its cause are similar to that above described in the text. 



