Barrett — Note on Combination- Tones. 369 



showed they were reinforced by resonators ; now, resonators are 

 only able to reinforce a tone when pendular vibrations actually 

 exist in the air ; they have no effect on tones which exist only in 

 the auditory apparatus. Several distinguished acousticians have, 

 however, criticised and dissented from Helmholtz's proofs ; the 

 matter is fully discussed in Ellis's translation of Helmholtz's 

 " Tonempfindungen," 1 and the general conclusion arrived at by 

 Ellis was that Helmholtz's views need reconsideration. In 1886 

 Prof. Lummer confirmed Helmholtz's experiments with resonators 

 by using instead a microphone transmitter and distant tele- 

 phone receiver ; and in 1895 Professor (now Sir A.) Eiicker 

 and E. Edser proved that both difference and summation-tones did 

 exist outside the ear. For this purpose they employed tuning-fork 

 resonators, their vibration being revealed not to the ear, but to the 

 eye by means of a delicate optical method ; thus the principal 

 objection raised to Helmholtz's experiments was removed. But 

 they could only detect the objective existence of combination - 

 tones when the primaries issued from a double siren, and were 

 peculiarly powerful. They could obtain no evidence of their 

 existence outside the ear when the primaries were two tuning-forks 

 or other sources of sound. 2 Nor could Helmholtz obtain any inde- 

 pendent proof of the external existence of combination-tones 

 when the primaries were two tuning-forks, or two violins, or two 

 singers, or two separate wind-instruments. The combination- 

 tones were heard in all these cases, but they doubtless arose 

 within the mechanism of the ear ; their origin was physiological 

 and not physical. Even in this case Helmholtz's explanation, 

 as Lord Eayleigh remarks, "is admissible only when the 

 generating sounds are loud — i.e., powerful as they reach the ear." 

 The foregoing facts render the experiments with the sensitive 

 flames and water- jets of peculiar interest, inasmuch as both flame 

 and water-jet responded to the difference-tone when the primaries 

 were not only distinct but feeble— in fact, hardly audible sources 

 of sound. Obviously Helmholtz's theory cannot here account for 

 the difference-tone ; for if the combination-tone were an effect of 



1 See Appendix xx., sect. l, of Helmholtz's " Sensations of Tone." Second 

 English edition, 1S85. 



2 Proc. Physical Society, vol.^xiii., p. 412. 



