1895.] 



on the Physical Work of von Helmholtz. 



491 



test the speed of the siren. When the proper note is reached the 

 spot of light will move, and if the difference tone is objective the 

 interference bands ought to disappear simultaneously. We tried this 

 experiment several times. An observer so placed that he could not 

 see the interference bands, lifted his hand when the spot of light 

 moved. It was quite extraordinary to note the absolute agreement 

 between his movements and the behaviour of the bands. 



By throwing the spot of light and the bands near together on the 

 screen, the coincidence can be watched by a number of persons. We 

 have tried whether the difference tone is objective in four cases, and 

 in all have detected it by the disappearance of the interference bands. 

 The details of the experiments are collected in the following table. 

 In the first two experiments the first difference tone is, and in the 

 last two it is not, coincident with Konig's lower beat note. 



Difference Tones. 



Of course the question at once arises whether, when it can be 

 distinguished separately, Konig's beat tone is also objective. I do 

 not wish to express a final opinion on this point, but I may say that 

 when the rows of eight and eighteen holes were opened, the speed of 

 the siren was increased till the notes corresponding to 256 and 576 

 vibrations were produced. Konig's note would in that case have a 

 frequency of 576 — 2 x 256 = 64. We tried twice to obtain this. On 

 the second occasion, especially, all the conditions were favourable, 

 and the experiment was carried on for a long time. On neither 

 occasion did we obtain the smallest sign of an effect on the fork and 

 interference bands. 



We must next turn to the summation tone which Helmholtz dis- 

 covered. It has beeu almost universally denied that this note is 

 objective. Without going into details, it is only necessary to remark 

 that the late Mr. Ellis, the translator of the ' Tonempfindungen,' 

 who took a dispassionate view of the controversy, thought that the 

 position assumed by Helmholtz had been disproved. To the state- 

 ment of Helmholtz that " it was formerly believed that the combina- 

 tional tones were purely subjective and were produced in the ear 

 itself," Ellis appended the note : " the result of Mr. Bosanquet's and 

 Prof. Preyer's quite recent experiments is to show that they are so." 



