354 Prof. Rucker and Mr. Edser on the 



When the pressure on the wind-chest was increased, the 

 rate of revolution being nevertheless maintained constant by 

 pressing lightly on the axle of the siren with a straw, the 

 effect of the partial was more marked, but it was always 

 produced when the holes appeared to move backwards. 



On the other hand, when both notes were sounded together 

 and when the pitch was gradually reduced to the desired 

 point, the disturbance always began when the holes moved 

 slowly forwards. If the pitch fell very slowly it was possible 

 to note a reduction of the disturbance, followed by an increase 

 when the holes appeared to move backwards. 



We thus convinced ourselves that the effects of the two 

 sources of disturbance could be distinguished, and that the 

 supposed summation-tone was not due to the partial of the 

 lower note. 



Experiment VII. 



We have also succeeded in demonstrating the reality of 

 the summation-tone with a mirror-resonator constructed by 

 Professor Boys to respond to a vibration-frequency of 576. 



The rows of 15 and 12 holes being opened, notes of 320 

 and 256 vibrations were produced. When they were sounded 

 separately, the mirror moved slightly. When they were 

 sounded together, the spot of light was driven off the scale 

 when the upper note coincided with that of a 320-vibration 

 fork, but immediately returned when this pitch was lost. 



The experiment was varied by using the 16 and 12 rows, 

 and also the 16 and 9 rows. The summation-tone corresponds 

 to 576 vibrations when the upper note is of 329*15 and 360 

 vibrations in these two cases respectively. The 320-fork 

 was used, and the disturbance occurred in the one case 

 when the pitch of the note was nearly the same as before, 

 and in the other when it was about a tone higher. 



We attach great importance to this corroboration of our 

 results by an instrument of a totally different construction 

 from that first employed. 



The attempt to obtain proof of the existence of a difference- 

 tone by means of the mirror-resonator of 161 vibrations has 

 not been successful. The instrument is much less affected 

 by the note to which it responds than is that which answers 

 to 576 vibrations, even when that note is produced directly 

 by the siren. It is, therefore, perhaps not wonderful that 

 it gives no reliable evidence of the existence of a difference- 

 tone. 



We now sum up the results we have obtained in two tables. 



