Objective Reality of Combination Tones, 347 



Precautions. 



It will not be thought remarkable that so delicate an appa- 

 ratus requires careful use. 



In the first place, it was necessary to be sure that the 

 disturbance ascribed to the joint action of two notes was 

 not produced by one or both of them when acting alone. In 

 all cases, therefore, in which it was possible to determine the 

 pitch of both notes separately, each note was sounded alone 

 and carried slowly past the particular pitch at which the 

 existence of a combination-tone was to be investigated. In 

 general, no effect was produced ; but after the apparatus 

 had been in use for some little time, we were troubled by the 

 fact that two notes which did not differ much from 256 and 

 320 vibrations respectively disturbed the bands when they 

 were produced separately. 



That these effects were spurious was evident from two facts. 

 Firstly, the same notes had previously been sounded for a 

 long time with the special object of determining whether 

 they produced any effect, and had produced none. Secondly, 

 the disturbances were evidently forced. When the fork has 

 been disturbed by a vibration of frequency 64. it continues to 

 oscillate for several seconds after the disturbing note has 

 ceased. The bands alternately appear and disappear several 

 times before the final state of rest is attained. In the case 

 of a " spurious " effect, the bands reappear instantaneously 

 with absolute clearness the moment the disturbing note 

 ceases, thereby indicating a forced vibration. It was, how- 

 ever, a more difficult task to discover the cause than to de- 

 termine the character of these disturbances. Finally, it was 

 found that since the apparatus was first set up some cracks 

 had opened in the wooden pyramidal collector. A tap on the 

 side of this will disturb the bands; and when one of the 

 cracked sides was loaded with weights the disturbance due to 

 the 320 note was much reduced. The cracks were then cut 

 out, the defects made good ; and since that time this dis- 

 turbance has entirely ceased. 



The effect produced by the 256 note was due to another 

 cause. When the pyramid was removed, the open ends of 

 the resonance-boxes of various tuning-forks (256, 320, 384) 

 were held near the square of wood which was attached to the 

 fork F. It was found that the 256 fork was the only one which 

 affected the bands, and that it only produced any result when 

 the sounding-box was held near that part of the wood which 

 projected beyond the end of the prong. It was therefore 



