1878.] Films under the Action of Sonorous Vibrations. 



75 



portions of the approximately identical figures due to either fork by 

 itself, took up, while both were sounding, a swaying movement about 

 their mean position ; one complete oscillation of figure synchronizing 

 exactly with each beat* heard. The resonance-boxes had, in this expe- 

 riment, been placed with their openings exactly opposite each other, 

 and the film, which was rectangular in shape, midway between, but in 

 a horizontal plane slightly above them. The swaying motion was, under 

 these circumstances, on the whole rectilinear, as though each fork 

 alternately gave the entire figure a pull in its own direction. The 

 behaviour of the vortices was still more remarkable. With vigorous 

 and equal bowing they rotated several times in one direction during 

 the first half of each beat, and the same number of times in the 

 opposite direction during the second half of it. If, instead of occupy- 

 ing the relative positions above described, in which the forks when 

 sounding singly gave rise to antagonistic vortices, they were both 

 placed on one side of the film, the result was rotation during one half- 

 beat and inaction during the next, followed again by similar alterna- 

 tions, but the direction of rotation remaining constant throughout. 

 In this case the vortices moved most rapidly during the maximum 

 and rested during the minimum of intensity. But in others it was 

 not so, and I even observed instances where in one part of the figure 

 the rotation coincided with the ma,ximum and the quiescence with the 

 minimum, while at another part of it the exactly contrary state of 

 things prevailed. 



In every experiment hitherto described the film was either acted on 

 by the resonance of a spherical air-cavity, which practically reinforces 

 only the fundamental tone of a compound sound in unison with it ; or 

 else the vibrations employed were exclusively those of a mounted 

 tuning-fork which follow the pendulum law. Hence no other kind of 

 movement was transmitted save that which gives rise to what Helm- 

 holtz calls a " simple tone." In order to examine the effects produced 

 by composite sounds, it was desirable to let their vibrations act on a 

 film unconnected with any resonant cavity. For this purpose one end 

 of a caoutchouc tube of large bore was fitted into a metal ring fixed in a 

 horizontal plane on which the film-bearing discs could be placed. Notes 

 of the human voice, of tuning forks, organ, pipes, &c, being sounded 

 into the tube, either directly or through a funnel in the shape of an 

 ordinary ear-trumpet, their effects on films of various forms and sizes 

 could be conveniently observed. A very wide field for research was 

 thus opened up, which I do not propose to enter upon here beyond 

 simply mentioning one result obtained in this manner which possesses 

 an independent interest. When two notes, identical in pitch and 



* The absence of an English equivalent for the Grernian Scliwebung , which denotes 

 the whole phenomena from one maximum of intensity (Sclilag) to the next, is most 

 inconvenient, and makes itself very perceptible here. 



