. Dr. A. M. Mayer's Researches in Acoustics. 271 



could issue between them and the surfaces of the rotating 

 disk, because the flanges entirely covered the hole in the disk 

 while this hole traversed the tubes T and T. A tube of 

 caoutchouc led from the tube T' to the ear. 



Although a film of oil was between the flanges and the 

 disk, and the disks were so accurately made that the greatest 

 departure from uniformity of thickness amounted to only 

 ^ millim., yet the sounds produced by the sliding of the 

 disk between the flanges caused much distraction in the 

 perception of the sound from the resonator, which was adapted 

 to the tube T, of J centim. in diameter. The results obtained 

 with this apparatus agreed with those given by apparatus [A] . 



[D.] I had formed great hopes of having the best apparatus 

 for the determination of the duration of a residual sensation in 



Fiff. 10. 



W 



:nx 



the one shown in fig. 10. A is the lower drum of a Helmholtz 

 double siren; D, the perforated disk of the siren, which was 

 rotated by the driving-wheel TV. The disk D was enclosed in a 

 cover, C, of the form shown in fig. 10, clamped to the drum of 

 the siren. The sound issued from the box thus formed, and was 

 conveyed to the ear by the tube E, to which was attached a tube 

 of caoutchouc. The sound to be experimented on was conveyed 

 from the fork, F, and resonator, R, through a long tube, T, to 

 the drum of the siren. By placing pulleys, P, of various dia- 

 meters on the axle of the disk of the siren, and by opening one 

 or another of the various circles of holes in the drum A, I had 



