320 Messrs. E. F. Herroun and G. F. Yeo. [Jan. 21 r 



the belief that a single wave could be transmitted, so as to excite 

 the terminals of the auditory nerve. 



On the other hand, if one complete vibration does not stimulate 

 the hearing apparatus, a distinct logical difficulty presents itself 

 when we attempt to explain how multiples of no stimulation can give- 

 rise to such an immense variety of definite effects on the cochlea and 

 the brain. If expressed as a formula, the inaudibility of single 

 vibrations appears absurd, 1 V.D. = but 528 Y.D. = C". 



Being satisfied that a single contraction of muscle could be heard 

 as a thud, and that the single thuds could be heard separately until a 

 rate was attained that fused them into a tone, it was thought 

 advisable to examine some physical instruments for the production of 

 sound, which admit of adjustment to various rates above and below 

 the lower limit of appreciation of tone. 



It is well known that, with organ pipes 32 feet long, or with a 

 monochord, the string of which by weighting 1 has been made to- 

 vibrate below thirty times per second, sounds are heard having the 

 character of an imperfectly fused rumble, in which the ear can dis- 

 tinctly detect the separate vibrations, but no distinctive tone. 



Some exception might be taken to the evidence furnished by the 

 use of pipes or strings as to the audibility of the separate waves on 

 account of the possibility of harmonic sub-division of the column of 

 air or string giving the octave as the chief harmonic. We, therefore, 

 preferred to use tuning forks, in the case of which this objection does 

 not obtain, as their vibrations are probably the most purely pendular, 

 and the first harmonic of the fork is so much higher in pitch (6j times 

 the vibration frequency of the fundamental tone), as to be always 

 readily recognisable if present. With a fork of 30 V.D. per second, 

 a sound is produced which is audible through the air, but much more 

 distinctly so by placing the base of the fork in contact with the head 

 or by fixing the fork firmly in a block of wood which was auscultated 

 by a binaural stethescope. The character of the sound remained the 

 same when transmitted by these different methods. At this ra.te of 

 vibration (thirty per second), the impression is just becoming con- 

 tinuous, but not sufficiently so as either to prevent the recognition of 

 the separate vibrations or to produce a distinctive tone. By the use of 

 brass weights, firmly clamped at different heights on the prongs of 

 the fork, its rate of vibration was reduced to 28, 24, and 20 V.D. per 

 second respectively, the rate being estimated by being recorded on a 

 smoked surface moving at a known velocity. The effect of this reduc- 

 tion of rate is that, while the intensity of the sound becomes enfeebled, 

 the separation of the constituent vibrations becomes more distinct and 

 unmistakable. Thus the audibility of the fork becomes less and less 

 as the rate is lowered, but the sound always preserves the same tone- 

 less and interrupted character. These effects of reducing the rate of 



