340 
On the Recognition of Noises. [ June 
only a noise, whilst another hears not merely a sound, but 
can indicate its place in the musical scale. (And those men 
for whom noise and sound are identical ?) 
We have also seen, in a series of explosive noises, that 
they approximate to quickly fading tones by means of the 
secondary vibrations of the air, and that, in accordance with 
the period of these vibrations, they are adapted to excite 
certain groups of nerve-fibres more strongly than others. 
For the secondary vibrations are by no means unimportant 
for the acoustic effedf. As far as these groups of nerve- 
fibres are concerned, they add their effedf to that of the 
primary wave. 
These explosive noises possess also the attributes of 
height and depth, and, as we found the same qualitative 
difference also in single waves of different times of undula- 
tion in which no secondary vibrations have been demon- 
strated, we concluded that, as theory demands, even the 
single wave may affedt the different fibres of the auditory 
nerve unequally, according to its period of vibration. 
The shrill shriek of a steam-whistle, even when so near 
that we shrink back from the painful impulse, does not con- 
vey to us the joint impression of a deep noise, not even at 
the beginning, because the single impulse does not agitate 
the more deeply-toned parts of the auditory organ in so far 
as to cause a sound-perception. 
The case is the same when two short steel cylinders are 
clashed together close by our ears, whilst the shot of a heavy 
cannon, miles away, which leaves the high-toned elements of 
our organ of hearing at perfect rest, occasions the perception 
of a deep noise. Whether the secondary vibrations (which by 
their period determine the groups of fibres to be excited) come 
into play or not, they can strengthen the effect of the former, 
but they cannot annul the adtion of the first impulse upon 
the other fibres if it has already set in. 
For the case in which the impulses lie too far apart from 
each other to occasion a sound, it is unimportant whether 
they are single or if they die away with a few vibrations 
determined by the period of the fibre-groups which are 
excited. The quality of the noise depends only on the 
wave-lengths as long as the noises are heard singly, and 
when the impulses approximate and begin to melt into each 
other the rapid change in the strength of the excitement 
produces a creaking, and not, in the first place, the sound 
which ought to be produced in accordance with the period 
in which the impulses succeed each other. 
There remain now, besides the momentary noises, explo- 
