1885.] 
On the Recognition of Noises. 33g 
effect still exists when the second impulse follows, and 
therefore a repetition of each impulse cannot produce a 
summation of the excitement of the deeper-toned nerve- 
tissues. When, therefore, acute sounds follow upon each 
other in long intervals, we hear them singly as noises, be- 
cause we hear them with parts of the internal ear which are 
tuned so high that they die away in a shorter time than the 
interval between the single noises. When this is not com- 
pletely the case, or when the perceptions of the single 
impulses melt into each other, we hear a chirping, or, if the 
impulse is stronger, a rattling or a creaking. 
Brticke corroborates this view by a series of experiments, 
with Savart’s wheel, and with children’s rattles : he shows that, 
for the experiments to be clear, every secondary vibration 
in any part of the apparatus must be eliminated. If this is 
not the case we obtain deep tones. When the secondary 
vibrations are checked as far as possible, a slow repetition 
of the impulse produces merely noises, but a rapid repetition 
gives distinct tones. Brucke obtains the same result with 
telephones, which produced noises if the eledtric impulses 
were slow, but tones if they were very rapid. 
In his researches Brucke arrives at the conclusion that 
we have no need to assume the existence of distinct nerves 
for the perception of noises. According to the consideration 
advanced by Prof, von Helmholtz concerning the cochlea, 
we suppose that tones are produced when certain groups of 
the cochlear fibres undergo prolonged excitement, and that 
the tones are the purer the more equable the excitement, 
and the less rapidly they change in intensity. We suppose, 
further, that noises are produced when the same nerves are 
excited only for a very short time, or when the excitements 
pass in irregular and rapid succession from one group of 
nerves to another, or when they affeft so many nerve-fibres 
simultaneously or in rapid succession, and in such alterna- 
tion that it is not possible to hear out of the whole impression 
a tone of definite pitch. 
In the view that the same nerves perceive noises if 
excited for a short time only is included the supposition that 
there is no fixed limit between tone (or sound) and noise. 
Between the sound which comes from striking a bell and the 
noise heard on striking a sheet of pasteboard there is a con- 
tinuous series in which the vibration dies away so rapidly 
that no sound is heard. Even then the noise is of different 
quality according as — chiefly or exclusively — the higher or 
the lower elements of the organ of hearing are excited. 
't he limit varies so much in different men that one perceives 
