The  general  character  of  these  limitations  may  be  illus-
trated by  drawing  a  pencil  across  the  teeth  of  a  comb,
slowly  at  first,  and  then  with  increasing  rapidity,  until
what  at  first  are  distinctly  separate  strokes  become
merged  into  one  rough  sound  with  distinct  pitch.  They
are  the  direct  analogue  of  our  ocular  limitations,  as  de-
monstrated by  Newton's  disc.  We  thus  have  an  auditory
unit  for  clinical  work,  determined  negatively  rather  than
positively ;  subjective,  not  objective ;  dependent  upon
reception  and  perception ;  absolutely  independent  of  in-
ception ;  but  a  most  fruitful  source  of  deception  ;  a  unit
which  derives  an  especial  and  unique  value  from  absolute
independence  of  the  origin  of  its  own  cause.  Yet  it  is  just
this  indefinite,  fluctuating  unit,  in  which  danger  and  value
are  each  the  cause  of  the  other,  this  unit  of  effect,  not
cause,  which  we  are  compelled  to  use  to  measure  sums  of
cause.  Much  confusion  and  many  errors  have  resulted
from  failure  to  recognize  this  relation,  and  the  conse-
quent necessity  that  any  sonorous  unit  which  contains
terms  of  multiple  origin,  and  depends  for  its  singularity
upon  fluctuating  auditory  acumen,  cannot,  per  se,  possess
any  fixed  elements.
