344  Prof.  C.  A.  Young  on  Recurrent  Vision* 
have  seen  noticed.  The  machine  gives,  easily,  intense  Leyden- 
jar  sparks  from  7  to  9  inches  in  length,  and  of  most  dazzling 
brilliance,  at  the  rate  of  seventy  a  minute.  When,  in  a  darkened 
room,  the  eye  is  screened  from  the  direct  light  of  the  spark,  the 
illumination  produced  is  sufficient  to  render  every  thing  in  the 
apartment  perfectly  visible;  and,  what  is  remarkable,  every  con- 
spicuous object  is  seen  twice  at  least,  with  an  interval  of  a  trifle 
less  than  a  quarter  of  a  second — the  first  time  vividly,  the  second 
time  faintly ;  often  it  is  seen  a  third,  and  sometimes  (but  only 
with  great  difficulty)  even  a  fourth  time.  The  appearance  is 
precisely  as  if  the  object  had  been  suddenly  illuminated  by  a 
light  at  first  bright,  but  rapidly  fading  to  extinction,  and  as  if, 
while  the  illumination  lasted,  the  observer  were  winking  as  fast 
as  possible. 
I  see  it  best  by  setting  up  in  front  of  the  machine,  at  a  dis- 
tance of  8  or  10  feet,  a  white  screen  having  upon  it  a  black 
cross,  with  arms  about  3  feet  long  and  1  foot  wide,  made  of 
strips  of  cambric.  That  the  phenomenon  is  really  subjective, 
and  not  due  to  a  succession  of  sparks,  is  easily  shown  by  swing- 
ing the  screen  from  side  to  side.  The  black  cross,  at  all  the 
periods  of  visibility,  occupies  the  same  place,  and  is  apparently 
stationary.  The  same  is  true  of  a  stroboscopic  disk  in  rapid  re- 
volution :  it  is  seen  several  times  by  each  spark,  but  each  time 
in  the  same  position.  There  is  no  apparent  multiplication  of  a 
moving  object  of  any  sort. 
The  interval  between  the  successive  instants  of  visibility  was 
measured  roughly  as  follows : — A  tuning-fork,  making  92J  vi- 
brations per  second,  was  adjusted  so  as  to  record  its  motion  upon 
the  smoked  surface  of  a  revolving  cylinder ;  and  an  electromagnet 
was  so  arranged  as  to  record  any  motion  of  its  armature  upon 
the  trace  of  the  fork ;  a  key  connected  with  this  magnet  was  in 
the  hands  of  the  observer.  An  assistant  turned  the  machine 
slowly,  so  as  to  produce  a  spark  once  in  two  or  three  seconds, 
while  the  observer  manipulated  the  key. 
In  my  own  case  the  mean  of  a  dozen  experiments  gave  0*22 
second  as  the  interval  between  the  first  and  second  seeing  of  the 
cross  upon  the  screen,  the  separate  results  varying  from  0'17  to 
030  second.  Another  observer  found  0*24  second  as  the  result 
of  a  similar  series. 
Whatever  the  true  explanation  may  turn  out  to  be,  the  phe- 
nomenon at  least  suggests  the  idea  of  a  reflection  of  the  nervous 
imjiulse  at  the  nerve  extremities — as  if  the  intense  impression 
upon  the  retina,  after  being  the  first  time  propagated  to  the 
brain,  were  there  reflected,  returned  to  the  retina,  and,  travelling 
again  from  the  retina  to  the  brain,  renewed  the  sensation.  I 
have  ventured  to  call  the  phenomenon  "  Recurrent  vision." 
