﻿538 
  

  

  On 
  the 
  Action 
  of 
  Light 
  on 
  Selenium, 
  

  

  [June 
  17, 
  

  

  deflected 
  to 
  the 
  other 
  side, 
  as 
  the 
  heat 
  radiated 
  from 
  the 
  Bunsen 
  burner 
  

   was 
  absorbed 
  by 
  the 
  selenium. 
  

  

  On 
  making 
  the 
  flame 
  luminous, 
  the 
  needle 
  was 
  suddenly 
  deflected 
  off 
  

   the 
  scale 
  with 
  great 
  rapidity. 
  

  

  With 
  the 
  10 
  shunt 
  to 
  the 
  galvanometer 
  there 
  was 
  no 
  deflection 
  on 
  ex- 
  

   posure 
  to 
  the 
  ordinary 
  Bunsen 
  flame 
  ; 
  but 
  with 
  the 
  luminous 
  flame 
  there 
  

   was 
  a 
  sudden 
  deflection, 
  which 
  increased 
  to 
  250 
  divisions 
  of 
  the 
  scale 
  in 
  a 
  

   few 
  seconds. 
  

  

  This 
  corresponded 
  to 
  a 
  change 
  of 
  resistance 
  in 
  E 
  of 
  about 
  1250 
  ohms. 
  

  

  This 
  experiment 
  was 
  repeated 
  in 
  a 
  slightly 
  different 
  way. 
  The 
  sele- 
  

   nium 
  was 
  balanced, 
  and 
  before 
  exposure 
  to 
  the 
  luminous 
  Bunsen 
  flame, 
  

   E 
  was 
  diminished 
  by 
  1000 
  ohms. 
  On 
  making 
  contact 
  and 
  exposing 
  at 
  

   the 
  same 
  instant, 
  there 
  was 
  a 
  slight 
  deflection, 
  showing 
  that 
  the 
  sudden 
  

   effect 
  was 
  equivalent 
  to 
  rather 
  less 
  than 
  1000 
  ohms 
  ; 
  but 
  in 
  a 
  very 
  few 
  

   seconds 
  the 
  needle 
  was 
  at 
  rest 
  at 
  zero, 
  and 
  to 
  keep 
  the 
  needle 
  at 
  zero 
  

   the 
  resistance 
  was 
  further 
  diminished 
  by 
  300 
  ohms. 
  

  

  The 
  resistance 
  had 
  been 
  diminished 
  by 
  one 
  fourth 
  of 
  its 
  whole 
  amount 
  

   in 
  less 
  than 
  one 
  minute 
  in 
  consequence 
  of 
  the 
  exposure. 
  

  

  Exposure 
  to 
  an 
  ordinary 
  wax 
  taper 
  diminished 
  the 
  resistance 
  of 
  the 
  

   selenium 
  by 
  300,000 
  ohms, 
  or 
  about 
  one 
  eighth 
  part 
  of 
  its 
  whole 
  resist- 
  

   ance. 
  

  

  The 
  illuminating 
  powers 
  of 
  these 
  sources 
  of 
  light 
  were 
  compared 
  by 
  

   means 
  of 
  the 
  Bunsen 
  photometer. 
  

  

  The 
  light 
  of 
  the 
  ordinary 
  Bunsen 
  flame 
  could 
  scarcely 
  be 
  measured, 
  but 
  

   was 
  somewhere 
  about 
  part 
  of 
  a 
  candle, 
  and 
  of 
  the 
  luminous 
  Bunsen 
  

   flame 
  about 
  10 
  candles, 
  whilst 
  the 
  light 
  from 
  the 
  taper 
  was 
  at 
  its 
  best 
  

   rather 
  more 
  than 
  one 
  fourth 
  of 
  a 
  candle. 
  

  

  The 
  heating 
  effects 
  of 
  these 
  three 
  sources 
  were 
  compared 
  by 
  means 
  of 
  

   the 
  thermo-electric 
  pile 
  and 
  delicate 
  astatic 
  galvanometer. 
  

  

  At 
  a 
  distance 
  of 
  one 
  foot 
  from 
  the 
  face 
  of 
  the 
  pile 
  the 
  deflection 
  

   produced 
  by 
  the 
  ordinary 
  Bunsen 
  flame 
  was 
  46 
  g°, 
  and 
  by 
  the 
  luminous 
  

   Bunsen 
  flame 
  was 
  52°, 
  whilst 
  the 
  taper 
  produced 
  no 
  effect 
  which 
  could 
  be 
  

   measured. 
  

  

  These 
  experiments 
  clearly 
  show 
  that 
  very 
  little 
  effect 
  is 
  produced 
  by 
  

   the 
  radiation 
  of 
  obscure 
  heat, 
  but 
  that 
  the 
  effect 
  is 
  due 
  almost 
  entirely, 
  if 
  

   not 
  entirely, 
  to 
  light. 
  

  

  As 
  the 
  effects 
  produced 
  were 
  measured 
  in 
  deflections 
  of 
  the 
  needle, 
  

   some 
  series 
  of 
  experiments 
  were 
  made 
  to 
  determine 
  the 
  value 
  in 
  resist- 
  

   ances 
  equivalent 
  to 
  the 
  divisions 
  of 
  the 
  scale, 
  from 
  which 
  it 
  appeared 
  

   that 
  with 
  the 
  10 
  shunt 
  to 
  the 
  galvanometer 
  and 
  with 
  30 
  cells, 
  20 
  divisions 
  

   of 
  the 
  scale 
  were 
  equivalent 
  to 
  100 
  ohms 
  resistance 
  ; 
  and 
  without 
  the 
  

   shunt, 
  100 
  divisions 
  of 
  the 
  scale 
  were 
  equivalent 
  to 
  from 
  100 
  to 
  110 
  

   ohms. 
  

  

  Experiments 
  were 
  also 
  made 
  to 
  determine 
  whether 
  moonlight 
  would 
  

   produce 
  any 
  change 
  in 
  the 
  electrical 
  resistance 
  of 
  selenium. 
  The 
  experi- 
  

  

  