﻿248 
  Prof. 
  Wood 
  on 
  the 
  Fluorescence, 
  Magnetic 
  Rotation, 
  

  

  untenable. 
  The 
  spectroscope 
  was 
  directed 
  so 
  as 
  to 
  view 
  the 
  

   image 
  of 
  the 
  aluminium 
  spark 
  reflected 
  at 
  the 
  inner 
  surface 
  

   of 
  the 
  bulb, 
  everything 
  else 
  being 
  screened 
  off 
  by 
  closing 
  the 
  

   iris 
  diaphragm 
  of 
  the 
  instrument. 
  The 
  vapour 
  was 
  given 
  

   the 
  density 
  which 
  showed 
  the 
  brightest 
  visible 
  fluorescence. 
  

   If 
  the 
  2536 
  radiations 
  were 
  present, 
  going 
  o:tt* 
  as 
  a 
  regular 
  

   (i. 
  e. 
  not 
  diffuse) 
  wave, 
  the 
  line 
  ought 
  to 
  appear 
  under 
  

   these 
  conditions, 
  but 
  no 
  trace 
  of 
  it 
  appeared 
  on 
  the 
  

   plate, 
  

  

  I 
  see 
  no 
  way 
  of 
  explaining 
  its 
  failure 
  to 
  appear 
  by 
  

   absorption, 
  for 
  the 
  whole 
  surface 
  of 
  the 
  bulb 
  on 
  one 
  side 
  

   is 
  illuminated 
  by 
  the 
  light 
  of 
  the 
  spark, 
  and 
  the 
  spectrograph 
  

   receives 
  the 
  light 
  from 
  the 
  outermost 
  surface 
  of 
  the 
  vapour 
  

   mass. 
  

  

  I 
  have 
  not 
  yet 
  determined 
  whether 
  an 
  elevation 
  of 
  tem- 
  

   perature 
  causes 
  the 
  disappearance 
  of 
  the 
  line, 
  as 
  it 
  does 
  of 
  

   the 
  continuous 
  spectrum_, 
  for 
  the 
  bulbs 
  which 
  I 
  now 
  have 
  

   are 
  not 
  well 
  adapted 
  to 
  the 
  complete 
  investigation 
  of 
  the 
  

   temperature 
  eft'ect. 
  I 
  am 
  having 
  two 
  connecting 
  bulbs 
  

   made, 
  one 
  of 
  which 
  can 
  be 
  raised 
  to 
  a 
  very 
  high 
  temperature, 
  

   the 
  other 
  being 
  held 
  at 
  a 
  lower 
  temperature, 
  the 
  density 
  of 
  

   the 
  vapour 
  being 
  the 
  same 
  in 
  both. 
  This 
  bulb 
  should 
  furnish 
  

   more 
  precise 
  information. 
  

  

  The 
  failure 
  of 
  the 
  2536 
  line 
  to 
  appear 
  in 
  the 
  fluorescent 
  

   spectrum 
  when 
  the 
  vapour 
  has 
  a 
  considerable 
  density, 
  and 
  

   the 
  complete 
  destruction 
  of 
  the 
  visible 
  fluorescence 
  by 
  the 
  

   elevation 
  of 
  temperature, 
  I 
  regard 
  as 
  the 
  two 
  most 
  important 
  

   points 
  brought 
  out 
  by 
  the 
  work. 
  

  

  The 
  2536 
  line 
  is 
  connected 
  in 
  some 
  way 
  with 
  an 
  absorption- 
  

   band 
  in 
  the 
  vicinity 
  of 
  the 
  wave-length 
  1860, 
  for 
  it 
  appears 
  

   only 
  when 
  the 
  vapour 
  is 
  stimulated 
  by 
  the 
  last 
  aluminium 
  

   line, 
  and 
  the 
  cadmium 
  lines 
  which 
  are 
  completely 
  stopped 
  

   by 
  2 
  cms. 
  of 
  quartz. 
  It 
  is 
  not 
  brought 
  out 
  by 
  stimulation 
  

   with 
  the 
  other 
  brilliant 
  cadmium 
  and 
  zinc 
  lines, 
  which 
  

   excite 
  a 
  very 
  powerful 
  visible 
  fluorescence. 
  Stimulation 
  by 
  

   the 
  2536 
  line 
  from 
  the 
  mercury 
  arc 
  probably 
  brings 
  out 
  the 
  

   2536 
  fluorescent 
  line, 
  though 
  I 
  have 
  not 
  proved 
  this 
  as 
  yet, 
  for 
  

   it 
  is 
  difficult 
  to 
  be 
  sure 
  that 
  we 
  have 
  removed 
  all 
  possibility 
  

   of 
  diffused 
  light. 
  The 
  fact 
  that 
  we 
  get 
  selective 
  reflexion 
  

   of 
  this 
  light 
  when 
  the 
  vapour 
  is 
  dense, 
  indicates 
  that 
  in 
  all 
  

   probability 
  we 
  have 
  a 
  diffuse 
  radiation 
  at 
  lesser 
  densities. 
  

   If 
  the 
  ultra-violet 
  spectrum 
  of 
  the 
  cadmium 
  spark 
  is 
  focussed 
  

   upon 
  the 
  bulb, 
  it 
  is 
  found 
  that 
  the 
  brightest 
  fluorescence 
  is 
  

   excited 
  by 
  the 
  lines 
  which 
  are 
  most 
  strongly 
  absorbed, 
  which 
  

   is 
  what 
  we 
  should 
  expect. 
  

  

  