﻿Improvement 
  in 
  Method 
  of 
  Determining 
  Visibility 
  Curves. 
  395 
  

  

  action 
  of 
  penetrating 
  rays 
  coming 
  from 
  external 
  

   sources. 
  

  

  6. 
  By 
  various 
  tests 
  it 
  was 
  found 
  impossible 
  to 
  trace 
  the 
  

  

  activity 
  of 
  potassium 
  salts 
  to 
  the 
  existence 
  in 
  them 
  of 
  

   minute 
  quantities 
  of 
  any 
  of 
  the 
  known 
  radioactive 
  

   elements 
  or 
  their 
  active 
  products. 
  

  

  7. 
  Of 
  all 
  the 
  elements 
  in 
  the 
  alkali 
  group, 
  potassium 
  alone 
  

  

  has 
  been 
  found 
  to 
  exhibit 
  marked 
  radioactivity. 
  Both 
  

   sodium 
  and 
  all 
  of 
  its 
  salts 
  were 
  found 
  to 
  be 
  quite 
  

   inactive, 
  and 
  although 
  a 
  sample 
  of 
  rubidium 
  alum 
  

   exhibited 
  an 
  activity 
  which 
  was 
  extremely 
  small, 
  and 
  

   a 
  specimen 
  of 
  CEesium 
  chloride 
  one 
  which 
  was 
  only 
  

   just 
  measurable, 
  sufficient 
  evidence 
  has 
  not 
  been 
  

   adduced 
  to 
  support 
  exclusively 
  the 
  view 
  that 
  the 
  

   activities 
  observed 
  were 
  due 
  to 
  a 
  physical 
  property 
  of 
  

   the 
  metallic 
  constituents 
  of 
  these 
  salts. 
  

  

  In 
  conclusion- 
  we 
  wish 
  to 
  express 
  our 
  very 
  great 
  indebted- 
  

   ness 
  to 
  Prof. 
  Lang 
  and 
  to 
  Prof. 
  Lash 
  Miller, 
  as 
  well 
  as 
  to 
  

   other 
  members 
  of 
  the 
  staff 
  in 
  the 
  Department 
  of 
  Chemistry, 
  

   for 
  their 
  kindness 
  in 
  placing 
  at 
  our 
  disposal 
  a 
  number 
  of 
  the 
  

   specimens 
  of 
  the 
  salts 
  examined. 
  

  

  Physical 
  Laboratory, 
  University 
  of 
  Toronto, 
  v 
  

  

  August 
  5 
  ; 
  1908. 
  

  

  XXXII. 
  Note 
  on 
  an 
  Improvement 
  in 
  the 
  Method 
  of 
  Deter- 
  

   mining 
  Visibility 
  Curves. 
  By 
  C. 
  S. 
  Weight, 
  B. 
  A., 
  1851 
  

   Exhibition 
  Scholar^ 
  1908, 
  University 
  of 
  Toronto*. 
  

  

  ONE 
  of 
  the 
  chief 
  objects 
  of 
  Michelson 
  in 
  designing 
  the 
  

   interferometer 
  which 
  bears 
  his 
  name, 
  was 
  to 
  analyse 
  a 
  

   source 
  of 
  light 
  and 
  determine 
  if 
  possible 
  the 
  distribution 
  of 
  

   intensity 
  in 
  that 
  source. 
  This 
  is 
  arrived 
  at 
  by 
  estimating 
  

   the 
  visibility 
  of 
  the 
  fringes 
  formed 
  by 
  the 
  interference 
  of 
  

   the 
  tw^o 
  beams 
  of 
  light 
  in 
  the 
  interferometer 
  for 
  various 
  

   differences 
  in 
  path 
  of 
  the 
  two 
  rays. 
  

  

  The 
  method 
  of 
  observation 
  consists 
  simply 
  in 
  moving 
  back 
  

   one 
  interferometer 
  plate 
  till 
  the 
  difference 
  in 
  path 
  A 
  between 
  

   the 
  two 
  interfering 
  beams 
  is 
  such 
  as 
  to 
  give 
  a 
  visibility 
  of 
  

   one 
  half 
  the 
  maximum 
  — 
  the 
  latter 
  being 
  obtained 
  for 
  A 
  — 
  0. 
  

   The 
  half-width 
  of 
  the 
  spectral 
  line 
  used 
  as 
  the 
  source 
  of 
  

  

  0*29 
  X 
  2 
  

   illumination 
  is 
  then 
  given 
  by 
  — 
  -jr 
  — 
  • 
  

  

  While 
  determining 
  the 
  visibility 
  curves 
  for 
  the 
  spectral 
  

   lines 
  of 
  some 
  of 
  the 
  elements, 
  it 
  occurred 
  to 
  the 
  writer 
  that 
  

  

  * 
  Communicated 
  by 
  Prof. 
  J. 
  C, 
  McLennan. 
  

  

  