﻿Notices 
  respecting 
  New 
  Books. 
  279 
  

  

  and 
  short-circuited 
  until 
  there 
  is 
  no 
  difference 
  of 
  potential 
  between 
  

   them. 
  One 
  of 
  the 
  plates 
  is 
  then 
  exposed 
  either 
  to 
  white 
  light 
  or 
  

   to 
  a 
  portion 
  of 
  the 
  spectrum, 
  and 
  the 
  difference 
  of 
  potential 
  pro- 
  

   duced 
  between 
  the 
  plates 
  is 
  compared 
  with 
  a 
  standard 
  Daniell 
  cell 
  

   by 
  a 
  potentiometer 
  method. 
  Tables 
  and 
  curves 
  are 
  given, 
  showing 
  

   the 
  behaviour 
  of 
  various 
  substances 
  when 
  illuminated, 
  and 
  the 
  

   author 
  sums 
  up 
  the 
  results 
  of 
  his 
  investigation 
  as 
  follows 
  : 
  — 
  

  

  1. 
  Plates 
  of 
  oxide, 
  sulphide, 
  fluoride, 
  chloride, 
  bromide, 
  or 
  iodide 
  

   of 
  copper, 
  plates 
  of 
  oxide 
  or 
  sulphide 
  of 
  tin, 
  and 
  of 
  sulphide 
  of 
  

   silver, 
  may, 
  under 
  certain 
  conditions, 
  form 
  actinoineters 
  of 
  great 
  

   sensitiveness. 
  

  

  2. 
  For 
  a 
  given 
  actinometer, 
  other 
  things 
  being 
  equal, 
  the 
  

   electromotive 
  force 
  depends 
  on 
  the 
  electrolyte. 
  

  

  3. 
  In 
  the 
  spectrum 
  each 
  actinometer 
  shows 
  a 
  maximum 
  of 
  

   sensitiveness 
  for 
  a 
  wave-length 
  depending 
  only 
  on 
  the 
  nature 
  

   of 
  the 
  sensitive 
  plate, 
  and 
  independent 
  of 
  the 
  electrolyte. 
  

  

  4. 
  When 
  the 
  plates 
  of 
  an 
  actinometer 
  of 
  oxide, 
  fluoride, 
  chloride, 
  

   bromide, 
  or 
  iodide 
  of 
  copper 
  are 
  immersed 
  in 
  solutions 
  of 
  different 
  

   colouring-matters, 
  the 
  electromotive 
  force 
  produced 
  by 
  light 
  of 
  

   given 
  intensity 
  is 
  increased. 
  In 
  the 
  spectrum 
  the 
  electromotive 
  

   force 
  is 
  increased 
  for 
  radiation 
  of 
  all 
  wave-lengths, 
  but 
  the 
  

   maximum 
  of 
  sensitiveness 
  corresponds 
  to 
  a 
  wave-length 
  depending 
  

   on 
  the 
  colouring-matter 
  chosen. 
  The 
  position 
  of 
  this 
  maximum 
  is 
  

   independent 
  of 
  the 
  copper 
  salt 
  of 
  which 
  the 
  actinometer 
  is 
  formed. 
  

  

  5. 
  In 
  the 
  spectrum, 
  for 
  an 
  actinometer 
  with 
  coloured 
  plates, 
  the 
  

   maximum 
  electromotive 
  force 
  due 
  to 
  the 
  colouring-matter 
  occurs 
  

   always 
  with 
  longer 
  waves 
  than 
  those 
  corresponding 
  to 
  the 
  maximum 
  

   absorption 
  of 
  light 
  by 
  the 
  colouring 
  matter 
  itself. 
  

  

  The 
  author 
  points 
  out 
  several 
  possible 
  applications 
  of 
  electro- 
  

   chemical 
  actinometers, 
  their 
  great 
  advantage 
  being 
  that 
  their 
  

   electromotive 
  force 
  returns 
  to 
  zero 
  on 
  removal 
  of 
  the 
  illumination. 
  

  

  J. 
  L. 
  H. 
  

  

  Observational 
  Astronomy. 
  By 
  Aethuh 
  Mee, 
  F.ll.A.S. 
  

   2nd 
  Edition. 
  Western 
  Mail, 
  Limited 
  : 
  Cardiff 
  and 
  London. 
  

  

  The 
  first 
  edition 
  of 
  this 
  work 
  appeared 
  in 
  1893, 
  and 
  met 
  with 
  

   well-deserved 
  appreciation 
  from 
  amateur 
  astronomers. 
  It 
  has 
  now 
  

   been 
  thoroughly 
  revised, 
  partly 
  re- 
  written, 
  and 
  brought 
  up 
  to 
  

   date. 
  Profusely 
  illustrated 
  and 
  abounding 
  in 
  facts 
  tersely, 
  plea- 
  

   santly, 
  and 
  correctly 
  described, 
  the 
  new 
  volume 
  is 
  well 
  calculated 
  

   to 
  prove 
  a 
  safe 
  and 
  efficient 
  guide 
  to 
  the 
  young 
  telescopist, 
  

   Written 
  in 
  a 
  chatty 
  attractive 
  style, 
  the 
  information 
  imparted 
  

   holds 
  the 
  attention 
  ; 
  and 
  the 
  work 
  forms 
  not 
  only 
  a 
  compendium 
  

   of 
  astronomical 
  history, 
  but 
  a 
  comprehensive 
  handbook 
  of 
  in- 
  

   struction 
  for 
  beginners. 
  It 
  matters 
  not 
  in 
  what 
  direction 
  or 
  

   department 
  the 
  student 
  may 
  essay 
  to 
  achieve 
  distinction, 
  he 
  will 
  find 
  

   in 
  these 
  pages 
  the 
  stepping-stones 
  to 
  success 
  clearly 
  indicated. 
  

   The 
  author, 
  in 
  preparing 
  the 
  volume, 
  has 
  received 
  valuable 
  help 
  in 
  

   several 
  branches 
  from 
  specialists, 
  and 
  this, 
  in 
  combination 
  with 
  his 
  

  

  