﻿216 
  Scientific 
  Intelligence. 
  

  

  than 
  had 
  been 
  supposed. 
  The 
  great 
  decreases 
  in 
  absorption 
  at 
  \l> 
  

   and 
  O 
  were 
  found 
  to 
  occur 
  every 
  spring, 
  and 
  to 
  a 
  lesser 
  extent 
  

   every 
  fall 
  ; 
  but 
  were 
  occasionally 
  found 
  in 
  the 
  winter 
  also, 
  but 
  

   never 
  in 
  the 
  summer. 
  Such 
  changes 
  were 
  found 
  sometimes 
  to 
  

   go 
  through 
  their 
  whole 
  cycle 
  in 
  a 
  week, 
  and 
  the 
  absorption 
  here 
  

   is 
  found 
  to 
  largely 
  increase 
  with 
  declining 
  sun. 
  

  

  The 
  effect 
  of 
  water 
  to 
  absorb 
  in 
  this 
  region 
  was 
  studied. 
  

   Narrow 
  cells 
  of 
  glass, 
  whose 
  absorption 
  was 
  known, 
  were 
  filled 
  

   with 
  water 
  and 
  placed 
  in 
  the 
  path 
  of 
  the 
  beam 
  in 
  taking 
  

   holographs 
  of 
  the 
  solar 
  spectrum. 
  It 
  was 
  thus 
  shown 
  that 
  

   " 
  liquid 
  " 
  water 
  absorbs 
  most 
  strongly 
  at 
  the 
  particular 
  regions 
  

   where 
  these 
  annual 
  variations 
  are 
  noticed. 
  A 
  fraction 
  of 
  a 
  

   millimeter 
  thickness 
  of 
  water 
  was 
  found 
  enough 
  to 
  produce 
  a 
  

   great 
  effect 
  beyond 
  the 
  wavelength 
  ].2/x, 
  and 
  2 
  millimeters 
  

   thickness 
  produced 
  almost 
  complete 
  absorption 
  of 
  the 
  solar 
  rays 
  

   beyond 
  1.2/x. 
  

  

  (4) 
  Constants 
  of 
  prisms. 
  — 
  In 
  the 
  course 
  of 
  the 
  reduction 
  of 
  

   measures 
  on 
  holographs 
  the 
  exact 
  index 
  of 
  refraction 
  at 
  the 
  A 
  

   line 
  for 
  the 
  salt 
  and 
  fluorite 
  prisms 
  was 
  required. 
  Several 
  

   measurements 
  of 
  these 
  quantities 
  made 
  at 
  different 
  times 
  fail- 
  

   ing 
  of 
  satisfactory 
  agreement, 
  several 
  interesting 
  things 
  came 
  

   out 
  in 
  finding 
  the 
  sources 
  of 
  error. 
  

  

  Rock 
  salt 
  prisms 
  have 
  not 
  constant 
  angles. 
  It 
  was 
  shown 
  that 
  

   with 
  a 
  rising 
  temperature 
  the 
  faces 
  of 
  the 
  prism, 
  if 
  at 
  first 
  flat, 
  

   became 
  convex, 
  and 
  all 
  three 
  angles 
  of 
  the 
  prism 
  increase, 
  unless 
  

   they 
  be 
  determined 
  at 
  the 
  centers 
  of 
  the 
  faces. 
  After 
  the 
  

   discovery 
  of 
  the 
  very 
  appreciable 
  value 
  of 
  this 
  change 
  with 
  our 
  

   great 
  prisms, 
  it 
  was 
  at 
  once 
  found 
  that 
  their 
  faces, 
  when 
  polished 
  

   flat, 
  are 
  considerably 
  concave 
  alter 
  coming 
  to 
  their 
  constant 
  

   temperature. 
  This 
  is 
  because 
  of 
  the 
  surface 
  heating 
  caused 
  by 
  

   the 
  friction 
  of 
  the 
  polisher. 
  It 
  is 
  now 
  the 
  practice 
  to 
  leave 
  the 
  

   faces 
  polished 
  very 
  slightly 
  convex, 
  to 
  allow 
  for 
  this 
  temperature 
  

   change. 
  

  

  Effect 
  of 
  this 
  on 
  the 
  accuracy 
  of 
  holographs 
  and. 
  on 
  the 
  

   determination 
  of 
  prism 
  angles. 
  — 
  After 
  a 
  very 
  considerable 
  

   amount 
  of 
  analytical 
  investigation 
  it 
  was 
  shown 
  that 
  such 
  

   concavity 
  of 
  prism 
  faces 
  as 
  was 
  present 
  when 
  the 
  holographs 
  

   were 
  taken 
  need 
  introduce 
  no 
  error 
  of 
  appreciable 
  magnitude 
  in 
  

   the 
  relative 
  deviations 
  of 
  the 
  holographs, 
  provided 
  the 
  angle 
  of 
  

   the 
  prism 
  was 
  determined 
  to 
  within 
  10 
  seconds 
  of 
  arc 
  at 
  a 
  point 
  

   a 
  little 
  nearer 
  the 
  back 
  of 
  the 
  prism 
  than 
  the 
  center 
  of 
  the 
  faces, 
  

   and 
  this 
  was 
  done 
  in 
  practice. 
  However, 
  in 
  determining 
  absolute 
  

   quantities 
  like 
  the 
  index 
  of 
  refraction 
  at 
  A, 
  it 
  was 
  found 
  neces- 
  

   sary 
  to 
  be 
  far 
  more 
  particular. 
  

  

  (5) 
  Measurement 
  of 
  refractive 
  indices 
  at 
  A. 
  — 
  Diaphragms 
  just 
  

   wide 
  enough 
  for 
  visual 
  resolution 
  of 
  A 
  were 
  placed 
  symmetrically 
  

   on 
  the 
  faces, 
  and 
  the 
  prism 
  was 
  so 
  placed 
  on 
  the 
  prism 
  table 
  

   that 
  exactly 
  the 
  same 
  beam 
  of 
  light 
  entered 
  the 
  diaphragm 
  in 
  

   one 
  position 
  of 
  the 
  prism 
  as 
  in 
  the 
  other, 
  in 
  measuring 
  both 
  the 
  

   angle 
  and 
  the 
  minimum 
  deviation. 
  In 
  this 
  way 
  very 
  excellent 
  

  

  