﻿412 
  Scientific 
  Intelligence. 
  

  

  impossible 
  to 
  obtain 
  a 
  suitable 
  cement 
  for 
  fastening 
  together 
  the 
  

   transparent 
  parts 
  of 
  the 
  various 
  t} 
  r 
  pes 
  of 
  cells 
  required. 
  Cements 
  

   cause 
  trouble 
  in, 
  at 
  least, 
  three 
  different 
  ways, 
  (a) 
  They 
  deform 
  

   or 
  strain 
  glass 
  which 
  has 
  been 
  figured 
  plane 
  or 
  spherical 
  to 
  the 
  

   highest 
  degree 
  of 
  accuracy, 
  (b) 
  ~No 
  one 
  cement 
  is 
  immune 
  to 
  

   chemical 
  action 
  when 
  all 
  kinds 
  of 
  liquids 
  at 
  various 
  temperatures 
  

   come 
  in 
  contact 
  with 
  it. 
  (c) 
  It 
  is 
  very 
  difficult, 
  if 
  not 
  practically 
  

   impossible, 
  to 
  cement 
  optically 
  plane 
  plates 
  to 
  a 
  separating 
  piece 
  

   so 
  as 
  to 
  obtain 
  perfect 
  parallelism 
  between 
  the 
  surfaces. 
  Certain 
  

   experiments 
  recently 
  performed, 
  in 
  the 
  research 
  laboratories 
  of 
  

   Adam 
  Hilger, 
  by 
  R. 
  G. 
  Parker 
  and 
  A. 
  J. 
  Dalladay 
  give 
  excel- 
  

   lent 
  promise 
  of 
  overcoming 
  all 
  of 
  the 
  disadvantages 
  pertaining 
  to 
  

   the 
  use 
  of 
  cements. 
  The 
  basic 
  idea 
  consists 
  in 
  heating 
  the 
  pieces 
  

   of 
  glass 
  — 
  while 
  under 
  pressure 
  and 
  in 
  optical 
  contact 
  — 
  until 
  they 
  

   unite 
  and 
  form 
  one 
  single 
  object. 
  

  

  The 
  necessary 
  conditions 
  of 
  perfect 
  figuring 
  and 
  absolute 
  clean- 
  

   liness 
  present 
  no 
  insurmountable 
  difficulties. 
  The 
  problem 
  there- 
  

   fore 
  resolves 
  itself 
  into 
  a 
  careful 
  study 
  of 
  the 
  proper 
  thermal 
  

   treatment. 
  The 
  temperature 
  for 
  complete 
  union 
  must 
  not 
  only 
  

   be 
  below 
  the 
  melting 
  point 
  of 
  the 
  particular 
  kind 
  of 
  glass 
  under 
  

   investigation, 
  but 
  it 
  must 
  also 
  be 
  inferior 
  to 
  the 
  annealing 
  tem- 
  

   perature, 
  for, 
  even 
  at 
  the 
  lower 
  temperature 
  slight 
  non-uniform 
  

   stress 
  will 
  give 
  rise 
  to 
  permanent 
  deformation 
  of 
  the 
  optically 
  per- 
  

   fect 
  surfaces. 
  By 
  keeping 
  two 
  pieces 
  of 
  the 
  same 
  kind 
  of 
  glass 
  

   in 
  optical 
  contact, 
  for 
  about 
  one 
  hour, 
  (under 
  suitably 
  applied 
  

   pressure) 
  at 
  a 
  temperature 
  60° 
  C. 
  or 
  70° 
  C. 
  below 
  the 
  annealing 
  

   temperature, 
  it 
  was 
  found 
  that 
  complete 
  union, 
  without 
  deforma- 
  

   tion 
  or 
  appreciable 
  strain, 
  resulted. 
  

  

  This 
  significant 
  result 
  was 
  deduced 
  from 
  the 
  empirical 
  formula 
  

  

  T-Ti 
  

  

  established 
  by 
  the 
  investigators. 
  They 
  found 
  that 
  S 
  T 
  =S 
  Tl 
  A 
  N 
  , 
  

   where 
  S 
  T 
  and 
  S 
  Tl 
  are 
  the 
  "mobilities" 
  at 
  the 
  temperatures 
  T 
  and 
  

   T\, 
  respectively, 
  — 
  measured 
  by 
  the 
  rate 
  of 
  deformation 
  of 
  glass 
  

   under 
  constant 
  stress, 
  — 
  and 
  A 
  and 
  N 
  are 
  constants 
  for 
  the 
  special 
  

   sort 
  of 
  glass 
  concerned. 
  The 
  authors 
  say: 
  "The 
  quantity 
  N 
  is 
  

   comparatively 
  small, 
  so 
  that 
  for 
  quite 
  small 
  values 
  of 
  T 
  — 
  T 
  1 
  , 
  

  

  T-T 
  

  

  the 
  index 
  — 
  — 
  — 
  } 
  would 
  become 
  equal 
  to 
  1, 
  2, 
  3, 
  . 
  . 
  . 
  If, 
  for 
  ex- 
  

   ample, 
  we 
  take 
  !N" 
  = 
  8, 
  A 
  = 
  2, 
  and 
  compare 
  the 
  values 
  of 
  the 
  

   mobility 
  of 
  the 
  particular 
  glass 
  at 
  temperatures 
  of 
  T 
  = 
  560° 
  C. 
  

   and 
  T, 
  = 
  552° 
  C, 
  544°, 
  536° 
  successively, 
  we 
  find 
  the 
  respective 
  

   values 
  of 
  the 
  mobility 
  to 
  be 
  

  

  |^- 
  = 
  2 
  1 
  when 
  I\ 
  =z 
  552° 
  C. 
  

  

  T 
  ' 
  = 
  2 
  2 
  = 
  4 
  when 
  T 
  1 
  = 
  544° 
  C. 
  

   = 
  2 
  3 
  = 
  8 
  when 
  T\ 
  = 
  536° 
  C. 
  

  

  That 
  is, 
  the 
  glass 
  becomes 
  twice 
  as 
  " 
  soft 
  " 
  for 
  every 
  rise 
  of 
  

   temperature 
  of 
  8° 
  C. 
  Therefore, 
  at 
  60° 
  or 
  70° 
  C. 
  below 
  the 
  

  

  