﻿446 
  Scientific 
  Intelligence. 
  

  

  density 
  and 
  elasticity 
  to 
  a 
  consideration 
  of 
  the 
  choice 
  of 
  materials 
  

   for 
  various 
  commercial 
  uses. 
  Chapter 
  ix 
  treats 
  of 
  thermal 
  and 
  

   elastic 
  alterations 
  which 
  depend 
  upon 
  the 
  recent 
  history 
  of 
  the 
  

   substance 
  in 
  question 
  ; 
  this 
  includes 
  a 
  discussion 
  of 
  the 
  best 
  

   materials 
  for 
  thermometers, 
  a 
  field 
  in 
  which, 
  as 
  is 
  well 
  known, 
  

   extraordinary 
  improvement 
  has 
  been 
  attained. 
  Chapter 
  x 
  is 
  

   given 
  to 
  the 
  chemical 
  behavior 
  of 
  the 
  surfaces 
  of 
  the 
  various 
  types 
  

   of 
  glass. 
  Chapter 
  xi 
  contains 
  a 
  discussion 
  of 
  the 
  electrical 
  and 
  

   magneto-optical 
  properties 
  of 
  glasses, 
  including 
  a 
  description 
  of 
  a 
  

   glass 
  designed 
  for 
  maximum 
  transparency 
  for 
  Rontgen 
  rays. 
  

  

  Turning 
  to 
  the 
  portion 
  which 
  treats 
  more 
  particularly 
  of 
  opti- 
  

   cal 
  questions, 
  we 
  find 
  in 
  the 
  first 
  chapter 
  an 
  interesting 
  history 
  of 
  

   the 
  investigations 
  undertaken 
  by 
  Dr. 
  Schott 
  with 
  the 
  direct 
  aim 
  

   of 
  exhausting 
  all 
  possible 
  means 
  for 
  varying 
  the 
  range 
  and 
  

   improving 
  the 
  properties 
  of 
  optical 
  glasses. 
  This 
  is 
  followed 
  by 
  

   chapters 
  reviewing 
  the 
  optical 
  constants 
  of 
  the 
  large 
  number 
  of 
  

   products 
  thus 
  secured 
  ; 
  treating 
  of 
  the 
  history 
  and 
  advantages 
  of 
  

   Abbe's 
  remarkable 
  invention 
  of 
  the 
  apochromatic 
  microscope 
  ; 
  

   the 
  improvements 
  in 
  the 
  photographic 
  camera 
  objective 
  directly 
  

   consequent 
  on 
  the 
  enlarged 
  range 
  of 
  materials 
  at 
  the 
  command 
  of 
  

   the 
  modern 
  optician, 
  and, 
  finally, 
  in 
  Chapter 
  vi, 
  the 
  general 
  dis- 
  

   cussion 
  of 
  telescopes. 
  In 
  the 
  last 
  a 
  description 
  of 
  the 
  successful 
  

   application 
  of 
  Porro's 
  ingenious 
  invention 
  of 
  the 
  inverting 
  prism 
  

   to 
  binoculars 
  by 
  the 
  Zeiss 
  firm 
  is 
  of 
  interest 
  ; 
  but 
  undoubtedly 
  

   the 
  scientific 
  optician 
  will 
  turn 
  with 
  the 
  liveliest 
  interest 
  to 
  that 
  

   section 
  which 
  treats 
  of 
  the 
  promise 
  of 
  improvement 
  of 
  the 
  astro- 
  

   nomical 
  telescope. 
  Here, 
  it 
  must 
  be 
  confessed, 
  disappointment 
  

   awaits 
  us. 
  It 
  is 
  true 
  that 
  a 
  combination 
  of 
  phosphate 
  crown 
  and 
  

   borate 
  flint 
  is 
  shown 
  to 
  yield 
  a 
  binary 
  objective 
  of 
  greatly 
  dimin- 
  

   ished 
  secondary 
  color 
  aberration, 
  but 
  the 
  mechanical 
  and 
  chem- 
  

   ical 
  properties 
  of 
  these 
  varieties 
  of 
  glass 
  quite 
  preclude 
  their 
  

   employment 
  in 
  large 
  lenses. 
  A 
  combination 
  of 
  apparently 
  unex- 
  

   ceptionable 
  crown 
  and 
  flint 
  glasses 
  is 
  highly 
  praised 
  by 
  Prof. 
  

   Wolff 
  as 
  practically 
  free 
  from 
  secondary 
  color 
  as 
  far 
  as 
  the 
  eye 
  

   can 
  recognize, 
  but 
  as 
  the 
  excessive 
  ratio 
  of 
  focal 
  length 
  of 
  21/1 
  

   is 
  requisite 
  in 
  order 
  to 
  sufficiently 
  reduce 
  the 
  chromatic 
  difference 
  

   of 
  spherical 
  aberration 
  even 
  for 
  a 
  telescope 
  of 
  about 
  eight 
  inches 
  

   aperture, 
  the 
  real 
  gain 
  is 
  problematical. 
  Large 
  telescopes 
  of 
  

   triple 
  combinations 
  are 
  probably 
  still 
  less 
  promising, 
  not 
  only 
  on 
  

   account 
  of 
  excessive 
  length 
  but 
  also 
  from 
  more 
  numerous 
  reflec- 
  

   tions, 
  increased 
  absorption, 
  and 
  enhanced 
  cost. 
  It 
  is 
  somewhat 
  

   remarkable 
  that 
  a 
  binary 
  combination 
  described 
  in 
  this 
  Journal 
  

   for 
  April, 
  1889, 
  is 
  not 
  alluded 
  to 
  here, 
  for 
  it 
  is 
  without 
  doubt 
  

   from 
  all 
  practical 
  standpoints 
  the 
  most 
  efficient 
  double 
  objective 
  

   yet 
  constructed. 
  This 
  readily 
  admits 
  of 
  the 
  ordinary 
  ratio 
  of 
  

   length 
  to 
  aperture 
  and 
  has 
  been 
  in 
  continuous 
  use 
  since 
  its 
  inven- 
  

   tion 
  ; 
  but 
  it 
  is 
  true 
  that 
  large 
  objectives 
  of 
  its 
  type 
  are 
  probably 
  

   impracticable. 
  c. 
  s. 
  h. 
  

  

  